UNIVERSITY  OF  CALIFORNIA. 


Class 


QDIO/P7  v.  i 


WORKS  OF  ALFRED  I.  CORN 

PUBLISHED   BY 

JOHN   WILEY  &  SONS. 


Indicators  and  Test-papers. 

Their  Source,  Preparation,  Application,  and  Tests 
for  Sensitiveness.  With  Tabular  Summary  of  the  Ap- 
plication of  Indicators.  Second  Edition,  Revised  and 
Enlarged.  i2mo,  ix  +  267  pages.  Cloth,  $2.00. 

Tests  and  Reagents. 

Chemical  and  Microscopical,  known  by  their 
Authors'  Names;  together  with  an  Index  of  Subjects. 
8vo,  iii  +  383  pages.  Cloth,  $3.00. 

TRANS  LA  TIONS. 

Fresenius's  Quantitative  Chemical  Analysis. 

New  Authorized  Translation  of  the  latest  German 
Edition.  In  two  volumes.  By  Alfred  I.  Conn, 
Phar.D.  Recalculated  on  the  basis  of  the  latest  atomic 
weights,  and  also  greatly  amplified  by  the  translator. 
8vo,  2  vols.,  upwards  of  2000  pages,  280  figures.  Cloth, 
$12.50. 

Techno-Chemical  Analysis. 

By  Dr.  G.  LUNGE,  Professor  at  the  Eidgenossische 
Polytechnische  Schule,  Zurich.  Authorized  Transla- 
tion by  Alfred  I.  Cohn,  Phar.D.  i2ino,  vii  +  i36  pages, 
16  figures.  Cloth,  $1.00. 

Toxins  and  Venoms  and  Their  Antibodies. 

By  EM.  Pozzi-EscoT.  Authorized  Translation  by 
Alfred  I.  Cohn,  Phar.D.  iamo,  vii+  101  pages.  Cloth, 
$1.00,  net. 


QUANTITATIVE 
CHEMICAL  ANALYSIS 


BY  THE  LATE 


DR.  0.  EEMIGIUS  FRESENIUS 

PRIVY  AULIC  COUNSELLOR; 

DIRECTOR  OF  THK  CHEMICAL  LABORATORY   AT  WIESBADEN 


AUTHORIZED    TRANSLATION   OF   THE   SIXTH  GERMAN    EDITION, 

GREATLY  AMPLIFIED  AND  REVISED 

BY 

ALFRED   I.   COHN 

AUTHOR  OF  "INDICATORS  AND  TEST-PAPERS,"  AND  "TESTS  AND  REAGENTS.** 

MEMBER  OF  THE  AMERICAN  CHEMICAL  SOCIETY;  SOCIETY  OF 

CHEMICAL  INDUSTRY;  VEREIN  DEUTSCHER 

CHEMJKER;  ETC. 


VOL.  I 


NEW  YORK 

'  JOHN  WTLEY   &   SONS 
43-45    EAST   NINETEENTH    STREET 

1907. 


/><  f-c 


Copyright,  1903, 

BY 

ALFRED  I.  COHN, 


ROBERT  DRUMMOND,    PRINTER,   NEW  VOHK. 


PREFACE. 


THE  great  advances  made  in  analytical  chemistry  since  the 
publication  of  the  previous  edition  of  this  work,  and  the  intro- 
duction' of  numerous  new  methods  of  analysis  and  improvements 
upon  older  ones,  have  necessitated  a  new  translation  of  the  most 
recent  German  edition.  This  translation  is  here  presented. 

While  the  German  text,  however,  has  been  adhered  to  as 
closely  as  possible  in  the  translation,  the  requirements  of  the 
skilled  analyst,  as  well  as  of  the  student,  have  been  borne  in  mind, 
and  hence  all  the  values  in  the  book  excepting  those  of  Appendices 
I  and  II  have  been  recalculated  on  the  basis  of  the  table  of  atomic 
weights  published  by  Prof.  F.  W.  CLARKE  in  the  Journal  of  the 
American  Chemical  Society,  March,  1902;  furthermore,  all  the 
old-style  formulas  and  equations  have  been  made  to  conform 
to  the  chemical  notation  and  nomenclature  now  in  use. 

It  had  been  the  translator's  intention  to  greatly  amplify  the 
work  by  inserting  very  many  of  the  more  recent  and  approved 
methods  of  chemical  analysis,  but  as  the  work  in  hand  progressed 
it  was  found  inexpedient  to  carry  out  the  original  intention  with- 
out too  greatly  enlarging  the  size  of  the  volumes;  and  the  more 
so  as  the  appearance  in  the  interim  of  the  works  by  CLASSEN, 
SMITH,  and  others,  in  part  rendered  unnecessary  any  very  extended 
amplification.  Nevertheless,  quite  a  number  of  new  methods 
have  been  added  by  the  translator,  and  have  been  appropriately 
designated. 

The  matter  inserted  by  Professors  ALLEN  and  JOHNSON  in 
the  previous  edition  has  been  retained  in  this,  unless  superseded 
by  more  recent  and  improved  methods.  In  addition,  and  be- 
cause of  their  undoubted  value,  it  has  been  deemed  useful  to  add 
in  the  form  of  appendices  the  official  methods  of  analysis  adopted 
by  the  Association  of  Official  Agricultural  Chemists,  and  con- 

iii 


1 93937 


IV  PREFACE. 

stituting  Bulletin  No.  46,  revised  edition,  of  the  U.  S.  Dept.  of 
Agriculture,  1899,  because  of  the  legal  recognition  they  receive; 
and  also  the  excellent  treatise,  on  "Some  Principles  and  Methods 
of  Rock  Analysis,"  constituting  Bulletin  No.  176  of  the  U.  S. 
Geological  Survey,  1900. 

The  table  of  factors  and  their  multiples  has  also  been  entirely 
recalculated,  and  the  translator  has  moreover  added  the  loga- 
rithmic values  of  the  factors.  Two  new  tables  of  weights  of  gases 
per  litre  have  been  added  to  Table  IX  by  the  translator,  both 
calculated  on  the  atomic  values  used  in  the  book;  the  object  being 
to  have  all  the  values  in  each  given  table  agree  among  themselves, 
whereby  more  consistent  and  uniform  results  may  be  obtained 
and  fewer  discrepancies  are  likely  to  occur. 

Particular  care  has  been  bestowed  upon  the  index  so  as  to 
render  it  as  complete  and  comprehensive  as  possible,  and  in  order 
that  the  work  may  prove  of  maximum  service  to  the  user. 

ALFRED  I.  COHN. 
NEW  YORK,  November,  1903. 


CONTENTS. 


PAOB 

INTRODUCTION. 1 

PART  I. 

GENKRAL    F»ART. 

SECTION  I. 

Operations,  §  1 • 11 

L  Determination  of  quantity,  §  2 11 

1.  Weighing,  §  3 11 

a.  The  balance 12 

Accuracy,  §  4 12 

Sensibility,  §  5 14 

Testing,  §  6 15 

§  7 17 

6.  The  weights,  §  8 19 

C.  The  process  of  weighing,  §  9 21 

Rules,  §  10 23 

2.  Measuring,  §  11 26 

a.  The  measuring  of  gases,  §  12 27 

Correct  reading-off,  §  13 30 

Influence  of  temperature,  §  14 33 

Influence  of  pressure,  §  15 33 

Influence  of  moisture,  §  16 34 

&  The  Measuring  of  fluids    §  17 36 

a.  Measuring  vessels  graduated  to  hold  certain  volumes 

of  fluid. 

aa.  Vessels  serving  to  measure  out  one  definite  volume 
of  fluid. 

1.  Measuring  flasks,  §  18 36 

H>.  Vessels  serving  to  measure  out  different  volumes  of 

fluid. 

2.  The  graduated  cylinder,  §  19 30 

v 


CONTENTS. 


ft.  Measuring  vessels  graduated  to  deliver  certain  vol- 

umes of  fluid. 
aa.  Vessels  serving  to  measure  out  one  definite  volume 

of  fluid. 

3.  The  graduated  pipette,  §  20  ................  39 

bb.  Vessels  serving  to  measure  out  different  volumes  of 

fluid. 

4.  The  Burette. 

I.  Mohr's  burette,  §  21  ..................  42 

II.  Gay-Lussac's  burette,  §  22  ........  .....  48 

III.  Geissler's  burette,  §  23  ................  49 

H.  Preliminary  operations.     Preparation  of  substances  for  the  pro- 
cesses of  quantitative  analysis. 

1.  Selection  of  the  sample,  §  24  .................  .  .........  50 

2.  Mechanical  division,  §  25  ...............................  51 

3.  Drying,  §  26  ..........................................  54 

Desiccators,  §  27  ....................................  56 

Water-baths,  §  28  .........  ..........................  58 

Air-baths,  §  29  ......................................  62 

Oil-baths,  §  30  ......................................  66 

Drying-disk,  §  31  ...................................  67 

III.  General  procedure  in  quantitative  analysis,  §  32  ...............  69 

1.  Weighing  the  substance,  §  33  ...........................  70" 

2.  Estimation  of  water,  §  34  ..............................  72 

a.  Estimation  of  water  by  loss  of  weight,  §  35  ..........  72 

b.  Estimation  of  water  by  direct  weighing,  §  36  .........  75 

3.  Solution  of  substances,  §  37  ............................  79 

a.  Direct  solution,  §  38  ..........................  ....  79 

b.  Decomposition  by  fluxing,  §  39  ..............  .......  80 

4.  Conversion  of  the  dissolved    substance    into   a  weighable 

form,  §  40  ......................................  81 

a.  Evaporation,  §  41  ................................  81 

Weighing  of  residues,  §  42  .......................  89 

b.  Precipitation,  §  43  ...............................  91 

a.  Separation  of  precipitates  by  decantation,  §  44..  93 

0.  Separation  of  precipitates  by  filtration,  §  45.  ...  94 

aa.  Ordinary  filtration,  §  45  ................  94 

aa.  Filtering  apparatus  ...............  94 

jlfi.    Rules  to  be  observed  in  the  process 

of  filtration  ...................  .  96 

7-7-.    Washing  of  precipitates,  §  46  ......  98 

bb.  Filtration  by  suction,  §  47.  .  .............  100 

•f.  Separation  of  precipitates  by  decantation  and  fil- 

tration combined,  §  l"8  ......................  108 


CONTENTS.  Vil 

FACE 

Further  treatment  of  precipitates  preparatory  to 

weighing,  §  49 109 

aa.  Drying  of  precipitates,  §  50 110 

bb.  Ignition  of  precipitates,  §  51.  . ..- 112 

First  method,  §  52 116 

Second  method,  §  53 118 

Asbestos  filters  with  Bunsen's  apparatus,  §  53,  a  ...  120 

5.  Volumetric  analysis,  §  54 122 

SECTION  II. 

Reagents,  §  55 127 

A.  Reagents  for  gravimetric  analysis  in  the  wet  way. 

I.  Simple  solvents,  §  56 127 

II.  Acids  and  halogens. 

a.  Oxygen  acids,  §  57 128 

6.  Hydrogen  acids  and  halogens,  §  58 129 

c.   Sulpho-acids 131 

III.  Bases  and  metals. 

a.  Oxygen  bases  and  metals. 
a.  Alkalies,  and 

£.  Alkaline  earths,  §  59 131 

f.  Heavy  metals  and  oxides  of  heavy  metals,  §  60 132 

6.   Sulpho-bases 134 

IV.  Salts. 

a.  Salts  of  the  alkalies,  §  61 135 

6.  Salts  of  the  alkali-earth  metals,  §  62 137 

c.   Salts  of  the  heavy  metals,  §  63 139 

B.  Reagents  for  gravimetric  analysis  in  the  dry  way,  §  64 140 

C.  Reagents  for  volumetric  analysis,  §  65 144 

D.  Reagents  for  organic  analysis,  §  66 151 

SECTION  III. 

Forms  and  combinations  in  which  substances  are  separated  from  each 

other,  or  weighed,  §  67 / 158 

A.  BASIC  RADICALS. 

FIRST    GROUP. 

1.  Potassium,  §  68 161 

2.  Sodium,  §  69 164 

3.  Ammonium,  §  70 . 167 

SECOND    GROUP. 

1.  Barium,  §  71 '. 168 

2.  Strontium,  §  72 171 

3.  Calcium,  §  73 173 

4.  Magnesium,  §  74 176 


Vlll  CONTENTS. 

THIRD   GROUP. 

PAGB 

1.  Aluminium,  §  75 179 

2.  Chromium,  §  76 181 

FOURTH   GROUP. 

1.  Zinc,  §  77 182 

2.  Manganese,  §  78 185 

3.  Nickel,  §  79 189 

4.  Cobalt,  §  80 191 

5.  Ferrous  iron;  and  6.  Ferric  iron,  §  81 194 

FIFTH    GROUP. 

1.  Silver,  §  82 198 

2.  Lead,  §  83 201 

3.  Mercury  in  mercurous;  and  4.  in  mercuric  compounds,  §  84 205 

5.  Copper,  §  85 208 

6.  Bismuth,  §  86 211 

7.  Cadmium,  §  87 213 

SIXTH   GROUP. 

1.  Gold,  §  88 215 

2.  Platinum,  §  89 215 

3.  Antimony,  §  90 ' 216 

4.  Tin  in  stannous;  and  5.  in  stannic  compounds,  §  91 219 

6.  Arsenous  acid ;  and  7.  Arsenic  acid,  §  92 221 

B.  ACIDS. 

FIRST   GROUP,    §  93. 

1.  Arsenous  and  arsenic  acids. 

2.  Chromic  acid 225 

3.  Sulphuric  acid 225 

4.  Phosphoric  acid 226 

5.  Boric  acid 232 

6.  Oxalic  acid 232 

7.  Hydrofluoric  acid 232 

8.  Carbonic  acid 233 

9.  Silicic  acid 233 

SECOND    GROUP,    §  94. 

1.  Hydrochloric  acid 235 

2.  Hydrobromic  acid 235 

3.  Hydriodic  acid 236 

4.  Hydrocyanic  acid 237 

5.  Hydrosulphuric  acid 237 

THIRD    GROUP,    §  95. 

1.  Nitric  acid 238 

2.  Chloric  acid 238 


CONTENTS. 


SECTION  IV. 

PAGE 

Determination  of  radicals,  §  96  ...................................  239 

I.  Determination  of  basic  radicals  .................................  242 

FIRST  GROUP. 

1.  Potassium.  §  97  ..........................................  •  242 

2.  Sodium,  §  98  .............................................  248 

3.  Ammonium,  §  99  .........................................  251 

Supplement  to  first  group,  §  100. 

4.  Lithium  .................................................  258 

SECOND    GROUP. 

1.  Barium  ,  §  101  ............................................  262 

2.  Strontium,  §  102  .........................................  265 

3.  Calcium,  §  103  ...........................................  268 

4.  Magnesium,  §  104  ........................................  274 

THIRD    GROUP. 

1.  Aluminium,  §  105  ........................................  277 

2.  Chromium,  §  106  .........................................  280 

Supplement  to  third  group,  §  107. 

3.  Titanium  ................................................  284 

FOURTH    GROUP. 

1.  Zinc,  §  108  ..............................................  286 

2.  Manganese,  §  109  ................  ,  ........................  291 

3.  Nickel,  §  110  ................................  .  ............  301 

4.  Cobalt,  §  111  .............................................  305 

5.  Ferrous  iron,  §  112  .......................................  310 

6.  Ferric  iron,  §  113  ....................  .'  ....................  321 

Supplement  to  fourth  group,  §  114. 

7.  Uranium  ................................................  335 

FIFTH    GROUP. 

1.  Silver,  §  115  .............................................  337 

2.  Lead,  §  116  ..............................................  351 

3.  Mercury  in  mercurous  compounds,  §  117  .....................  361 

4.  Mercury  in  mercuric  compounds,  §  118  ......................  363 

5.  Copper,  §  119  ............................................  370 

6.  Bismuth,  §  120  ...........................................  382 

7.  Cadmium,  §  121  ..........................................  387 

Supplement  to  fifth  group,  §  122. 

8.  Palladium.  .  389 


CONTENTS. 


SIXTH   GROUP. 

PAGE 

1.  Gold,  §123 , 391 

2.  Platinum,  §  124 393 

3.  Antimony,  §  125 395 

4.  Tin  in  stannous;   and  5.  in  stannic  compounds,  §  126 403 

6.  Arsenous  acid ;  and  7.  Arsenic  acid,  §  127 409 

Supplement  to  sixth  group,  §  128. 

8.  Molybdic  acid 1 1 , 1 1 .  t  •  1 1 . » t .  1 1 » , 420 

JL  Estimation  of  the  acids. 


FIRST  GROUP. 

First  Division. 

1.  Arsenous  and  arsenic  acids,  §  129 422 

2.  Chromic  acid,  §  130 422 

Supplement,  §  131. 

1.  Selenous  acid 429 

2.  Sulphurous  acid 431 

3.  Thiosulphuric  acid 432 

4.  lodic  acid 432 

5.  Nitrous  acid 433 

Second  Division. 

Sulphuric  acid,  §  132 434 

Supplement,  §  133. 

Hydrofluosilicic  acid 442 

Third  Division. 

1.  Phosphoric  acid. 

I.  Determination,  §  134 444 

II.  Separation  from  the  bases,  §  135 457 

2.  Boric  acid,  §  136 465 

3.  Oxalic  acid,  §  137 470 

4.  Hydrofluoric  acid,  §  138 472 

Fourth  Division.  . 

1.  Carbonic  acid,  §  139 479 

2.  Silicic  acid,  §  140 505 

SECOND   GROUP. 

1.  Chlorine  (Hydrochloric  acid),  §  141 521 

Supplement:  free  chlorine,  §  142 529 

2.  Bromine  (Hydrobromic  acid),  §  143 532 

Supplement:  free  bromine,  §  144 536 

3.  Iodine  (Hydriodic  acid),  §  145 536 

Supplement:  free  iodine,  §  146 542 

4.  Cyanogen  (Hydrocyanic  acid),  §  147 548 

5.  Sulphur  (Hydrosulphuric  acid),  §  148 558 


CONTENTS.  XI 

THIRD    GEOUP. 

PAGE 

1.  Nitric  acid,  §149. 571 

2.  Cholricacid,  §150. 393 

SECTION  V. 
Separation  of  bodies,  §  151 596 

I.    SEPARATION    OF   BASIC   RADICALS   FROM   EACH   OTHER. 
FIRST    GROUP. 

Separation  of  the  alkalies  from  each  other,  §  152 599 


SECOND    GROUP. 


L  Separation  of  the  basic  radicals  of  the  second  group  from  those  of 

first,  §  153 607 

II.  Separation  of  the  basic  radicals  of  the  second  group  from  each  other, 

§  154 615 


THIRD   GROUP. 

I.  Separation  of  metals  of  the  third  group  from  the  alkalies,  §  155. ...  622 
II.  Separation  of  metals  of  the  third  group  from  the  alkali-earth  metals, 

§  156 623 

III.  Reparation  of  metals  of  the  third  group  from  each  other,  §  157. . . .  630 

FOURTH  GROUP. 

I.  Separation  of  the  metals  of  the  fourth  group  from  the  alkalies,  §  158    631 
II.  Separation  of  the  metals  of  the  fourth  group  from  those  of  the 

second,  §  159 633 

III.  Separation  of  the  motals  of  the  fourth  group  from  those  of  the  third 

and  from  each  other,  §  160 639 

IV.  Separation  of  iron,  aluminium,  manganese,  calcium,  magnesium, 

potassium,  and  sodium,  §  161.  -. 666 

Separation  of  uranium  from  the  metals  of  groups  I. -IV. 672 

FIFTH   GROUP. 

I.  Separation  of  the  metals  of  the  fifth  group  from  those  of  the  preced- 
ing four  groups,  §  162 676 

IL  Separation  of  the  metals  of  the  fifth  group  from  each  other,  §  163 .     685 

SIXTH   GROUP. 

I.  Separation  of  the  metals  of  the  sixth  group  from  those  of  the  first 

five  groups,  §  164 699 

II.  Separation  of  the  metals  of  the  sixth  group  from  each  other,  §  165     715 


Xll  CONTENTS. 


H.   SEPARATION  OF  ACIDS   FROM  EACH  OTHER. 

FIRST    GROUP. 

PAGE 

Separation  of  the  acids  of  the  first  group  from  each  other,  §  166. .     730 

SECOND    GROUP. 

I.  Separation  of  the  acids  of  the  second  group  from  those  of  the  first, 

§  167 739 

Supplement. — Analysis  of  compounds  containing  sulphides  of  the 

alkali  metals,  carbonates,  sulphates,  and  thiosulphates,  §  168. .     742 
II.  Separation  of  the  acids  of  the  second  group  from  each  other,  §  169. .     744 

THIRD    GROUP. 

I.  Separation  of  the  acids  of  the  third  group  from  those  of  the  two 

first  groups,  §  170 757 

IL  Separation  of  the  acids  of  the  third  group  from  each  other 759 


INTRODUCTION. 


Chemical  analysis  comprises  two  branches,  viz.,  qualitative 
analysis  and  quantitative  analysis,  the  object  of  the  former 
being  to  ascertain  the  nature,  that  of  the  latter  to  determine  the 
amount,  of  the  several  component  parts  of  any  compound. 

By  QUALITATIVE  ANALYSIS  we  convert  the  unknown  constituents 
of  a  body  into  certain  "known  forms  and  combinations,  whereby  we 
are  enabled  to  draw  correct  inferences  respecting  the  nature  of 
these  unknown  constituents.  In  QUANTITATIVE  ANALYSIS  the  object 
is  attained,  according  to  circumstances,  often  by  very  different 
ways ;  the  two  methods  most  widely  differing  from  each  other  are 
analysis  by  weight,  or  gravimetric  analysis,  and  analysis  by 
measure,  or  volumetric  analysis. 

GRAVIMETRIC  ANALYSIS  has  for  its  object  the  conversion  of  the 
"known  constituents  of  a  substance  into  forms  or  combinations 
which  will  admit,  of  the  most  exact  determination  of  their  weight, 
and  of  which,  moreover,  the  composition  is  accurately  known. 
These  new  forms  or  combinations  may  be  either  educts  from  the 
analyzed  substance,  i.e.,  bodies  that  were  present  as  such  in  the 
analyzed  substance,  as  water  in  crystallized  sodium  sulphate  or 
charcoal  in  gunpowder;  or  they  may  be  products,  i.e.,  substances 
that  have  formed  from  the  constituents  of  the  analyzed  substance 
by  the  addition  of  other  elements,  e.g.,  carbonic  acid  and  water  by 
the  combustion  of  paraffin,  or  barium  sulphate  on  bringing  together 
barium-chloride  solution  and  sulphuric  acid.  In  the  former  case 
the  ascertained  weight  of  the  eliminated  substance  is  the  direct"  ex- 
pression of  the  amount  in  which  it  existed  in  the  compound  under 
cxnmination;  in  the  latter  case,  that  is,  when  we  have  to  deal  with 
2)roducts,  the  quantity  in  which  the  eliminated  constituent  was 
originally  present  in  the  analyzed  compound  has  to  be  deduced  by 


2  INTRODUCTION. 

calculation  from  the  quantity  in  which,  it  exists  in  its  new  combi- 
nation. 

The  following  example  will  serve  to  illustrate  these  points: 
Suppose  we  wish  to  determine  the  quantity  of  mercury  contained 
in  mercuric  chloride.  We  may  do  this,  either  by  precipitating  the 
metallic  mercury  from  the  solution  of  the  chloride,  say  by  means 
of  stannous  chloride,  or  we  may  attain  our  object  by  precipitating 
the  solution  by  hydrogen  sulphide  and  weighing  the  precipi- 
tated mercuric  sulphide.  100  parts  of  mercuric  chloride  consists 
of  73*83  of  mercury  and  26-17  of  chlorine;  consequently  if  the 
process  is  conducted  •  with  absolute  accuracy,  the  precipitation  by 
stannous  chloride  of  the  mercury  in  100  parts  of  mercuric  chloride 
will  yield  73 '83  parts  of  metallic  mercury.  With  equally  exact 
manipulation  the  other  method  yields  85 '67  parts  of  mercuric 
sulphide. 

Now,  in  the  former  case  we  find  the  numbers  73*83  directly ;' 
in  the  latter  case  we  have  to  deduce  it  by  calculation  (100  parts, 
of  mercuric  sulphide  contain  86*18  parts  of  mercury;  how  much 
mercury  do  85*67  parts  contain?): 

100  :  85*67  :  :  86*18  :  x  .  •.  x  =  73*83. 

As  already  hinted,  it  is  absolutely  indispensable  that  the  forms 
into  which  bodies  are  converted  for  the  purpose  of  estimation  by 
weight  phould  fulfil  two  conditions.  First,  they  must  be  capable  of 
being  weighed  exactly ;  secondly,  they  must  be  of  known  composi- 
tion, for  it  is  quite  obvious,  on  the  one  hand,  that  accurate  quan- 
titative analysis  must  be  altogether  impossible  if  the  substance  the 
quantity  of  which  it  is  intended  to  ascertain  does  not  admit  of 
correct  weighing ;  and  on  the  other  hand,  it  is  equally  evident  that 
if  we  do  not  know  the  exact  composition  of  a  new  product,  we  lack 
the  necessary  basis  of  our  calculation. 

YOLUMETKIC  ANALYSIS  is  based  upon  a  principle  very  different 
from  that  of  gravimetric  analysis ;  viz.,  it  effects  the  quantitative 
determination  of  a  body  by  converting  it  from  a  certain,  definite 
state  to  another  equally  definite  state  by  means  of  a  fluid  of  accu- 
rately known  power  of  action,  and  under  circumstances  which  per- 
mit the  analyst  to  mark  with  rigorous  precision  the  exact  point 
when  the  conversion  is  accomplished.  The  following  example  will 
serve  to  illustrate  the  principle  of  this  method:  Potassium  per- 


INTRODUCTION.  3 

manganate  added  to  a  solution  of  ferrous  sulphate  acidulated  with 
sulphuric  acid  immediately  converts  the  ferrous  sulphate  into  fer- 
ric sulphate,  the  permanganic  acid,  characterized  by  its  intense 
color,  yielding  up  oxygen  and  forming  with  the  free  sulphuric  acid 
present  colorless  manganous  sulphate.  If,  therefore,  to  an  acidu- 
lated fluid  containing  a  ferrous  salt  we  add,  drop  by  drop,  a  solu- 
tion of  potassium  permanganate,  its  red  color  continues  for  some 
time  to  disappear  upon  stirring;  but  at  last  a  point  is  reached 
when  the  coloration  imparted  to  the  fluid  by  the  last  drop  added 
remains.  This  point  marks  the  termination  of  the  conversion  of 
the  ferrous  salt  into  a  ferric  salt. 

If  we  now  accurately  determine  the  effective  value  of  the  per- 
manganate solution,  which  may  be  done  by  noting  its  action  on  a 
known  quantity  of  dissolved  ferrous  sulphate,  we  are  in  a  position 
to  determine,  by  means  of  this  solution,  the*  quantity  of  ferrous  salt 
in  any  solution  of  unknown  strength.  For  instance,  suppose  there 
were  required  just  100  parts  of  the  permanganate  solution  to  com- 
pletely oxidize  2  parts  of  ferrous  salt  in  solution.  If,  therefore, 
we  used  only  50  parts  of  the  permanganate  solution,  only  1  part  of 
ferrous  salt  would  be  indicated  as  being  present,  etc.  Accordingly, 
by  measuring  the  quantity  of  permanganate  solution,  the  propor- 
tional quantity  of  ferrous  salt  is  at  once  determined. 

Since  the  quantity  of  active  fluid  used  is  determined  by  measur- 
ing, and  not  by  weighing,  this  method  of  analysis  is  termed  volu- 
metric analysis.  The  object  in  view  is  usually  much  more 
rapidly  attained  by  its  means  than  by  gravimetric  analysis. 

To  this  brief  intimation  of  the  general  purport  and  object  of 
quantitative  analysis,  and  the  general  mode  of  proceeding  in  ana- 
lytical researches,  there  must  be  added  that  certain  qualifications  are 
essential  to  those  who  would  devote  themselves  successfully  to  the 
pursuit  of  this  branch.  These  qualifications  are,  1,  theoretical 
knowledge ;  2,  skill  in  manipulation ;  and  3,  strict  conscientious- 
ness. 

The  preliminary  knowledge  required  consists  in  an  acquaintance 
with  qualitative  analysis,  the  stoichiometric  laws,  and  simple  arith- 
metic. Thus  prepared,  we  shall  understand  the  method  by  which 
bodies  are  separated  and  determined,  and  v/e  shall  be  in  a  position 
to  perform  our  calculations,  by  which,  on  the  one  hand,  the  formu- 
las of  compounds  are  deduced  from  the  analytical  results,  and,  on 


4  INTRODUCTION. 

the  other  hand,  the  correctness  of  the  adopted  methods  is  tested, 
and  the  results  obtained  are  controlled.  To  this  knowledge  must 
be  joined  the  ability  to  perform  the  necessary  practical  operations. 
This  axiom  generally  holds  good  for  all  applied  sciences,  but  if  it  is 
true  of  one  more  than  another,  quantitative  analysis  is  that  one. 
'The  most  extensive  and  solid  theoretical  acquirements  will  not 
enable  us,  for  instance,  to  determine  the  amount  of  common  salt 
present  in  a  solution  if  we  are  without  the  requisite  dexterity  to 
transfer  a  fluid  from  one  vessel  to  another  without  the  smallest 
loss  by  spirting,  running  down  the  side,  etc.  The  various  opera- 
tions of  quantitative  analysis  demand  great  aptitude  and  manual 
skill,  which  can  be  acquired  only  by  practice ;  but  even  the  pos- 
session of  the  greatest  practical  skill  in  manipulation,  joined  to  a 
thorough  theoretical  knowledge,  will  still  prove  insufficient  to  in- 
sure a  successful  pursuit  of  quantitative  researches  unless  also 
combined  with  a  sincere  love  of  truth  and  a  firm  determination 
to  accept  none  but  thoroughly  confirmed  results. 

Every  one  who  has  been  engaged  in  quantitative  analysis  knows 
that  cases  will  sometimes  occur,  especially  when  commencing  the 
study,  in  which  doubts  may  be  entertained  as  to  whether  the  result 
will  turn  out  correct,  or  in  which  even  the  operator  is  positively 
convinced  that  it  cannot  be  quite  correct.  Thus,  for  instance,  a 
small  portion  of  the  substance  under  investigation  may  be  spilled 
or  some  of  it  lost  by  decrepitation ;  or  the  analyst  may  have  rea- 
son to  doubt  the  accuracy  of  his  weighing ;  or  it  may  happen  that 
two  analyses  of  the  same  substance  do  not  exactly  agree.  In  all 
such  cases  it  is  indispensable  that  the  operator  should  be  conscien- 
tious enough  to  repeat  the  whole  process  over  again.  He  who  is 
not  possessed  of  this  self-command — who  shirks  trouble  where 
truth  is  at  stake — who  would  be  satisfied  with  mere  assumptions 
and  guess-work  where  the  attainment  of  positive  certainty  is  the 
object — must  be  pronounced  just  as  deficient  in  the  necessary  quali- 
fications for  quantitative  analytical  researches  as  he  who  is  wanting 
in  knowledge  or  skill.  He,  therefore,  who  cannot  fully  trust  his 
work,  who  cannot  swear  to  the  correctness  of  his  results,  may  in- 
deed occupy  himself  with  quantitative  analysis  by  way  of  practice, 
but  lie  ought  on  no  account  to  publish  or  use  his  results  as  if  they 
were  positive,  since  such  proceeding  could  not  conduce  to  his  own 


INTRODUCTION.  5 

advantage,  and  would  certainly  be  mischievous  as  regards  the 
science. 

The  domain  of  quantitative  analysis  may  be  said  to  extend  over 
all  matter — that  is,  in  other  words,  anything  corporeal  may  become 
the  object  of  quantitative  investigation.  The  present  work,  how- 
ever, is  intended  to  embrace  only  the  substances  used  in  pharmacy, 
arts,  trades,  and  agriculture. 

Quantitative  analysis  may  be  subdivided  into  two  branches, 
viz.,  analysis  of  mixtures,  and  analysis  of  chemical  compounds. 
This  division  may  appear  at  first  sight  of  very  small  moment,  yet  it 
is  necessary  that  we  should  establish  and  maintain  it  if  we  would 
form  a  clear  conception  of  the  value  and  utility  of  quantitative 
research.  The  quantitative  analysis  of  mixtures,  too,  has  not  the 
same  aim  as  that  of  chemical  compounds ;  and  the-  method  applied 
to  secure  the  correctness  of  the  results  in  the  former  case  is  differ- 
ent from  that  adopted  in  the  latter.  The  quantitative  analysis  of 
chemical  compounds  also  rather  subserves  the  purposes  of  the  sci- 
ence, while  that  of  mixtures  belongs  to  the  practical  purposes  of 
life.  If,  for  instance,  we  analyze  the  salt  of  an  acid,  the  result  of 
the  analysis  will  give  the  constitution  of  that  acid,  its  combining 
proportion,  saturating  capacity,  etc.  ;  or,  in  other  words,  the 
results  obtained  will  enable  us  to  answer  a  series  of  questions  of 
which  the  solution  is  important  for  the  theory  of  chemical  science. 
But  if,  on  the  other  hand,  we  analyze  gunpowder,  alloys,  medicinal 
mixtures,  ashes  of  plants,  etc.,  etc.,  we  have  a  very  different 
object  in  view.  It  is  not  intended  in  such  cases  to  apply  the 
results  obtained  to  the  solution  of  any  theoretical  question  in 
chemistry,  but  we  want  to  render  a  practical  service  either  to  the 
arts  and  industries  or  to  some  other  science.  If  in  the. analysis  of 
a  chemical  compound  we  wish  to  control  the  results  obtained,  we 
may  do  this  in  most  cases  by  means  of  calculations  based  on 
stoichiometric  data,  but  in  the  case  of  a  mixture,  a  second  analysis 
is  necessary  to  confirm  the  correctness  of  the  results  afforded  by 
the  first. 

The  preceding  remarks  clearly  show  the  immense  importance 
of  quantitative  analysis.  It  may,  indeed,  be  averred  that  chem- 
istry owes  to  this  branch  its  elevation  to  the  rank  of  a  science,  since 
quantitative  researches  have  led  us  to  discover  and  determine  the 
laws  which  govern  the  combinations  and  transpositions  of  the  ele- 


6  INTRODUCTION. 

ments.  Stoichiometry  is  entirely  based  upon  the  results  of  quanti- 
tative investigations;  all  rational  views  respecting  the  constitution 
of  compounds  rest  upon  them  as  the  only  safe  and  solid  basis. 

Quantitative  analysis,  therefore,  forms  the  strongest  and  most 
powerful  lever  for  chemistry  as  a  science,  and  not  less  so  for 
chemistry  in  its  applications  to  the  practical  purposes  of  life,  to 
trades,  arts,  manufactures,  and  likewise  in  its  application  to  other 
sciences.  It  teaches  the  mineralogist  the  true  nature  of  minerals,  and 
suggests  to  him  principles  and  rules  for  their  recognition  and  classi- 
fication. It  is  an  indispensable  auxiliary  to  the  physiologist,  and 
agriculture  has  already  derived  much  benefit  from  it;  but  far 
greater  benefits  may  be  predicted.  We  need  not  expatiate  here  upon 
the  advantages  which  medicine,  pharmacy,  and  every  branch  of 
industry  derive,  either  directly  or  indirectly,  from  the  practical 
application  of  its  results.  On  the  other  hand,  the  benefit  thus  be- 
stowed by  quantitative  analysis  upon  the  various  sciences,  arts,  etc., 
has  been  in  a  measure  reciprocated  by  some  of  them.  Thus,  while 
Stoichiometry  owes  its  establishment  to  quantitative  analysis,  the 
stoichiometric  laws  afford  us  the  means  of  controlling  the  results 
of  our  analyses  so  accurately  as  to  justify  the  reliance  which  we 
now  generally  place  on  them.  Again,  while  quantitative  analysis 
has  advanced  the  progress  of  arts  and  industry,  our  manufacturers 
in  return  supply  us  with  the  most  perfect  platinum,  glass,  and  por- 
celain vessels,  and  with  articles  of  india-rubber,  without  which  it 
would  be  next  to  impossible  to  conduct  our  analytical  operations 
with  the  minuteness  and  accuracy  which  we  have  now  attained. 

Although  the  aid  which  quantitative  analysis  thus  derives  from 
Stoichiometry  and  the  arts  and  manufactures  greatly  facilitates  its 
practice,1  and  although  many  determinations  are  considerably  abbre- 
viated by  volumetric  analysis,  it  must  be  admitted,  notwithstanding, 
that  the  pursuit  of  this  branch  of  chemistry  requires  considerable 
expenditure  of  time.  This  remark  applies  especially  to  those  who 
are  commencing  the  study,  for  they  must  not  allow  their  attention 
to  be  divided  upon  many  things  at  one  time,  otherwise  the  accuracy 
of  their  results  will  be  more  or  less  injured.  I  would  therefore 
advise  every  one  desirous  of  becoming  an  analytical  chemist  to  arm 
himself  with  a  considerable  share  of  patience,  reminding  him  that 
it  is  not  at  one  bound,  but  gradually,  and  step  by  step,  that  the. 
student  may  hope  to  attain  the  necessary  certainty  in  his  work,  the 


INTRODUCTION.  7 

indispensable  self-reliance  which  can  alone  be  founded  on  one's 
own  results.  However  mechanical,  protracted,  and  tedious  the 
operations  of  quantitative  analysis  may  appear  to  be,  the  attain- 
ment of  accuracy  will  amply  compensate  for  the  time  and  labor 
bestowed  upon  them;  while,  on  the  other  hand,  nothing  can  be 
more  disagreeable  than  to  find,  after  a  long  and  laborious  process, 
that  our  results  are  incorrect  or  uncertain.  Let  him,  therefore, 
who  would  render  the  study  of  quantitative  analysis  agreeable  to 
himself,  from  the  very  outset  endeavor,  by  strict,  nay,  scrupulous 
adherence  to  the  conditions  laid  down,  to  attain  correct  results  at 
any  sacrifice  of  time.  I  scarcely  know  a  better  and  more  imme- 
diate reward  of  labor  than  that  which  springs  from  the  attainment 
of  accurate  results  and  perfectly  corresponding  analyses.  The  satis- 
faction enjoyed  at  the  success  of  our  efforts  is  surely  in  itself  a 
sufficient  motive  for  the  necessary  expenditure  of  time  and  labor, 
€ven  without  looking  to  the  practical  benefits  which  we  may 
derive  from  our  operations. 

The  following  are  the  substances  treated  of  in  this  work : 

I.   METALLOIDS. 

Oxygen,  Hydrogen,  Sulphur,  [Selenium~\,  Phosphorus,  Chlo- 
rine, Iodine,  Bromine,  Fluorine,  Nitrogen,  Boron,  Silicon, 
Carbon. 

II.   METALS. 

Potassium,  Sodium,  [Lithium, ~\  Barium,  Strontium,  Calcium, 
Magnesium,  Aluminium,  Chromium,  [Titanium^]  Zinc,  Manga- 
nese, Nickel,  Cobalt,  Iron,  [Uranium,'}  Silver,  Mercury,  Leady 
Copper,  Bismuth,  Cadmium,  {Palladium,}  Gold,  Platinum, 
Tin,  Antimony,  Arsenic,  {Molybdenum. ,] 

(The  elements  enclosed  within  brackets  are  considered  in  sup- 
plementary paragraphs,  and  more  briefly  than  the  rest.) 


The  subject  has  been  divided  into  three  parts.  In  the  first 
quantitative  analysis  generally  is  treated  of,  describing  the  execu- 
tion of  analysis.  In  the  second  is  given  a  detailed  description  of 
several  special  analytical  processes.  In  the  third  a  number  of 


8  INTRODUCTION. 

carefully  selected  examples,  which  may  serve  as  exercises  for  the 
groundwork  of  the  study  of  quantitative  analysis. 

The  following  table  will  afford  the  reader  a  clear  and  definite 
notion  of  the  contents  of  the  whole  work : 

I.  GENEEAL  PAET. 

A.  Execution  of  analysis. 

1.  Operations. 

2.  Reagents. 

3.  Forms  and  combinations  in  which  bodies  are  separated  from 
others,  or  in  which  their  weight  is  determined. 

4.  Determination  of  bodies  in  simple  compounds. 

5.  Separation  of  bodies. 

6.  Organic  elementary  analysis. 

B.  Calculation  of  the  results. 

II.  SPECIAL  PAET. 

1.  Analysis  of  waters,  more  especially  mineral  waters. 

2.  Analysis  of  such  minerals  and  technical  products  as  are  most 
frequently  the  object  of  chemical  investigation,  including  methods 
for  ascertaining  their  commercial  value. 

3.  Analysis  of  plant  ashes. 

4.  Analysis  of  soils. 

5.  Analysis  of  manures. 

6.  Analysis  of  atmospheric  air. 

III.  EXEECISES   FOE   PEACTICE. 

APPENDIX. 

1.  Analytical  experiments. 

2.  Tables  for  calculating  analytical  results. 


PART  I. 


GENERAL    PART. 


THE    EXECUTION    OF   ANALYSIS. 


SECTION"  I. 
OPEKATIONS. 

* 

§-4 
JL» 

MOST  of  the  operations  performed  in  quantitative  research  are  the 
same  as  in  qualitative  analysis,  and  have  been  accordingly  described 
in  my  work  on  that  branch  of  analytical  science.  "With  respect  to 
such  operations  I  shall,  therefore,  confine  myself  here  to  pointing 
out  any  modifications  they  may  require  to  adapt  them  for  applica- 
tion in  the  quantitative  branch ;  but  there  will,  of  course,  be  giren 
a  full  description  of  such  as  are  resorted  to  exclusively  in  quanti- 
tative investigations.  Operations  forming  merely  part  of  certain 
specific  processes  will  be  found  described  in  the  proper  place, 
under  the  head  of  such  processes. 

I.  DETERMINATION  OF  QUANTITY. 

§2. 

The  quantity  of  solids  is  usually  determined  by  weight;  -the 
quantity  of  gases  and  fluids,  in  many  cases  by  measure;  and 
upon  the  care  and  accuracy  with  which  these  operations  are  per- 
formed, depend  the  value  of  all  our  results.  We  shall  therefore 
dwell  minutely  upon  them. 

§3. 

1.   WEIGHING. 

To  enable  us  to  determine  with  precision  the  correct  weight  of 
a  substance,  it  is  indispensable  that  we  should  possess,  1st,  a 
BALANCE,  and  2d,  accurate  WEIGHTS. 


OPERATIONS. 


[§4. 


a.  THE  BALANCE. 

Fig.  1  represents  a  form  of  balance  well  adapted  for  analytical 
purposes.  There  are  several  points  respecting  the  construction 
and  properties  of  a  good  balance,  which  it  is  absolutely  necessary  for 
every  chemist  to  understand.  The  usefulness  of  this  instrument 
depends  upon  two  points  :  1st,  its  accuracy,  and  2d,  its  sensibility 
or  delicacy. 

M- 

The  ACCURACY  of  a  balance  depends  upon  the  following  condi- 
tions : — 

a.  The  fulcrum  or  the  point  on  which  the  beam  rests  must  lie 
above  the  centre  of  gravity  of  the  balance. 


Fig.  1. 

This  is  in  fact  a  condition  essential  to  every  balance.  If  the 
fulcrum  were  placed  in  the  centre  of  gravity  of  the  balance,  the 
beam  would  not  oscillate,  but  remain  in  any  position  in  which  it  is 
placed,  assuming  the  scales  to  be  equally  loaded.  If  the  fulcrum 
be  placed  below  the  centre  of  gravity,  the  balance  will  be  overset 
by  the  slightest  impulse. 

When  the  fulcrum  is  above  the  centre  of  gravity  the  balance 
represents  a  pendulum,  the  length  of  which  is  equal  to  that  of  the 
line  uniting  the  fulcrum  with  the  centre  of  gravity,  and  this  line 
forms  right  angles  with  the  beam,  in  whatever  position  the  latter 
may  be  placed.  Now,  if  we  impart  an  impetus  to  a  ball  suspended 
by  a  thread,  the  ball,  after  having  terminated  its  vibrations,  will 


§4] 


WEIGHING. 


invariably  rest  in  its  original  perpendicular  position  under  the 
point  of  suspension.  It  is  the  same  with  a  properly  adjusted  bal- 
ance —  impart  an  impetus  to  it,  and  it  will  oscillate  for  some  time, 
but  it  will  invariably  return  to  its  original  position  ;  in  other 
words,  its  centre  of  gravity  will  finally  fall  back  into  its  perpen- 
dicular position  under  the  fulcrum,  and  the  beam  must  consequently 
ivussuine  the  horizontal  position. 

But  to  judge  correctly  of  the  force  with  which  this  is  accom- 
plished, it  must  be  borne  in  mind  that  a  balance  is  not  a  simple 
pendulum,  but  a  compound  one,  i.  e.,  a  pendulum  in  which  not 
one,  but  many  material  points  move  round  the  turning  point.  The 
inert  mass  to  be  moved  is  accordingly  equal  to  the  sum  of  these 
points,  and  the  moving  force  is  equal  to  the  excess  of  the  material 
points  below,  over  those  above  the  fulcrum. 

p.  The  points  of  suspension  of  the  scales  must  be  on  an  exact 
level  with  the  fulcrum.  If  the  •fulcrum  be  placed  below  the  line 
joining  the  points  of  suspension,  increased  loading  of  the  scales 
will  continually  tend  to  raise  the  centre  of  gravity  of  the  whole 
system,  so  as  to  bring  it  nearer  and  nearer  the  fulcrum  ;  the  weight 
which  presses  upon  the  scales  combining  in  the  relatively  high- 
placed  points  of  suspension  ;  at  last,  when  the  scales  have  been 
loaded  to  a  certain  degree,  the  centre  of  gravity  will  shift  alto- 
gether to  the  fulcrum,  and  the  balance  will  consequently  cease  to 
vibrate  —  any  further  addition  of  weight  will  finally  overset  the 
beam  by  placing-  the  centre  of  gravity  above  the  fulcrum.  If,  on 
the  other  hand,  jthe  fulcrum  be  placed  above  the  line  joining  the 
points  of  suspension,  the  centre  of  gravity  will  become  more  and 
more  depressed  in  proportion  as  the  loading  of  the  scales  is  in- 
creased ;  the  line  of  the  pendulum  will  consequently  be  length- 
ened, and  a  greater  force  will  be  required  to  produce  an  equal 
turn  ;  in  other  words,  the  balance  will  grow  less  sensitive  the 
greater  the  load.  But  when  the  three  edges  are  in  one  plane,  in- 
creased loading  of  the  scales  will,  indeed,  continually  tend  to  raise 
the  centre  of  gravity  towards  the  fulcrum,  but  the  former  can  in 
this  case  never  entirely  reach  the  latter,  and  consequently  the  bal- 
ance will  never  altogether  cease  to  vibrate  upon  the  further  addi- 
tion of  weight,  nor  will  its  sensibility  be  lessened  ;  on  the  contra  ry 
—  speaking  theoretically  —  a  greater  degree  of  sensibility  is  im- 
parted to  it.  This  increase  of  sensibility  is,  however,  compensated 
for  by  other  circumstancas.  (See  §  5.) 


14  OPERATIONS.  [§  5. 

y.  The  beam  must  be  sufficiently  rigid  to  bear  without  bend- 
ing  the  greatest  weight  that  the  construction  of  the  balance  admits 
of.  The  bending  of  the  beam  would  of  course  depress  the  points 
of  suspension  so  as  to  place  them  below  the  fulcrum,  and  this 
would,  as  we  have  just  seen,  tend  to  diminish  the  sensibility  of  the 
balance  in  proportion  to  the  increase  of  the  load.  It  is,  therefore, 
necessary  to  avoid  this  fault  by  a  proper  construction  of  the  beam. 
The  form  best  adapted  for  beams  is  that  of  an  isosceles  obtuse- 
'angled  triangle,  or  of  a  rhombus. 

d.  The  arms  of  the  balance  must  Ue  of  equal  length,  i.  e.,  the 
points  of  suspension  must  be  equidistant  from  the  fulcrum,  for  if 
the  arms  are  of  unequal  length  the  balance  will  not  be  in  equili- 
brium, supposing  the  scales  to  be  loaded  with  equal  weights,  but 
there,  will  be  preponderance  on  the  side  of  the  longer  arm. 

§  5. 

The  SENSIBILITY  of  a  balance  depends  principally  upon  the  three 
following  conditions : — 

a.  The  friction  of  the  edges  upon  their  supports  must  be  as 
slight  as  possible.  The  greater  or  less  friction  of  the  edges  upon 
their  supports  depends  upon  both  the  form  and  material  of  those 
parts  of  the  balance.  The  edges  must  be  made  of  good  steel,  the 
supports  may  be  made  of  the  same  material ;  it  is  better,  however, 
that  the  centre  edge  at  least  should  rest  upon  an  agate  plane.  To 
form  a  clear  conception  of  how  necessary  it  is  that  even  the  end 
edges  should  have  as  little  friction  as  possible,  we  need  simply 
reflect  upon  what  would  happen  were  we  to  fix  the  scales  immov- 
bly  to  the  beam  by  means  of  rigid  rods.  Such  a  contrivance 
would  at  once  altogether  annihilate  the  sensibility  of  a  balance,  for 
if  a  -weight  were  placed  upon  one  scale,  this  certainly  would  have 
a  tendency  to  sink ;  but  at  the  same  time  the  connecting  rods  be- 
ing compelled  to  form  constantly  a  right  angle  with  the  beam,  the 
weighted  scale  would  incline  inwards,  whilst  the  other  scale  would 
turn  outwards,  and  thus  the  arms  would  become  unequal,  the 
shorter  arm  being  on  the  side  of  the  weighted  scale,  whereby  the 
tendency  of  the  latter  to  sink  would  be  immediately  compensated 
for.  The  more  considerable  the  friction  becomes  at  the  end  edges 
of  a  balance,  the  more  the  latter  approaches  the  state  just  now 
described,  and  consequently  the  more  is  its  sensibility  impaired. 

yd   The  centre  of  gravity  must  be  as  near  as  possible  to  theful- 


.^  ;j)  6.]  WEIGHING.  15 


The  nearer  the  centre  of  gravity  approaches  the  fulcrum, 
the  shorter  becomes  the  pendulum.  If  we  take  two  balls,  the  one 
suspended  by  a  short  and  the  other  by  a  long  thread,  and  impart 
the  same  impetus  to  both,  the  former  will  naturally  swing  at  a  far 
greater  angle  from  its  perpendicular  position  than  the  latter.  The 
same  must  of  course  happen  with  a  balance  ;  the  same  weight  will 
cause  the  scale  upon  which  it  is  placed  to  turn  the  more  rapidly 
mid  completely,  the  shorter  the  distance  between  the  centre  of 
gravity  and  the  fulcrum.  We  have  seen  above,  that  in*  a  balance 
where  the  three  edges  are  on  a  level  with  each  other,  increased 
loading  of  the  scales  will  continually  tend  to  raise  the  centre  of 
gravity  towards  the  fulcrum.  A  good  balance  will  therefore  be- 
come more  delicate  in  proportion  to  the  increase  of  weights  placed 
upon  its  scales;  but,  on  the  other  hand,  its  sensibility  will  be  di- 
minished in  about  the  same  proportion  by  the  increment  of  the 
muss  to  be  moved,  and  by  the  increased  friction  attendant  upon 
the  increase  of  load  ;  in  other  words,  the  delicacy  of  a  good  balance 
will  remain  the  same,  whatever  may  be  the  load  placed  upon  it. 
The  nearer  the  centre  of  gravity  lies  to  the  fulcrum,  the  slower  are 
the  oscillations  of  the  balance.  Hence  in  regulating  the  position 
of  the  centre  of  gravity  we  must  not  go  too  far,  for  if  it  ap- 
proaches the  fulcrum  too  nearly,  the  operation  of  weighing  will 
take  too  much  time. 

y.  The  learn  must  ~be  as  light  as  possible.  The  remarks  which 
we  have  just  now  made  will  likewise  show  how  far  the  weight  of 
the  beam  may  influence  the  sensibility  of  a  balance.  We  have  seen 
that  if  a  balance  is  not  actually  to  become  less  delicate  on  increased 
loading,  it  must  on  the  one  hand  have  a  tendency  to  become  more 
delicate  by  the  continual  approach  of  the  centre  of  gravity  to  the 
fulcrum.  ~Now  it  is  evident,  that  the  more  considerable  the  weight 
of  the  beam  is,  the  less  will  an  equal  load  placed  upon  both  scales 
alter  the  centre  of  gravity  of  the  whole  system,  the  more  slowly 
will  the  centre  of  gravity  approach  the  fulcrum,  the  less  will  the 
increased  friction  be  neutralized,  and  consequently  the  less  sensi- 
bility will  the  balance  possess.  Another  point  to  be  taken  into 
account  here  is,  that  the  moving  forces  being  equal,  a  less  mass 
or  weight  is  more  readily  moved  than  a  greater.  (§  4  a.) 

§6. 

We  will  now  proceed,  first,  to  give  the  student  a  few  general 


16  OPERATIONS.  [§  6. 

rules  to  guide  him  in  the  purchase  of  a  balance  intended  for  the 
purposes  of  quantitative  analysis ;  and,  secondly,  to  point  out  the 
best  method  of  testing  the  accuracy  and  sensibility  of  a  balance. 

1.  A  balance  able  to  bear  TO  or  80  grammes  in  each  scale,  suf- 
fices for  most  purposes. 

2.  The  balance  must  be  enclosed  in  a  glass  case  to  protect  it 
from  dust.      This  case  ought  to  be  sufficiently  large,  and,  more 
especially,  its  sides  should  not  approach  too  near  the  scales.     It 
must  be  constructed  in  a  manner  to  admit  of  its  being  opened  and 
closed  with  facility,  and  thus  to  allow  the   operation  of  weighing 
to  be  effected  without  any  disturbing   influence    from    currents 
of  air.     Therefore,  either  the  front  part  of  the  case  should   consist 
of  three  parts,  viz.,  a  fixed  centre  part  and  two  lateral  parts,  open- 
ing like  doors ;  or,  if  the  front  part  happens  to  be  made  of  one 
piece,  and  arranged  as  a  sliding-door,  the  two  sides  of  the  case  must 
be  provided  each  with  a  door. 

3.  The  balance  must  be  provided  with  a  proper  contrivance  to 
render  it  immovable  whilst  the  weights  are  being  placed  upon  the 
scale.     This  is  most  commonly  effected  by  an  arrangement  which 
enables  the  operator  to  lift  up  the  beam  and  thus  to  remove  the 
middle  edge  from  its  support,  whilst  the  scales  remain  suspended. 

Older  contrivances  check  the  scales  without  lifting  the  middle 
edge  from  its  support.  It  is  very  convenient  to  have  a  stop  both 
for  the  scale-pans  and  the  beam.  The  newer  balances  are  almost 
always  so  provided.  The  usual  device  for  checking  the  scale- 
pans  consists  of  two  supports  immediately  below  them,  which  slide 
up  and  down,  and  are  provided  with  crossed  silk  ribbons  or  camel's- 
hair  brushes.  The  supports  must  move  with  such  perfect  steadi- 
ness that,  when  carefully  removed  from  the  scale-pans,  the  latter 
do  not  shake  in  the  least.  This  arrangement  is  of  advantage  in 
facilitating  the  loading  of  the  scale-pans,  besides  enabling  an  im- 
mediate stop  to  be  put  to  trembling  or  shaking  of  the  scales,  and 
is  convenient  also  because  in  cases  where  the  same  body  has  to  be 
weighed  repeatedly,  the  weights  may  be  left  on  the  scale-pan 
without  risk  to  the  balance.  Stops  that  check  the  beam  and  scale- 
pans  by  one  action  (a  turn)  appear  to  me  less  practical,  because 
the  checking  of  the  scale-pans  after  every  addition  of  a  small  weight 
is  purposeless, while  it  impairs  the  rapidity  of  weighing.  It  is 
highly  advisable  to  have  the  checking  contrivances  arranged  to  be 
manipulated  from  without,  while  the  glass  case  remains  closed. 


§  7.]  WEIGHING.  17 

4.  It  is  necessary  that  the  balance  be  provided  with  an  index 
or  pointer  to  mark  its  oscillations  on  a  graduated  arc ;   and  it  is 
more  advantageous  to  have  the  index  beneath  the  axis  rather  than 
at  the  side  of  the  balance. 

5.  The  balance  must  be   provided  with  a  pendulum  or  spirit 
level  in  order  that  the  three  edges  may  be  placed  on   an  exactly 
horizontal  level;  it  is  hence  practical  to  have  the  case  rest  on  three 
screws. 

6.  It  is  very  convenient  and  time-saving  for  the  beam  to  be 
graduated    decimally,    so   that   by   means   of   a    centi- 
gramme   u  rider,"    Fig.    2,    milligrammes    and    their 
fractions  may  be  weighed.       Modern   balances  are  so 
constructed  that  the  rider  may  be  shifted  to  any  posi- 
tion on  the  beam,  without  opening  the  case,  by  means        Fig.  2. 
of  a  movable  arm  passing  through  the  side  of  the  case.* 

7.  The  balance  must  be  provided  with  a  screw  to  regulate  the 
centre  of  gravity,  and  likewise  with  two  screws  to  regulate  the 
equality  of  the  arms,  and  finally  with  screws  to  restore  the  equi- 
librium of  the  scales,  should  this  have  been  disturbed. 

§7. 

The  following  experiments  serve  to  test  the  accuracy  and  sensi- 
bility of  a  balance : 

1.  The  balance  is,  in  the  first  place,  accurately  adjusted,  if 
necessary,  either  by  the  regulating  screws,  or  by  means  of  tinfoil, 
and  a  milligramme  weight  is  then  placed  in  one  of  the  scales.  A 
good  and  practically  useful  balance  must  turn  very  distinctly  with 
this  weight;  a  delicate  chemical  balance  should  indicate  0.1  milli- 
gramme distinctly.  It  should  be  noted  here  that  the  mere  point- 
ing of  the  index  to  zero  is  not  sufficient  evidence  of  equilibrium. 
It  is  much  better  to  observe  the  oscillations  of  the  pointer,  which 
may  be  effected,  if  necessary,  by  a  slight  move  of  the  hand  near 
one  of  the  scale-pans  so  as  to  produce  a  slight  wind.  The  vibra- 
tion must  be  nearly  equal  on  both  sides,  growing  less  with  each 
vibration,  until  the  pointer  finally  comes  to  rest  at  zero. 

*  HEMPEL,  of  Paris,  puts  a  very  complete  arrangement  for  placing  small 
weights  and  shifting  the  rider,  on  his  balances  (see  Zeitschr.  f.  analyt.  Chern.,  iv, 
83).  I  have  had  no  personal  experience  with  it,  however. 


18  OPERATIONS.  [§  7. 

2.  Both  scales  are  loaded  with  the  maximum  weight  the  con- 
struction of  the  balance  will  admit  of;   the  balance  is  then  accu« 
rately  adjusted,  and  a  milligramme  added  to  the  weight  in  the  one 
scale.      This  ought  to  cause  the  balance  to  turn  to  the  same  extent 
as  in  1.     In  most  balances,  however,  it  shows  somewhat  less  on 
the  index.      It  follows  from   §  5  ft  that  the.  balance  will  oscillate 
more  slowly  in  this  than  in  the  first  experiment. 

3.  The  balance  is  accurately  adjusted  (should  it  be  necessary 
to  establish  a  perfect  equilibrium  between  the  scales  by  loading  the 
one  with  a  minute  portion  of  tinfoil,  this  tinfoil  must  be  left  re- 
maining upon  the  scale   during  the  experiment) ;    both  scales  are 
then  equally  loaded,  say,  with  fifty  grammes  each,  and,  if  neces- 
sary,  the    balance    is    again    adjusted    (by   the    addition    of    small 
weights).      The  load  of  the  two  scales  is  then  interchanged,  so  as 
to  transfer  that  of  the  right  scale  to  the  left,  and  vice  versa.     A 
balance  with  perfectly  equal  arms  must  maintain  its  absolute  equi- 
librium upon  this  interchange  of  the  weights  of  the  two  scales. 

4.  The  balance  is  accurately  adjusted ;   it  is  then  arrested  and 
again  set  in  motion ;   the  same  process  should  be  repeated  several 
times.      A  good  balance  must  invariably  reassume  its  original  equi- 
librium.    A  balance  the  end  edges  of  which  afford  too  much  play 
to  the  hook  resting  upon  them,  so  as  to  allow  the  latter  slightly  to 
alter   its   position,    will   show   perceptible    differences   in   different 
trials.     This  fault,  however,  is  possible  only  with  balances  of  defec- 
tive construction.* 

A  balance  to  be  practically  useful  for  the  purposes  of  quantita- 
tive analysis  must  stand  the  first,  second,  and  last  of  these  tests. 
A  slight  inequality  of  the  arms  is  of  no  great  consequence,  as  the 
error  that  it  would  occasion  may  be  completely  prevented  by  the 
manner  of  weighing. 

As  the  sensibility  of  a  balance  will  speedily  decrease  if  the  steel 
edges  are  allowed  to  get  rusty,  delicate  balances  should  never  be 
kept  in  the  laboratory,  but  always  in  a  separate  room.'  It  is  also 
advisable  to  place  within  the  case  of  the  balance  a  vessel  half  filled 
with  calcined  potassium  carbonate,  to  keep  the  air  dry.  I  need 
hardly  add  that  this  salt  must  be  recalcined  as  soon  as  it  gets  moist. 


*  G.  WESTPHAL,  of  Celle,  has  described  a  mode  of  construction  which  abso- 
lutely excludes  the  possibility  of  this  fault  (Zeitschr.f.  analyt.  Chem  ,  vii,  294). 


§  8.]  WEIGHTING.  19 


5.   THE  WEIGHTS. 

Intrinsically,  it  is  quite  immaterial  what  unit  of  weight  is 
adopted.  Most  chemists,  however,  use  the  gramme  weights 
because  of  convenience  in  recording  as  well  as  in  calculating. 
With  regard  to  the  set  of  weights,  it  is  generally  a  matter  of  indif- 
ference for  scientific  purposes  whether  the  gramme,  its  multiples, 
and  fractions  are  really  and  perfectly  equal  to  the  accurately 
adjusted  normal  weights  of  the  corresponding  denominations;*  but 
it  is  absolutely  necessary  that  they  should  agree  perfectly  among 
themselves,  i.e.,  the  centigramme  weight  must  be  exactly  the  one- 
hundredth  part  of  the  gramme  weight  of  the  set,  etc.,  etc. 

Before  describing  the  testing  of  the  weights  as  to  their  accuracy, 
attention  must  be  called  to  the  following  points: 

1.  A   set  of  weights  ranging  from   one  milligramme  to   fifty 
grammes  fully  suffices  for  most  purposes. 

2.  The  set  of  weights  should  be  kept  in  a  suitable,  well- closing 
box ;  and  it  is  desirable  likewise  that  a  distinct  compartment  be 
appropriated  to  every  one,  even  of  the  smaller  weights. 

3.  As  to  the  shape  best  adapted  for  weights,  I  think  that  of 
short  frusta  of  cones  inverted,  with  a  handle  at  the  top,  the  most 
convenient  and  practical  form  for  the  large  weights;  square  pieces 
of  foil,  turned  up  at  one  corner,  are  best  adapted  for  the  small 
weights.      The  foil  used  for  this  purpose  should  not  be  too  thin, 
and    the   compartments  adapted  for  the  reception  of  the  several 
smaller  weights  in  the  box  should  be  large  enough  to  admit  of  their 
contents  being  taken  out  of  them  with  facility,  or  else  the  smaller 
weights  will  soon  become  crumpled  and  defaced.     Every  one  of 
the  weights  (with  the  exception  of  the  milligramme)  should  be  dis- 
tinctly marked. 

4.  So  far  as  the  material  most  suitable  for  weights  is  con- 
cerned, rock  crystal,  though  best  adapted  for  normal  weights,  is 
unsuitable  for  ordinary  weights  because  of  its  high  cost  and  the 
inconvenient  form  the  weights  would  have.      Platinum,  were  it  not 
so   costly,   would   be   surely  adopted  generally,  on  account  of  its 
unchangeability.    As  a  rule,  however,  platinum  is  used  for  weights 

*  It  were  desirable  that  makers  of  analytical  weights  endeavor  to  procure 
normal  weights.  It  is  very  annoying,  in  many  cases,  to  find  notable  differences 
between  weights  of  the  same  denomination,  but  coming  from  different  makers, 
as  I  have  frequently  observed. 


20  OPERATIONS.  [§  8. 

smaller  than  1  gramme  or  0*5  gramme,  while  brass  is  used  for  all 
the  higher  denominations.  Brass  weights  must  be  carefully 
shielded  from  the  action  of  acid  or  other  vapors,  or  their  correct- 
ness will  be  impaired ;  nor  should  they  ever  be  touched  with  the 
fingers,  but  always  with  small  pincers.  It  is  an  erroneous  notion 
to  suppose  that  weights  slightly  tarnished  are  unfit  for  use.  In 
fact,  it  is  scarcely  possible  to  keep  weights  for  any  very  great 
length  of  time  from  becoming  slightly  tarnished.  I  have  carefully 
examined  many  weights  of  this  description,  and  have  found  them 
to  correspond  as  exactly  with  one  another  in  their  relative  propor- 
tions as  they  did  when  first  used.  The  tarnishing  coat  is  so 
extremely  thin  that  even  a  very  delicate  balance  will  generally  fail 
to  point  out  any  perceptible  difference  in  the  weight.  It  will 
nevertheless  be  found  advantageous  to  gild  the  brass  weights  pre- 
vious to  their  final  adjustment. 

The  following  is  the  proper  way  of  testing  the  weights : 
One  scale  of  a  delicate  balance  is  loaded  with  a  one-gramme 
weight,  and  the  balance  is  then  completely  equipoised  by  taring 
with  small  pieces  of  brass,  and  finally  tinfoil  (not  paper,  since  this 
absorbs  moisture).  The  weight  is  then  removed,  and  replaced  suc- 
cessively by  the  other  gramme  weights,  and  afterwards  by  the  same 
amount  of  weight  in  pieces  of  lower  denominations. 

The  balance  is  carefully  scrutinized  each  time,  and  any  devia- 
tion from  the  exact  equilibrium  marked.  In  the  same  way  it  is 
seen  whether  the  two-gramme  piece  weighs  as  much  as  two  single 
grammes,  the  five-gramme  piece  as  much  as  three  single  grammes 
and  the  two-gramme  piece,  etc.  In  the  comparison  of  the  smaller 
weights  thus  among  themselves,  they  must  not  show  the  least  dif- 
ference on  a  balance  turning  with  0*1  milligramme.  In  comparing 
the  larger  weights  with  all  the  small  ones,  differences  of  0*1  to  0"2 
milligramme  may  be  passed  over.  If  you  wish  them  to  be  more 
accurate,  you  must  adjust  them  yourself.  In  the  purchase  of 
weights  chemists  ought  always  to  bear  in  mind  that  an  accurate 
weight  is  truly  valuable,  whilst  an  inaccurate  one  is  absolutely 
worthless."55'  It  is  the  safest  way  for  the  chemist  to  test  every 
weight  he  purchases,  no  matter  how  high  the  reputation  of  the 
maker. 


*  Compare  W.  CROOKES,  on  adjusting  chemical  weights  (Zeitschr.  f.  Analyt 
Chem.,  vi,  431),  and  K.  L.  BAUER  (ibid.,  vin,  390). 


§  9.J  WEIGHING.  21 

§9. 
c.   THE  PKOCESS  OF  WEIGHING. 

There  are  two  different  methods  of  determining  the  weight  of 
substances ;  the  one  might  be  termed  direct  weighing,  the  other  is 
called  weighing  by  substitution. 

In  direct  weighing,  the  substance  is  placed  upon  one  scale,  and 
the  weight  upon  the  other.  If  we  possess  a  balance,  the  arms  of 
which  are  of  equal  length,  and  the  scales  in  a  perfect  state  of 
equilibrium,  it  is  indifferent  upon  which  scale  the  substance  is 
placed  in  the  several  weighings  required  during  an  analytical  pro- 
cess ;  i.e.,  we  may  weigh  upon  the  right  or  upon  the  left  side,  and 
change  sides  at  pleasure,  without  endangering  the  accuracy  of  our 
results.  But  if,  on  the  contrary,  the  arms  of  our  balance  are  not 
perfectly  equal,  or  if  the  scales  are  not  in  a  state  of  perfect  equili- 
brium, we  are  compelled  to  weigh  invariably  upon  the  same  scale, 
otherwise  the  correctness  of  our  results  will  be  more  or  less  materi- 
ally impaired. 

Suppose  we  want  to  weigh  one  gramme  of  a  substance,  and  to 
divide  this  subsequently  into  two  equal  parts.  Let  us  assume 
our  balance  to  be  in  a  state  of  perfect  equilibrium,  but  with 
unequal  arms,  the  left  being  99  millimeters,  the  right  100 
millimeters,  long;  we  place  a  gramme  weight  upon  the  left  scale, 
and  against  this,  on  the  right  scale,  as  much  of  the  substance  to  be 
weighed  as  will  restore  the  equilibrium  of  the  balance. 

According  to  the  axiom,  "  masses  are  in  equilibrium  upon  a 
lever,  if  the  products  of  their  weights  into  their  distances  from  the 
fulcrum  are  equal,"  we  have  consequently  upon  the  right  scale  0'99 
grm.  of  substance,  since  99  X  1*00=:  100  X  0'99.  If  we  now,  for  the 
purpose  of  weighing  one  half  the  quantity,  remove  the  whole 
weight  from  the  left  scale,  substituting  a  0*5  grm.  weight  for  it, 
and  then  take  off  part  of  the  substance  from  the  right  scale,  until 
the  balance  recovers  its  equilibrium,  there  will  remain  0*495  grm. ; 
and  this  is  exactly  the  amount  we  have  removed  from  the  scale : 
we  have  consequently  accomplished  our  object  with  respect  to  the 
relative  weight ;  and  as  we  have  already  remarked,  the  absolute 
weight  is  not  generally  of  so  much  importance  in  scientific  work. 
But  if  we  attempted  to  halve  the  substance  which  we  have  on  the 
right  scale,  by  first  removing  both  the  weight  and  the  substance 


22  OPERATIONS.  [§  9. 

from  the  scales,  and  placing  subsequently  a  0'5  grm.  weight  upon 
the  right  scale,  and  part  of  the  substance  upon  the  left,  until  the 
balance  recovers  its  equilibrium,  we  should  have  0*505  grm.  of 
substance  upon  the  left  scale,  since  100  X  0*5  =  99  X  0'505; 
and  consequently,  instead  of  exact  halves,  we  should  have  one 
part  of  the  substance  amounting  to  0'505,  the  other  only  to  0-485, 
grm. 

If  the  balance  is  equal-armed,  but  the  scale-pans  are  not  in  a 
state  of  absolute  equilibrium,  we  are  obliged  to  weigh  our  sub- 
stances in  vessels  to  insure  accurate  results  (although  the  arms  of 
the  balance  be  perfectly  equal).  It  is  self-evident  that  the  weights 
in  this  case  must  likewise  be  invariably  placed  upon  one  and  the 
same  pan,  and  that  the  difference  between  the  two  scale-pans 
must  not  vary  during  the  course  of  a  series  of  experiments. 

From  these  remarks  result  the  two  following  rules : 

1.  It  is,  under  all  circumstances,  advisable  to  place  the  sub-, 
stance  invariably  upon  one  and  the  same  pan — most  conveniently 
upon  the  left. 

2.  If  the  operator  happens  to  possess  a  balance  for  his  own 
private  and  exclusive  use,  there  is  no  need  that  he  should  adjust  it 
at  the  commencement  of  every  analysis ;  but  if  the  balance  be  used 
in  common  by  several  persons,  it  is  absolutely  necessary  to  ascer- 
tain, before  every  operation,  whether  the  state  of  absolute  equili- 
brium may  not  have  been  disturbed. 

Weighing  by  substitution  yields  not  only  relatively,  but  also 
absolutely  accurate  results ;  no  matter  whether  the  arms  of  the 
balance  be  of  exactly  equal  lengths  or  not,  or  whether  the  scales  be 
in  perfect  equipoise  or  not. 

The  process  is  conducted  as  follows :  The  material  to  be 
weighed — say  a  platinum  crucible — is  placed  upon  one  scale,  and 
the  other  scale  is  accurately  counterpoised  against  it.  The  plati- 
num crucible  is  then  removed,  and  the  equilibrium  of  the  balance 
restored  by  substituting  weights  for  the  removed  crucible.  It  is 
perfectly  obvious  that  the  substituted  weights  will  invariably 
express  the  real  weight  of  the  crucible  with  absolute  accuracy. 
We  weigh  by  substitution  whenever  we  require  the  greatest  pos' 
sible  accuracy;  as,  for  instance,  in  the  determination  of  atomic 
weights.  The  process  may  be  materially  shortened  by  first  placing 
a  tare  (which  must  of  course  be  heavier  than  the  substance  to  be 
weighed)  upon  one  scale,  say  the  left,  and  loading  the  other  scale 


§  10.]  WEIGHING.  23 

with  weights  until  equilibrium  is  produced.  This  tare  is  always 
retained  on  the  left  scale.  The  weights  after  being  noted  are  re- 
moved. The  substance  is  placed  on  the  right  scale,  together  with 
the  smaller  weights  requisite  to  restore  the  equilibrium  of  the 
balance.  The  sum  of  the  weights  added  is  then  subtracted  from 
the  noted  weight  of  the  counterpoise :  the  remainder  will  at  once 
indicate  the  absolute  weight  of  the  substance.  Let  us  suppose,  for 
instance,  we  have  on  the  left  scale  a  tare  requiring  a  weight  of  fifty 
grammes  to  counterpoise  it.  We  place  a  platinum  crucible  on  the 
right  scale,  and  find  that  it  requires  an  additional  weight  of  10 
grammes  to  counterpoise  the  tare  on  the  left.  Accordingly,  the 
crucible  weighs  50  minus  10,  i.e. ,  40  grammes. 

§  10. 

The  following  rules  will  be  found  useful  in  performing  the 
process  of  weighing : 

1.  The  balance  should  be  kept  in  a  dry  place,  protected  from 
acid  vapors,  etc.,  and,  if  possible,  not  exposed  to  direct  sunlight. 
It  should  be  placed  on  a  firm  support  and  in  a  level  position ;   nor 
must  it  be  too  near  the  source  of  heat,  should  the  room  be  heated, 
otherwise  it  may  be  unequally  heated, 

2.  The  safest  and  most  expeditious  method  of  ascertaining  the 
exact  weight  of  a  substance,  is  to  avoid  trying  weights  at  random ; 
instead  of  this,  a  strictly  systematic  course  ought  to  be  pursued  in 
counterpoising  substances  on  the  balance.      Suppose,  for  instance, 
we  want  to  weigh  a  crucible,  the  weight  of  which  subsequently 
turns  out  to  be  6 '627  grammes;   we  first  place  10  grammes  on  the 
other  scale  against  it,  and  we  find  this  is  too  much ;   we  place  the 
weight  next  in  succession,  i.e.,  5  grammes,  and  find  this  too  little; 
next  Y,  too  much  ;  6,  too  little ;  6*5,  too  little ;  6 -7,  too  much  ;  6 -6, 
too  little;  6*65,  too  much;  6'62,  too  little;  6'63,  too  much;  6-625, 
too  little;   6 '627,  right. 

For  the  sake  of  illustration,  a  most  complicated  case  has  been 
selected ;  this  systematic  way  of  laying  on  the  weights  will,  how- 
ever, in  most  instances  lead  to  the  desired  end,  in  half  the  time 
required  when  weights  are  tried  at  random.  After  a  little  practice 
a  few  minutes  will  suffice  to  ascertain  the  weight  of  a  substance  to 
within  O'l  milligramme,  provided  the  balance  does  not  oscillate  too 
slowly. 


24  OPERATIONS.  L§10. 

3.  The  milligrammes  and  fractions  of  milligrammes  are  deter- 
mined by  a  centigramme  rider  (to  be  placed  on  or  between  the 
divisions  on  the  beam)  far  more    expeditiously  and    conveniently 
than  by  the  use  of  the  weights  themselves,  and  at  the  same  time 
with  equal  accuracy. 

4.  Particular  care  and  attention  should  be  bestowed  on  enter- 
ing the  weights  in  the  book.      The  best  way  is  to  write  down  the 
weights  first  by  inference  from  the  blanks,  or  gaps  in  the  weight 
box,  and  to  control  the  entry  subsequently  by  removing  the  weights 
from  the  scale,  and  replacing  them  in  their  respective  compartments 
in  the  box.      The  student  should  from  the  commencement  make  it 
a  rule  to  enter  the  number  to  be  deducted  in  the  lower  line;  thus, 
in  the  upper  line,  the  weight  of  the  crucible  +  the  substance ;   in 
the  lower  line,  the  weight  of  the  empty  crucible. 

5.  The  balance  ought  to  be  arrested  every  time  any  change  is 
contemplated,  such  as  removing  weights,  substituting  one  weight 
for  another,  etc.  etc. ,  or  it  will  soon  be  spoiled. 

6.  Substances  (except,  perhaps,  pieces  of  metal,  or  some  other 
bodies  of  the  kind)  must  never  be  placed  directly  upon  the  scales, 
but  ought  to  be  weighed  in  appropriate  vessels  of  platinum,  silver, 
glass,  porcelain,  etc.,  never  on  paper  or  card,   since  these,  being 
liable  to  attract  moisture,  are  apt  to   alter  in  weight.     The  most 
common  method  is  to  weigh  in  the  first  instance  the  vessel  by  itself, 
and  to  introduce  subsequently  the  substance  into  it,  to  weigh  again, 
and  subtract  the  former  weight  from  the  latter.     In  many  instances, 
and  more  especially  where  several  portions  of  the  same  substance 
are  to  be  weighed,  the  united  weight  of  the  vessel  and  of  its  con- 
tents is  first  ascertained ;   a  portion  of  the  contents  is  then  shaken 
out,  and  the  vessel  weighed  again  ;   the  loss  of  weight  expresses  the 
amount  of  the  portion  taken  out  of  the  vessel. 

7.  Substances  prone  to  attract  moisture  from  the  air  must  be 
weighed  invariably  in    closed   vessels    (in    covered    crucibles,    for 
instance,  or  between  two  watch-glasses,  or  in  a  closed  glass  tube); 
fluids  are  to  be  weighed  in  small  bottles  closed  with  glass  stoppers. 

8.  A  vessel  ought  never  to  be  weighed  while  warm,  since  it 

£5  O 

will  in  that  case  invariably  weigh  lighter  than  it  really  is.  This  is 
owing  to  two  circumstances.  In  the  first  place,  every  substance 
condenses  upon  its  surface  a  certain  quantity  of  air  and  moisture,, 
the  amount  of  which  depends  upon  the  temperature  and  hygro- 


§  10.]  WEIGHING.  25 

scopic  state  of  the  air,  and  likewise  on  its  own  temperature.  Now 
suppose  a  crucible  has  been  weighed  cold  at  the  commencement 
of  the  operation,  and  is  subsequently  weighed  again  while  hot, 
together  with  the  substance  it  contains,  and  the  weight  of  which 
we  wish  to  determine.  If  we  subtract  for  this  purpose  the 
weight  of  the  cold  crucible,  ascertained  in  the  former  instance, 
from  the  weight  found  in  the  latter,  we  shall  subtract  too  much, 
and  consequently  we  shall  set  down  less  than  the  real  weight  for 
the  substance.  In  the  second  place,  bodies  at  a  high  temperature 
are  constantly  communicating  heat  to  the  air  immediately  around 
them ;  the  heated  air  expands  and  ascends,  and  the  denser  and 
colder  air,  flowing  towards  the  space  which  the  former  leaves,  pro- 
duces a  current  which  tends  to  raise  the  scale,  making  it  thus 
appear  lighter  than  it  really  is. 

9.  If  we  suspend  from  the  end  edges  of  a  correct  balance 
respectively  10  grammes  of  platinum  and  10  grammes  of  glass,  by- 
wires  of  equal  weight,  the  balance  will  assume  a  state  of  equili- 
brium ;  but  if  we  subsequently  immerse  the  platinum  and  glass 
completely  in  water,  this  equilibrium  will  at  once  cease,  owing  to 
the  different  specific  gravity  of  the  two  substances ;  since,  as  i& 
well  known,  substances  immersed  in  water  lose  of  their  weight 
a  portion  equal  to  the  weight  of  their  own  bulk  of  water.  If 
this  be  borne  in  mind,  it  must  be  obvious  to  every  one  that 
weighing  in  the  air  is  likewise  defective,  inasmuch  as  the  bulk 
of  the  substance  weighed  is  not  equal  to  that  of  the  weight 
used.  This  defect,  however,  is  so  very  insignificant,  owing 
to  the  trifling  specific  gravity  of  the  air  in  proportion  to  that 
of  solid  substances,  that  we  may  generally  disregard  it  alto- 
gether in  analytical  experiments.  In  cases,  however,  where 
altfiolutcly  accurate  results  are  required,  the  bulk  both  of 
the  substance  examined,  and  of  the  weight,  must  be  taken 
into  account,  and  the  weight  of  the  corresponding  volume 
of  air  added  respectively  to  that  of  the  substance  and  of 
the  weight,  making  thus  the  process  equivalent  to  weighing 
in  vacuo. 


26  OPERATIONS.  [§  11. 

§11. 

2.   MEASURING. 

The  process  of  measuring  is  confined  in  analytical  researches 
mostly  to  gases  and  liquids.  The  method  of  measuring  gases  has 
been  brought  to  such  perfection  by  BUNSEN,  REGNAULT  and  REISET, 
FRANKLAND  and  WARD,  WILLIAMSON  and  RUSSELL,  and  by  others, 
that  it  may  be  said  to  equal  in  accuracy  the  method  of  weighing. 
However,  such  accurate  measurements  demand  an  expenditure  of 
time  and  care  which  can  be  bestowed  only  on  the  nicest  and  most 
delicate  scientific  investigations.* 

The  measuring  of  liquids  in  analytical  investigations  was  resorted 
to  first  by  DESCROIZILLES  (Alkalimeter,  1806).  GAY-LUSSAG 
materially  improved  the  process,  and  indeed  brought  it  to  the 
highest  degree  of  perfection  (measuring  of  the  solution  of  sodium 
chloride  in  the  assay  of  silver  in  the  wet  way).  More  recently 
F.  MOHR  f  has  bestowed  much  care  and  ingenuity  upon  the  pro- 
duction of  appropriate  and  convenient  measuring  apparatus,  and 
has  added  to  our  store  the  eminently  practical  pinch-cock  burette. 
The  process  is  now  resorted  to  even  in  most  accurate  scientific 
investigations,  since  it  requires  much  less  time  than  the  process  of 
weighing. 

The  accuracy  of  all  measurings  depends  upon  the  proper  con- 

*  A  detailed  description  of  BUNSEN'S  method  is  to  be  found  in  the  Hand- 
worterbuch  der  Chemie,  by  LIEBIG,  POGGENDORFF,  and  WOHLER,  n,  1053 
(KOLBE'S  Eudiometer),  and  i,  2,  2d  edit.,  930  (Volumetric  Analysis  of  Gases,  by 
KOLBE  and  FRANKLAND).  BUNSEN,  further,  wrote  a  valuable  monograph  on 
this  subject  under  the  title  Gasometric  Methods,  and  published  by  FR.  VIEWEG 
&  SON,  Brunswick,  1857,  and  which  was  translated  by  ROSCOE.  The  methods 
of  gas  measurement  employed  by  KEGNAULT  and  REISET,  as  well  as  by  FRANK- 
LAND  and  WARD,  differ  from  the  improved  BUNSEN  method  in  that  in  the 
former  the  measuring  tubes  stand  in  cylinders  filled  with  water,  whereby  the 
temperature  of  the  gas  is  brought  in  a  few  minutes  to  that  of  the  water,  thus 
materially  shortening  the  time  required  in  gas  analysis.  In  the  FRANKLAND- 
WARD  method  the  gasometric  determination  is  also  independent  of  the  atmos- 
pheric pressure.  Both  methods,  as  a  matter  of  course,  require  complicated  and 
expensive  apparatus.  These  are  minutely  described  and  figured  in  the  above- 
mentioned  article  by  FRANKLAND.  The  WILLIAMSON-RUSSELL  gasometric 
apparatus  is  described  in  the  Jour.  Chem.  Soc.,  xvn,  238;  and  RUSSELL'S  modi- 
fication, ibid.  (2),  vi,  128,  also  in  Zeitschr.f.  analyt.  Chem.,  vn,  454. 

f  Lehrbuch  der  Titrirmethode,  Dr.  Fr.  MOHR. 


§  12.]  MEASURING   OF   GASES.  27 

strnction  of  the  measuring  vessels,  and  also  upon  the  manner  in 
which  the  process  is  conducted. 

§12. 
a.  THE  MEASURING  OF  GASES. 

We  use  for  the  measuring  of  gases  graduated  tubes  of  greater 
or  less  capacity,  made  of  strong  glass,  and  sealed  at  one  end,  which 
should  be  rounded.  The  following  tubes  will  be  found  sufficient 
for  all  the  processes  of  gas  measuring  required  in  organic  elementary 
analyses  : 

1.  A  bell-glass  capable  of  holding  from  150  to  250  c.  c.,  and 
about  4  centimetres  in  diameter,  and  divided  into  cubic  centimetres. 

2.  Five  or  six  glass  tubes  of  about  12  to  15  millimetres  bore 
diameter,  and  capable  of  holding  from  30  to  40  c.  c.  each,  divided 
into  0-2  c.  c. 

The  sides  of  these  tubes  should  be  fairly  thick,  otherwise  they 
will  be  liable  to  break,  especially  when  used  to  measure  over  mer- 
cury. The  sides  of  the  bell-glass  should  be  about  3,  of  the  tubes 
about  2,  millimetres  thick. 

The  most  important  point,  however,  in  connection  with  meas- 
uring instruments  is  that  they  be  correctly  graduated,  since  upon 
this  of  course  depends  the  accuracy  of  the  results.  For  the  method 
of  graduating  consult  BERZELIUS'  "  Lehrbuch  der  Chemie^  4th 
•ed.,  x,  under  Messenj  also  GREVILLE  WILLIAMS'  "  Chemical 
Manipulation.  '  ' 

In  testing  the  measuring  tubes  we  have  to  consider  three 
questions  : 

1.  Do  the  divisions  of  a  tube  correspond  with  each  other? 

3.  Do  the  divisions  of  each  tube  correspond  with  those  of  the 
other  tubes  ? 

3.  Do  the  volumes  expressed  by  the  graduation  lines  corre- 
spond with  the  weights  used  by  the  analyst  ? 

These  three  questions  are  answered  by  the  following  experi- 
ments : 

a.  The  tube  which  it  is  intended  to  examine  is  placed  in  a  per- 
pendicular position,  and  filled  gradually  with  accurately  measured 
small  quantities  of  mercury,  care  being  taken  to  ascertain  with  the 
utmost  precision  whether  the  graduation  of  the  tube  is  proportion- 
ate to  the  equal  volumes  of  mercury  poured  in.  The  measuring- 


OF   THE 

UNIVERSITY 


28  OPERATIONS.  [§  12, 

off  of  the  mercury  is  effected  by  means  of  a  small  glass  tube,  sealed 
at  one  end,  and  ground  perfectly  even  and  smooth  at  the  other. 
This  tube  is  filled  to  overflowing  by  immersion  under  mercury  y 
care  being  taken  to  allow  no  air  bubbles  to  remain  in  it ;  the 
excess  of  mercury  is  then  removed  by  pressing  a  small  glass  plate 
down  on  the  smooth  edge  of  the  tube.* 

~b.  Different  quantities  of  mercury  are  successively  measured 
off  in  one  of  the  smaller  tubes,  and  then  transferred  into  the  other 
tubes.  The  tubes  may  be  considered  in  perfect  accordance  with 
each  other,  if  the  mercury  reaches  invariably  the  same  divisional 
point  in  every  one  of  them. 

Such  tubes  as  are  intended  simply  to  determine  the  relative 
volume  of  different  gases,  need  only  pass  these  two  experiments ; 
but  in  cases  where  we  want  to  calculate  the  weight  of  a  gas  from 
its  volume,  it  is  necessary  also  to  obtain  an  answer  to  the  third 
question.  For  this  purpose — 

c.  One  of  the  tubes  is  accurately  weighed  and  then  filled  with 
distilled  water  of  a  temperature  of  17 '5°  to  the  last  mark  of  the 
graduated  scale ;  the  weight  of  the  water  is  then  accurately  deter- 
mined. If  the  tube  agrees  with  the  weight,  every  100  c.  c.  of 
water  at  17*5°  must  weigh  99 '78  grammes. f  Should  it  not  agree, 
no  matter  whether  the  error  is  due  to  faulty  weights  or  incorrect 
graduations,  we  must  apply  a  correction  to  the  volume  observed 
before  calculating  the  weight  therefrom.  For  instance,  if  100  c.  c. 
had  been  found  to  weigh  100  grammes — assuming  our  weight  to  be 
perfect — then  the  c.  c.  divisions  would  be  too  large,  and  to  convert 
100  c.  c.  into  normal  c.  c.  the  following  calculation  would  have  to 
be  made : 

99-78  :  100  ::  100  :  x. 

In  gas  analysis  proper  by  BUNSEN'S  methods  (the  simplest  and 
most  accurate)  a  suitable  eudiometer  is  indispensable.  BUNSEN'S 
eudiometer,  Fig.  3,  is  a  glass  tube  500  to  600  mm.  long,  with  a 
bore  of  20  mm.,  as  uniform  as  possible  throughout,  and  the  thick- 
ness of  the  glass  not  exceeding  2  mm.  At  the  upper,  closed  end 

*  As  warming  the  metal  is  to  be  carefully  avoided  in  this  process,  it  is  advis- 
able not  to  hold  the  tube  with  the  hand  in  immersing  it  in  the  mercury,  but  to 
fasten  it  in  a  small  wooden  holder. 

f  A  gramme  is  the  weight  of  1  c.  c.  of  water  in  vacuo  at  4°. 


§12.] 


MEASURING   OF   GASES. 


29 


of  the  tube  there  are  sealed  in  at  opposite  sides  two  fine  platinum 
wires.     These  wires  are  ^  bent  internally  to  lie  close  to  the  walls  of 

Fig.  3. 


Fig.  4. 

the  tube,  and  approach  each  other  at  the  apex  of  the  tube  until 
separated  by  a  distance  of  1  to  2  mm. 

The  tube  is  graduated  in  millimetre  divisions  by  means  of  an 
ingenious  divider.  The  volumes  corresponding  to  the  several  divi- 
sions are  then  determined  by  measurement  with  equal  volumes  of 
mercury,  and  noted  down  in  a  table.  This  method  of  dividing  and 
adjusting  is  unquestionably  the  most  accurate. 

Besides  this  large  eudiometer  there  is  required  also  a  short  one, 
Fig.  4,  similarly  graduated  in  millimetres,  and  slightly  curved  at 
the  lower  end.  Its  length  is  250  mm.,  and  its  bore  20  mm.  in 
diameter ;  the  glass  should  be  2  mm.  thick. 

BUNSEN'S  method  of  gas  analysis  requires  a  laboratory  with  a 
northern  exposure  and  uniform  temperature,  and  consumes  much 
time  because  of  the  slow  cooling  of  the  gases.  In  order  to  adapt 
the  method  for  the  use  of  those  who  do  not  possess  a  suitable  labo- 
ratory, and  to  shorten  the  time,  O.  KERSTEN*  recommends  that 
the  BUNSEN  eudiometer  be  provided  with  a  screw  stopper  like  that 
in  BUNSEN'S  absorptiometer  tube,f  and  that  the  readings  should  be 
taken  after  immersing  the  eudiometer  in  water.  The  same  result 
is  obtained  in  another  manner  in  the  eudiometer  recommended  by 
J.  P.  COOKE.J 

In  measuring  gases  attention  must  be  given  to  the  following 
points:  1.  Correct  reading  of  the  results;  2,  the  temperature  of 
the  gas ;  3,  the  pressure  under  which  the  gas  is  confined ;  and  4, 
the  circumstance  whether  the  gas  is  dry  or  moist. '  The  three  last 
points  will  be  readily  understood  when  it  is  remembered  that  a 

*Zeitschr.f.  analyt.  Chem.,  I,  281. 
fBuNSEN,  Gasometr.  Meth,,  p.  147. 
\Zeitschr.f.  analyt.  Chem.,  vn,  86. 


30  OPERATIONS.  [§  13, 

given  weight  of  gas  undergoes  considerable  alteration  in  volume  by 
changes  in  temperature  or  pressure,  as  well  as  from  greater  or  less 
tension  of  the  admixed  aqueous  vapor. 


§  13. 

1 .  CORRECT  READING  OF  RESULTS.  — "When  mercury  is  introduced 
into  a  tube,  it  exhibits  a  convex  surface,  because  of  its  cohesion, 
the  phenomenon  being  particularly  striking  in  a  narrow  glass  tube. 
On  the  other  hand  water,  under  similar  circumstances,  exhibits  a 
concave  surface,  owing  to  the  attraction  between  the  tube- wall  and 
the  water.  These  circumstances  render  accurate  reading-oil'  diffi- 
cult. The  tube  must  invariably  be  placed  perpendicularly,  with 
the  eye  on  a  level  with  the  surface  of  the  fluid.  This  is  effected 
by  the  aid  of  two  plummets  suspended  a  short  distance  from  the 
tube  and  at  a  proper  distance  from  each  other,  or  by  the  aid  of  any 
convenient  perpendicular  door-  or  window-edge.  A  small  mirror 
is  then  pressed  against  the  back  of  the  tube,  and  the  eye  lixed  on 
it  across  the  surface  of  the  liquid.  When  the  eye  has  thus  been 
placed  in  the  proper  position,  the  mirror  is  removed,  and  the 
reading  taken. 

Instead  of  a  mirror,  BUNSEN  generally  employs  a  horizontal 
telescope,  movable  vertically,  and  placed  at  a  distance  of  four  to 
six  paces  from  the  eudiometer.  This  arrangement,  while  very 
greatly  facilitating  the  reading,  is  of  especial  advantage  in  the 
measurement  of  gases,  in  that  the  observer  is  placed  some  distance 
from  the  eudiometer,  thus  avoiding  any  expansion  of  the  gases  such 
as  is  to  be  apprehended  by  the  close  proximity  required  in  using 
the  mirror. 

In  taking  the  reading  over  water,  the  middle  of  the  dark  zone 
formed  by  the  water  drawn  up  the  walls  of  the  tube,  is  to  be  taken 
as  the  actual  surface.  When  operating  with  mercury,  however, 
we  have  to  place  the  real  surface  in  a  plane  exactly  in  the  middle 
between  the  highest  point  of  the  surface  of  the  mercury,  and  the 
points  at  which  the  latter  is  in  actual  contact  with  the  walls  of  the 
tube.  However,  the  results  obtained  in  this  way  are  only  approx- 
imate. 

Absolutely  accurate  results  cannot  be  arrived  at,  in  measuring 
over  water  or  any  other  fluid  that  adheres  to  glass.  But  over  mer- 


§  13.]  MEASURING   OF   GASES.  31 

cury  they  may  be  arrived  at  if  the  error  of  the  meniscus  be  deter- 
mined and  the  mercury  be  read  off  at  the  highest  -point.  The 
determination  of  the  error  of  the  meniscus  is  performed  for  each 
tube,  once  for  all,  in  the  following  manner :  Some  mercury  is 
poured  into  the  tube,  and  its  height  read -off  right  on  a  level  with 
the  top  of  the  convex  surface  exhibited  by  it ;  a  few  drops  of  solu- 
tion of  mercuric  chloride  are  then  poured  on  the  top  of  the  metal ; 
this  causes  the  convexity  to  disappear.  The  height  of  the  mercury 
in  the  tube  is  now  read-off  again  and  the  difference  noted.  In  the 
process  of  graduation,  the  tube  stands  upright,  in  that  of  measur- 
ing gases,  it  is  placed  upside  down ;  the  difference  observed  must 
accordingly  be  doubled,  and  the  sum  added  to  each  volume  of  gas 
read-off. 

The  mercury  used  in  measuring  gases  must  be  as  pure  as  pos- 
sible, and  must  more  particularly  be  free  from  lead  and  tin,  as 
these  impart  to  it  the  property  of  adhering  to  glass.  Should  they 
be  present,  they  may  be  most  readily  removed  by  subjecting  the 
mercury  for  a  day  to  the  action  of  diluted  nitric  acid,  in  a  shallow 
porcelain  dish,  with  frequent  stirring.  Dust,  etc.,  maybe  removed 
by  filtration  through  a  cloth. 

As  a  pneumatic  trough,  that  devised  by  BUNSEN,  Fig.  5,  will 
be  found  convenient.    A  is  a  piece  of  pear-wood 
310   to  350  mm.  long,    80  to   86   mm.  wide, 
with  a  cavity  chiseled  in  it  240  to  250  mm. 
long,  50   mm.  wide,  and   50  mm.    deep.      The 
bottom  of  the  cavity  is  round,   except  at  one 
end,  where  a  flat  surface  32  mm.  wide  and  50 
mm.  long  is  prepared.     On  this  surface  is  ce- 
mented a  sheet 
of      vulcanized 
caoutchouc       3 
mm.  thick.     To 
A  are  joined  two 
end  pieces,  BB, 
each     19     mm. 
thick,  100  tol  10 
mm.   wide,  and 
150  to  155  mm.  Fig.  5. 

high.      They  serve  as  supports  for  A,  as  well  as  ends  for  a  further 


32 


OPERATIONS. 


[§13. 


trough,  the  side  walls  of  which  are  formed  of  stout  pieces  of  glass 
cemented  in' grooves  in  A  and  BE.  The  glass  plates  are  each  310 
to  320  mm.  long,  and  55  mm.  high.  They  are  placed  not  quite 
parallel,  the  lower  edges  being  separated  by  a  distance  of  67  to 
70  mm.,  the  upper  85  mm.  The  trough  stands  on  a  board,  DD, 
to  which  it  is  fastened  by  strips  of  wood,  e  e.  A  vertical  pillar,  F, 
screwed  into  D,  carries  the  inclined  channel,  G,  lined  with  felt; 
it  serves  to  support  the  measuring  tube,  h  is 
a  round,  slanting  cut  in  _Z?,  for  the  reception 
of  the  tube ;  i  is  an  incision  for  the  reception 
of  the  lower  end  of  the  tube ;  it  prevents  the 
latter  from  falling  into  the  lower  part  of  the 
trough.  In  use  the  trough  is  filled  to  within 
an  inch  of  the  upper  edges  of  the  glass  plates 
with  mercury,  of  which  30  [to  35  Ib.  will  be 
necessary.  In  order  that  the  mercury  may 
adhere  to  the  wooden  walls,  the  latter  are  first- 
rubbed  moist  then  dry  with  mercury  and 
mercuric-chloride  solution.  In  order  to 
transfer  gases  which  have  been  collected  in 
large  bottles,  a  similar  but  larger  trough  is 
employed. 

Lastly,  in  order  to  accurately  determine  the 
volume  of  a  gas  collected  over  mercury,  it  is 
above  all  necessary  that  the  tube  be  first  com- 
pletely filled  with  mercury,  and  with  the  entire 
exclusion  of  air  bubbles,  before  introducing  the 
gas.  To  this  end  the  tube  is  first  washed  with 
water  and  dried  with  filter  paper  by  aid  of  a 
wooden  rod,  Fig.  6,  the  upper  end  of  which 
is  provided  with  10  to  20  little  spikes.  Care 
must  be  taken  that  no  filaments  of  paper  be 
left  behind.  The  filling  with  mercury  is  accomplished  by  means  of 
a  funnel,  Fig.  7,  kept  filled  with  mercury,  and  having  a  long  stem, 
with  narrow  exit,  reaching  to  the  bottom  of  the  tube  to .  be  filled. 
The  metal  thus  introduced  from  below  presents  a  mirror-like  surface 
on  the  sides  of  the  tube.  If  such  a  funnel  as  described  is  not  at 
hand,  a  glass  tube  drawn  out  at  the  lower  end  may  be  fused  to  a 
small  funnel. 


Tig.  6.       Fig.  7. 


§§  14,  15.]  MEASURING    OF   GASES.  33 

§14. 

2.  INFLUENCE    OF   TEMPERATURE. — The  temperature  of    gases 
to  be  measured  is  determined  either  by  reducing  it  to  the  same 
temperature  as  the  confining  fluid,  or,  using  a  closed  eudiometer,  to 
that  of  the  water  in  the  cylinder  provided  for  that  purpose,  and 
then  measuring  the  temperature  of  the  liquid;   or  by  suspending  a 
sensitive  thermometer  by  the  side  of  the  gas,  and  noting  its  varia- 
tion. 

If  the  construction  of  the  pneumatic  apparatus  permits  the  total 
immersion  of  the  cylinder  in  the  confining  fluid,  uniformity  of 
temperature  between  the  latter  and  the  gas  which  it  is  intended  to 
measure  is  most  readily  and  speedily  obtained ;  but  in  the  reverse 
case,  the  operator  must  always,  after  every  manipulation,  allow 
half  an  hour,  or,  in  operations  combined  with  much  heating,  even 
an  entire  hour,  to  elapse,  before  proceeding  to  observe  the  state  of 
the  mercury  in  the  cylinder  and  in  the  thermometer. 

Proper  care  must  also  be  taken,  after  the  temperature  of  the  gas 
has  been  duly  adjusted,  to  prevent  re-expansion  during  the  reading- 
off;  all  injurious  influences  in  this  respect  must  accordingly  be  care- 
fully guarded  against,  and  the  operator  should,  more  especially, 
avoid  laying  hold  of  the  tube  with  his  hand  (in  pressing  it  down, 
for  instance,  into  the  confining  fluid) ;  making  use,  instead,  of  a 
wooden  holder. 

On  account  of  the  necessity  of  bringing  the  gas  and  surround- 
ing air  to  the  same  temperature,  every  sudden  change  in  the  latter 
is  prejudicial,  hence  it  is  advisable  to  select  for  gas  analysis  a  room 
having  a  northern  exposure,  and  sheltered  so  far  as  possible  from 
the  direct  influence  of  solar  heat. 
* 

§15. 

3.  INFLUENCE  OF  PRESSURE. — When  a  gas  is  confined  by  a  fluid, 
and  the  level  of  the  latter  is  the  same  inside  the  tube  as  outside, 
then  the  gas  is  under  the  prevailing  pressure  only,  that  of  the  atmos- 
phere ;   and  this  may  be  directly  found  by  a  barometric  reading. 
On  the  other  hand,   when  the  confining  fluid  within  the  tube  is 
higher  than  that  without,  then  the  gas  is  under  less  pressure ;   if 
lower,  it  is  under  greater  pressure  than  that  of  the  atmosphere.     In 


34 


OPERATIONS. 


[§16. 


the  latter  case,  the  level  may  be  equalized  by  raising  the  tube ;   in 
the  former,  by  sinking  it,  if  the  trough  be  deep  enough.      When 

operating  over  water,  the  level 
may  be  usually  secured  without 
difficulty;  when  operating  over 
mercury,  however,  this  is  very 
often  impossible,  particularly 
with  wide  tubes,  as  in  Fig.  8. 

In  this  case  the  gas  is  under 
the  pressure  of  the  atmosphere 
minus  the  pressure  of  a  column 
of  mercury  equal  in  height  to 
the  line  ab.  The  pressure  is 
accordingly  ascertained  by  accu- 
rately measuring  the  line  «J, 
and  subtracting  its  length  from 
the  observed  height  of  the 
barometer.  For  instance,  if  the  latter  is  758  mm.,  and  the  line 
ab  measures  100  mm.,  the  actual  pressure  upon  the  gas  will 
be  758  —  100  =  658  mm.  mercury. 

If  there  is  water  or  some  other  fluid,  e.g.,  potassa  solution,  con- 
fined over  the  mercury,  we  as  a  rule  proceed  just  as  if  there  were 
no  fluid  present  but  mercury,  but  either  bring  the  mercury  inside 
and  outside  the  tube  to  the  same  level,  or  else  measure  the  differ- 
ence between  the  two  surfaces  of  mercury.  The  pressure  of  the 
additional  column  of  water,  etc.,  is  usually  so  insignificant  that  it 
may  be  disregarded.  Properly  speaking,  the  fluid  ought  to  be 
measured,  and,  according  to  its  specific  gravity,  reduced  to  mercu- 
rial pressure,  and  this  subtracted  from  the  barometric  reading. 
This  correction  may,  however,  be  omitted  as  a  rule,  since,  as 
already  stated,  an  absolutely  accurate  measurement  is  impossible 
under  such  circumstances. 


Fig.  8. 


§16. 

4.  INFLUENCE  OF  MOISTURE. — When  a  gas  saturated  with 
aqueous  vapor  is  measured,  its  actual  volume  is  not  ascertained,  since 
the  aqueous  vapor,  because  of  its  tension,  exerts  a  pressure  on  the 
confining  fluid.  As,  however,  the  tension  of  aqueous  vapor  is 


§  16.]  MEASURING   OF   GASES.  35 

known  for  various  temperatures,  the  necessary  corrections  may  be 
readily  made.  This  is,  however,  only  then  possible  when  the  gas 
is  ftrlly  saturated  with  vapor.  Care  must  be  exercised,  hence, 
when  measuring  gases,  that  the  gas  be  either  fully  saturated  with 
aqueous  vapor,  or  else  is  absolutely  dry. 

The  drying  of  gases  confined  over  mercury  is  effected  by  means 
of  a  ball  of  fused  calcium  chloride  fixed  on  a  platinum  wire.  This 
may  be  prepared  by  inserting  the  end  of  a  platinum  wire,  bent  in 
the  form  of  a  hook,  into  a  bullet  mold  of  about  6  mm.  diameter, 
and  then  filling  the  latter  with  fused  calcium  chloride  free  from 
caustic  lime.  After  cooling,  the  neck  of  calcium  chloride  adhering 
to  the  wire  is  removed  by  means  of  a  knife.  In  order  to  dry  a  gas, 
the  ball  is  pushed,  by  means  of  the  wire,  up  through  the  mercury 
into  the  gas,  in  which  it  is  allowed  to  remain  for  about  an  hour, 
when  it  is  removed  from  the  now  fully  dried  gas.  While  the  ball 
is  in  contact  with  the  gas,  care  must  be  taken  that  the  end  of  the 
platinum  wire  is  entirely  submerged  in  the  mercury  in  the  trough, 
otherwise  there  will  inevitably  occur  a  diffusion  of  the  confined  gas 
and  outside  air  through  the  exposed  wire. 

It  is  more  convenient  to  measure  the  gas  in  the  moist  condition, 
when  this  may  be  done.  BUNSEN  effects  the  saturation  with  moisture 
by  introducing  a  drop  of  water  the  size  of  a  lentil  into  the  empty 
tube,  by  means  of  a  glass  rod,  taking  care  to  deposit  it  at  the  top 
of  the  tube,  and  without  touching  the  sides  of  the  tube.  This 
quantity  of  water  is  amply  sufficient  to  fully  saturate  with  aqueous 
vapor  at  the  ordinary  temperature  the  gas  subsequently  introduced. 


From  the  preceding  remarks,  it  will  be  quite  obvious  that  the 
volumes  of  gases  can  be  compared  only  when  reduced  to  the  same 
temperature,  pressure,  and  degree  of  moisture.  As  a  rule  the 
reductions  are  made  to  0°,  760  mm.  barometric  pressure,  and 
absolute  dryness.  The  methods  by  which  this  is  effected,  as  well 
as  the  manner  of  ascertaining  the  weights  of  gases  from  their  vol- 
umes, will  be  found  under  the  calculation  of  analyses. 


36  OPERATIONS.  [§§  17,  lt>. 

§  IT. 

5.  THE  MEASURING  OF  FLUIDS. 

In  consequence  of  the  vast  development  which  volumetric 
analysis  has  of  late  undergone,  the  measuring  of  fluids  has  become 
an  operation  of  very  frequent  occurrence.  According  to  the  dif- 
ferent objects  in  view,  various  kinds  of  measuring  vessels  are 
employed.  The  number  proposed  has  gradually  increased  to  such 
an  extent  that  all  the  forms  and  arrangements  recommended  can  not 
be  discussed  here,  but  only  those  will  be  described  that  have  been 
found  most  practical,  and  that  have  given  the  best  results  in  my 
work. 

The  operator  must,  in  the  case  of  every  measuring  vessel,  care- 
fully distinguish  whether  it  is  graduated  for  holding  or  for  deliver- 
ing the  exact  number  of  c.  c.  marked  on  it.  If  you  have  made  use 
of  a  vessel  of  the  former  description  in  measuring  off  100  c.  c.  of  a 
fluid,  and  wish  to  transfer  the  latter  completely  to  another  vessel, 
you  must,  after  emptying  your  measuring  vessel,  rinse  it,  and  add 
the  rinsings  to  the  fluid  transferred;  whereas,  if  you  have  made 
use  of  a  measuring  vessel  of  the  latter  description,  there  must  be  no 
rinsing. 

a.  MEASURING  VESSELS  GRADUATED  FOR  HOLDING  THE  EXACT  MEAS- 
URE OF  FLUID  MARKED  ON  THEM. 

aa.  Measuring  vessels  which  serve  to  measure  out  one  definite 
quantity  of  fluid. 

We  use  for  this  purpose — 

§18. 
1.  Measuring  Flasks. 

Fig.  9  represents  a  measuring  flask  of  the  most  practical  and 
convenient  form. 

Measuring  flasks  of  various  sizes  are  obtainable,  holding  re- 
spectively 200,  250,  500,  1000,  2000,  etc.,  c.  c.  As  a  general 
rule,  they  have  no  ground-glass  stoppers;  it  is,  however,  very 
desirable,  in  certain  cases,  to  have  measuring  flasks  with  ground 
stoppers.  The  flasks  must  be  made  of  well-annealed  glass  of  uni- 


§  18.]  MEASURING    OF   FLUIDS.  37 

form  thickness,  so  that  fluids  may  be  heated  in  them.  The  line- 
mark  should  be  placed  within  the  lower  third,  or  at  least  within 
the  lower  half,  of  the  neck. 

Measuring  flasks,  before  they  can  properly  be  employed  in 
analytical  operations,  must  first  be  carefully 
tested.  The  best  and  simplest  method  of 
effecting  this  is  to  proceed  thus :  Put  the 
flask,  perfectly  dry  inside  and  outside,  on 
the  one  scale  of  a  sufficiently  delicate  balance, 
together  with  a  weight  of  1000  grm.  in  the 
case  of  a  litre  flask,  500  grin,  in  the  case  of 
a  half -litre  flask,  etc.,  restore  the  equilibrium 
by  placing  the  requisite  quantity  of  shot  and 
tinfoil  on  the  other  scale,  then  remove  the 
flask  and  the  weight  from  the  balance,  place 
the  flask  on  a  perfectly  level  surface,  and 
pour  in  distilled  water  at  IT* 5  until  the  lower  border  of  the  dark 
zone  formed  by  the  top  of  the  water  around  the  inner  walls  corre- 
sponds with  the  line-mark.  After  having  thoroughly  dried  the 
neck  of  the  flask  above  the  mark,  replace  it  upon  the  scale;  if  this 
restores  the  perfect  equilibrium  of  the  balance,  the  water  in  the 
flask  weighs,  in  the  case  of  a  litre  measure,  exactly  1000  grm.  If 
the  scale  bearing  the  flask  sinks,  the  water  in  it  weighs  as  much 
above  1000  grm.  as  the  additional  weights  amount  to  which  you 
have  to  put  in  the  other  scale  to  restore  the  equilibrium ;  if  it  rises, 
on  the  other  hand,  the  water  weighs  as  much  less  as  the  weights 
amount  to  which  you  have  to  put  in  the  scale  with  the  flask  to  effect 
the  same  end. 

If  the  water  in  the  litre  measure  weighs  1000  grm.,  in  the 
half -litre  measure  500  grm.,  etc.,  the  measuring  flasks  are  correct. 
Differences  up  to  O'l  grm.  in  the  liter  measure,  up  to  0'07  grm. 
in  the  half-litre  measure,  and  up  to  0'05  grm.  in  the  quarter-litre 
measure,  are  not  taken  into  account,  as  one  and  the  same  measuring 
flask  will  be  found  to  offer  variation  to  the  extent  indicated,  in 
repeated  consecutive  weighings"  though  filled  each  time  exactly  up 
to  the  mark  with  water  of  the  same  temperature. 

Though  a  flask  should,  upon  examination,  turn  out  not  to  hold 
the  exact  quantity  of  water  which  it  is  stated  to  contain,  it  may  yet 
possibly  agree  with  the  other  measuring  vessels,  and  may  accord- 


38  OPERATIONS.  [§  18. 

ingly  still  be  perfectly  fit  for  use  for  most  purposes.  Two  meas- 
uring vessels  agree  among  themselves  if  the  marked  numbers  of 
c.  c.  bear  the  same  proportion  to  each  other  as  the  weights  found; 
thus,  for  instance,  supposing  your  litre  measure  to  hold  998  grm. 
water  at  17*5°,  and  your  50  c.  c.  pipette  to  deliver  49 -9  grm. 
water  of  the  same  temperature,  the  two  measures  agree,  since 

1000:50  =  998:49-9. 

To  prepare  or  correct  a  measuring  flask,  tare  the  dry  litre,  half- 
litre,  or  quarter-litre  flask,  and  then  weigh  into  it,  by  substitution 
(§  9),  1000  grin.,  or,  as  the  case  may  be,  the  half  or  quarter  of 
that  quantity  of  distilled  water  at  17'5°.  Put  the  flask  on  a  per- 
fectly horizontal  support,  place  your  eye  on  an  exact  level  with  the 
surface  of  the  water,  and  mark  the  lower  border  of  the  dark  zone 
by  two  little  dots  made  on  the  glass  with  a  point  dipped  into  thick 
asphaltum  varnish,  or  some  other  substance  of  the  kind.  Now 
pour  out  the  water,  place  the  flask  in  a  convenient  position,  and  cut 
with  a  diamond  a  fine  distinct  line  into  the  glass  from  one  dot  to 
the  other. 

Measuring  vessels  are  sometimes  graduated  also  for  delivery. 
These,  however,  can  only  be  used  where  very  accurate  measuring 
is  unnecessary,  because  the  quantity  of  water  remaining  in  the  flask 
varies  not  inconsiderably,  and  hence  in  repeated  measurings  with 
the  same  flask  notable  differences  may  arise.  The  graduation  or 
testing  of  such  flasks  is  effected  by  filling  the  flask  with  water,  then 
emptying  it  and  allowing  it  to  drain  for  a  minute,  and  then  weigh- 
ing into  it  the  weight  of  distilled  water  at  17 '5°  corresponding  to 
the  number  of  c.  c. 

In  none  of  these  weighings,  as  will  be  seen,  have  the  operations 
been  conducted  at  4°,  or  reduced  to  vacuum,  hence  the  measuring 
vessels  will  not  accurately  conform  to  the  standard.  If,  however, 
this  system  is  carried  out  with  all  the  vessels  used  for  measuring 
fluids,  as  proposed  by  FR.  MOHR,  the  measures  will  correspond  per- 
fectly among  themselves,  and  this  will  suffice  for  all  the  purposes 
of  volumetric  analysis.  In  the  exceptional  case  where  such  a 
measuring  vessel  is  used  in  measuring  a  gas,  the  proper  correction 
may  be  made  by  multiplying  the  c.  c.  by  1-0022,  as  pointed  out 
by  FR.  MOHR  (Zeitschr.  f.  Analyt.  Chem.,  vn,  287). 


£  19,  20.] 


MEASURING   OF  FLUIDS. 


39 


55.  Measuring  vessels  which  serve  to  measure  out  any  quan- 
tities of  fluid  at  will. 

§19. 
2.    The  Graduated  Cylinder. 

This  instrument,  represented  in  Fig.  10,  should  be  from  2  to  3 
cm.  wide,  of  a  capacity  of  100  to  300  c.  c.,  and  divided  into  single 
c.  c.  It  must  be  ground  at  the  top,  so  that  it  may 
be  covered  closely  with  a  ground-glass  plate.  The 
measuring  with  such  cylinders  is  not  quite  so  ac- 
curate as  with,  measuring  flasks,  as  in  the  latter 
the  volume  is  read  off  in  a  narrower  part.  The 
accuracy  of  measuring  cylinders  may  be  tested  in 
the  same  way  as  in  the  case  of  measuring  flasks, 
viz.,  by  weighing  into  them  water  at  17*5°;  or, 
also,  very  well,  by  letting  definite  quantities  of 
fluid  flow  into  the  cylinder  from  a  correct  pipette 
or  burette  graduated  for  delivering,  and  observ- 
ing whether  or  not  they  are  correctly  indicated  by 
the  scale  of  the  cylinder. 


100 

90 

80 
J70 

60 
1 50 


/3.  MEASURING  VESSELS  GRADUATED  FOR  DELIVER- 
ING THE  EXACT  MEASURE  OF  FLUID  MARKED  ON 
THEM. 

aa.  Measuring  vessels  which  serve  to  measure 
out  one  definite  quantity  of  fluid. 

§20. 
3.   The   Graduated  Pipette. 


40 
(30 
[20 

10 


Fig.  10. 


This  instrument  serves  to  remove  a  definite  volume  of  a  fluid 
from  one  vessel  and  to  transfer  it  to  another ;  it  must  accordingly 
be  of  a  suitable  shape  to  admit  of  its  being  freely  inserted  into 
flasks  and  bottles. 

We  use  pipettes  of  1,  5,  10,  20,  50,  100,  150,  and  200  c.  c. 
capacity.  The  proper  shape  for  pipettes  up  to  20  c.  c.  capacity 


40 


OPERATIONS. 


[§20. 


is  represented  in  Fig.  11 ;  Fig.  12  shows  the  most  practical  form 
for  larger  ones.  To  fill  a  pipette  suction  is  applied 
to  the  upper  aperture,  either  directly  with  the  lips 
or  through  a  caoutchouc  tube,  until  the  fluid 
stands  above  the  mark ;  the  upper  orifice  which  is 
somewhat  narrowed  and  ground)  is  then  closed 
with  the  first  finger  of  the  right  hand  (the  point  of 
which  should  be  a  little  moist) ;  the  outside  is  then 
wiped  dry,  if  required,  and,  the  pipette  being  held 
in  a  perfectly  vertical  direction,  the  fluid  is  allowed 
to  drop  out,  by  lifting  the  finger  a  little,  till  it  has 
fallen  to  the  required  level ;  the  loose  drop  is  care- 
fully wiped  off,  and  the  contents  of  the  tube  are  then 
finally  transferred  to  the  other  vessel.  In  this  pro- 
cess it  is  found  that  the  fluid  does  not  run  out 
completely,  but  that  a  small  portion  of  it  remains 
adhering  to  the  glass  in  the  point  of  the  pipette ;  after 
a  time,  as  this  becomes  increased  by  other  minute 
particles  of  fluid  trickling  down  from  the  upper  part 
of  the  tube,  there  gathers  at  the  lower  orifice  a  drop 
which  may  be  allowed  to  fall  off  from  its  own 
weight,  or  may  be  made  to  drop  off  by  a  slight 
shake.  If,  after  this,  the  point  of  the  pipette  be 
laid  against  a  moist  portion  of  the  inner  side  of  the- 
vessel,  another  minute  portion  of  fluid  will  trickle 
out,  and,  lastly,  another  trifling  droplet  or  so  may 
be  got  out  by  blowing  into  the  pipette.  Now, 
supposing  the  operator  follows  no  fixed  rule  in  this- 
respect,  letting  the  fluid,  for  instance,  in  one 
operation  simply  run  out,  whilst  in  another  operation  he  lets  it 
drain  afterwards,  and  in  a  third  blows  out  the  last  particles  of  it 
from  the  pipette ;  it  is  evident  that  the  respective  quantities  of  fluid 
delivered  in  the  several  operations  cannot  be  quite  equal.  I  prefer 
in  all  cases  the  second  method,  viz.,  to  lay  the  point  of  the  pipette,, 
whilst  draining,  finally  against  a  moist  portion  of  the  side  of  the 
vessel,  which  I  have  always  found  to  give  the  most  accurately  cor- 
responding measurements. 

The  correctness  of  a  pipette  is  tested  by  filling  it  up  to  the 
mark  with  distilled  water  at  17' 5°,  letting  the  water  run  out,  in 


Fig.  11.    Fig.  12. 


§  20.]  MEASURING   OF  FLUIDS.  41 

the  manner  just  stated,  into  a  tared  vessel,  and  weighing;  the 
pipette  may  be  pronounced  correct  if  100  c.  c.  of  water  at  17 '5° 
weigh  100  grm. 

Testing  in  like  manner  the  accuracy  of  the  measurements  made 
with  a  simple  hand  pipette,  we  find  that  one  and  the  same  pipette 
will,  in  repeated  consecutive  weighings  of  the  contents,  though 
filled  and  emptied  each  time  with  the  minutest  care,  show  differ- 
ences up  to  O'Ol  grin,  for  10  c.  c.  capacity,  up  to  0*04 grm.  for  50 
c.  c.  capacity. 

The  accuracy  of  the  measurements  made  with  a  pipette  may 
be  heightened  by  giving  the  instrument  the  form  and  construction 
shown  in  Fig.  13  and  fixing  it  in  a  holder. 

It  will  be  seen  from  the  drawing  that  these  pipettes 
are  emptied  only  to  a  certain  mark  in  the  lower  tube,  and 
that  they  are  provided  with  a  pinch-cock,  a  contrivance 
which  we  shall  have  occasion  to  describe  in  detail  when  on 
the  subject  of  burettes.  This  contrivance  reduces  the  dif- 
ferences of  measurements  with  one  and  the  same  50  c.  c. 
pipette  to  0'005  grin. 

Pipettes  are  used  more  especially  in  cases  where  it  is  in- 
tended to  estimate  different  constituents  in  separate  por- 
tions of  a  substance :  for  instance,  10  grm.  of  the  sub- 
stance under  examination  are  dissolved  in  a  250  c.  c. 
flask,  the  solution  is  diluted  up  to  the  mark,  shaken,  and 
2,  3,  or  4  several  portions  are  then  taken  out  with  -a  50  c.  c. 
pipette.  Each  portion  consists  of  I  part  of  the  whole,  and 
accordingly  contains  2  grin,  of  the  substance.  Of  course 
the  pipette  and  the  flask  must  be  in  perfect  harmony. 
Whether  they  are  may  be  ascertained  by,  for  instance, 
emptying  the  50  c.  c.  pipette  5  times  into  the  250  c.  c. 
flask,  and  observing  if  the  lower  edge  of  the  dark  zone  of 
fluid  coincides  with  the  mark.  If  it  does  not,  you  may 
make  a  fresh  mark,  which,  no  matter  whether  it  is  really 
correct  or  not,  will  bring  the  two  instruments  in  question 
into  conformity  with  each  other. 

Cylindrical  pipettes,  graduated  throughout  their  entire 
length,  may  be  used  also  to  measure  out  any  given  quanti- 
ties of  liquid;  however,  these  instruments  can  properly  be 
employed  only  in  processes  where  minute  accuracy  is  not 
indispensable,  as  the  limits  of  error  in  reading  off  the  divisions  in. 


V 


OPERATIONS. 


C 


the  wider  part  of  the  tube  are  not  inconsiderable.  For  smaller 
quantities  of  liquid  this  inaccuracy  may  be  avoided  by  making  the 
pipettes  of  tubes  of  uniform  width,  having  a  small  diameter  only. 
and  narrowed  at  both  ends.  (Fu.  MOHK'S  measuring  pipettes.) 

"When  a  fluid  runs  out  of  a  pipette,  drops  sometimes  remain 
here  and  there  adhering  to  the  tube ;   this  arises  from  a  film  of  fat 

on  the  inside  ;  it  may 
be  removed  by  filling 
the  instrument  with  a 
concentrated  solution 
of  potassium  bichro- 
mate mixed  with  sul- 
phuric acid,  or  with 
potassa  solution,  and, 
after  allowing  sufficient 
time  for  action,  thor- 
oughly washing. 

bb.  Me asur  -i  n g 
vessels  which  serve  to 
measure  out  quantities 
of  fluid  at  will. 

4.    The  Burette. 

Of  the  various 
forms  and  dispositions 
of  this  instrument,  the 
following  appear  to  me 
the  most  convenient :  * 

§21. 
I.   Molir^s  Burette. 

This    excellent 
measuring  apparatus  is 
represented  in  Fig.  14. 
It  consists  of  a  cylin- 
drical  tube,    narrower  towards  the  lower  end  for  about  an  inch, 

*In  regard  to  other  forms  see  F.  Moira,  Lehrbuch  der  Titrirmethode,  3d  edit,, 
§2  ;  G.  C.  WITTSTEIN,  Vierteljaliresschr.  f.  prakt.  Pharm.,  xvi,  567,  and  Zeitschr. 
/.  analyt.  Chem.,  vn,  84;  A.  GAWALOWSKY,  Zeitschr.  /.  Chem.  [N.  F.],  vi,  129, 
and  Zeitschr.  f.  analyt.  Chem.,  ix,  369  ;  GONDOLO,  Rev.  hebdomad.  deChim.,  Nov 
1869,  and  Zeitschr.  f.  analyt.  Chem.,  ix,  370. 


Fig.  14. 


§  21.]  MEASURING   OF   FLUIDS.  "  43 

with  a  slight  widening,  however,  at  the  extreme  point,  in  order 
that  the  caoutchouc  connector  may  take  a  firm  hold.  I  only  use 
burettes  of  two  sizes,  viz..,  of  30  c.  c. ,  divided  into  O'l  c.  c. ;  and 
of  50  c.  c.,  divided  into  0*5  c.  c.  The  former  are  employed 
principally  in  scientific,  the  latter  chiefly  in  technical,  investiga- 
tions. The  usual  length  of  my  30  c.  c.  burette  is  about  50  cm. ; 
the  graduated  portion  occupies  about  49  cm.  The  diameter  of  the 
tube  is  accordingly  about  10  mm.  in  the  clear;  the  upper  orifice 
is,  for  the  convenience  of  filling,  widened  in  form  of  a  funnel, 
measuring  20  mm.  in  diameter;  the  width  of  the  lower  orifice  is 
5  mm.  For  very  delicate  processes,  the  length  of  the  graduated 
portion  may  be  extended  to  50  or  52  cm.,  leaving  thus  intervals 
of  nearly  2  mm.  between  the  small  divisional  lines.  In  my  50  c.  c. 
burettes  the  graduated  portion  of  the  tube  is  generally  40  cm.  long. 

To  make  the  instrument  ready  for  use,  the  narrowed  lower  end 
of  the  tube  is  warmed  a  little,  and  greased  with  tallow;  a  caout- 
chouc tube,  about  30  mm.  long,  and  having  a  diameter  of  3  mm. 
in  the  clear,  is  then  drawn  over  it ;  into  the  other  end  of  this  is 
inserted  a  tube  of  pretty  thick  glass,  about  40  mm.  long,  and 
drawn  out  to  a  tolerably  fine  point;  it  is  advisable  to  slightly 
widen  the  upper  end  of  this 
tube  also,  and  to  covet-  it  with 
a  thin  coat  of  tallow ;  and  also 
to  tie  linen  thread,  or  twine, 
round  both  ends  of  the  con- 
nector, to  insure  perfect  tight- 
ness. 

The  space  between  the 
lower  orifice  of  the  burette  and  the  upper  orifice  of  the  small 
delivery  tube  should  be  about  15  mm.  The  India-rubber  tube  is 
now  pressed  together  between  the  ends  of  the  tubes  by  the  pinch- 
cock  (or  clip).  This  latter  instrument  is  usually  made  of  brass 
wire;  the  form  represented  in  Fig.  15  was  devised  by  MOHR. 

A  good  clip  must  pinch  so  tightly  that  not  a  particle  of  fluid 
can  make  its  way  through  the  connector  when  compressed  by  it ;  it 
must  be  so  constructed  that  the  analyst  may  work  it  with  perfect 
facility  and  exactness,  so  as  to  regulate  the  outflow  of  the  liquid 
with  the  most  rigorous  accuracy,  by  bringing  a  greater  or  less 
degree  of  pressure  to  bear  upon  it. 


44 


OPERATIONS. 


[§21. 


MOHR*  lias  also  devised  very  practical  clips  of  glass  (or  horn) 
and  rubber,  which  are  to  be  highly  recommended. 

Figs.  16  and  17  show  the  construction  of  these  clips,  which  are 
so  simple  that  any  one  can  readily 
make   them   by  following  MOHR'S 
instructions,  as  follows: 

Bend  two  pieces  of  flat  ther- 
mometer-tubing, 80  to  90  cm.  long, 
into  a  very  obtuse  angle,  and  place 
between  them,  in  the  middle,  a 
thin  piece  of  cork,  about  1£  to  2 
mm.  thick ;  pass  a  rubber  ring,  • 


Fig.  16. 


Fig.  17. 


cut  from  a  somewhat  wide  rubber  tube,  over  the  part  inclosing  the 
cork.  After  placing  the  rubber  tube  of  the  burette  between  the 
two  glass  tubes,  press  these  together  and  slip  another  rubber  ring 
over  the  ends  of  the  glass  tubes.  These  rings  serve  to  tightly  com- 
press and  close  the  rubber  tube  OH  the  burette.  On  pressing  together 
the  divergent  ends  of  the  glass  tubes,  however,  the  pressure  on  the 
rubber  tube  is  relieved,  and  the  liquid  flows  through  the  delivery 
tube.  On  releasing  the  pressure,  the  elastic  bands  again  completely 
close  the  connecting  tube. 

For  supporting  MOHR'S  burettes,  use  is  made  of  the  holder 
shown  in  Fig.  14;  this  instrument,  whilst  securely  confining  the 
tube,  permits  its  being  moved  up  and  down  with  perfect  freedom, 
and  also  its  being  taken  out,  without  interfering  with  the  pinch- 
cock.  The  position  of  the  burette  must  be  strictly  perpendicular, 
to  insure  which,  care  must  be  taken  to  have  the  grooves  of  the  cork 
lining,  which  are  intended  to  receive  the  tube,  perfectly  vertical, 
with  the  lower  board  of  the  stand  in  a  horizontal  position. 

The  arm  bearing  the  burette  I  now  have  made  movable  around 
the  upright,  so  that  first  one,  then  another  burette  may  readily  be 


*  MOHR,  Lehrbuch  der  Titrirmethode,  3d  edit.,  p.  7. 


§21.] 


MEASURING   OF   FLUIDS. 


45 


used.  If  it  is  desired  to  fix  the  arm,  a  screw  (wanting  in  the  illus- 
tration) serves  the  purpose.  A  similar  holder,  with  a  brass  clamp, 
is  shown  in  Fig.  18. 

To  best  charge  the  burette  for  a  volumetric  operation,  the 
point  of  the  instrument  is  immersed  in  the  liquid,  the  pinch-cock 
opened,  and  a  little  liquid,  suf- 
ficient at  least  to  reach  into  the 
burette  tube,  drawn  up  by  ap- 
plying suction  to  the  upper  end  ; 
the  cock  is  then  closed,  and  the 
liquid  poured  into  the  burette 
until  it  reaches  up  to  a  little 
above  the  top  mark.  The  burette 
having,  if  required,  been  duly 
adjusted  in  the  proper  vertical 
position,  the  liquid  is  allowed  to 
drop  out  to  the  exact  level  of 
the  top  mark.  The  instrument 
is  now  ready  for  use.  When  as 
much  liquid  has  flowed  out  as  is 
required  to  attain  the  desired 
object,  the  analyst,  before  pro- 
ceeding to  read  off  the  volume 
used,  should  wait  a  few  minutes, 
to  give  the  particles  of  fluid  ad- 
hering to  the  sides  of  the  emp- 
tied portion  of  the  tube  proper 
time  to  run  down.  This  is  an 
indispensable  part  of  the  opera- 
tion in  accurate  measurements, 
since,  if  neglected,  an  experiment 
in  which  the  standard  liquid  in  the  burette  is  added  slowly  to  the 
fluid  under  examination  (in  which,  accordingly,  the  minute  particles 
of  fluid  adhering  to  the  glass  have  proper  time  afforded  them  during 
the  operation  itself  to  run  down),  will,  of  course,  give  slightly  dif- 
ferent results  from  those  arrived  at  in  another  experiment,  where 
the  larger  portion  of  the  standard  fluid  is  applied  rapidly,  and  the 
last  few  drops  alone  are  added  slowly. 

The  manner  in  which  the  rcading-off  is  effected,  is  a  matter  of 


Fig.  18. 


46 


OPERATIONS. 


[ 


great  importance  in  volumetric  analysis;  the  first  requisite  is  to 
bring  the  eye  on  a  level  with  the  top  of  the  fluid.  "We  must 
consequently  settle  the  question — What  is  to  be  considered  the  top? 
If  you  hold  a  burette,  partly  filled  with  water,  between  the  eye 
and  a  strongly  illumined  wall,  the  surface  of  the  fluid  presents  the 
appearance  shown  in  Fig.  19 ;  if  you  hold  close  behind  the  tube  a 
sheet  of  white  paper,  with  a  strong  light  falling  on  it,  the  surface 
of  the  fluid  presents  the  appearance  shown  in  Fig.  20. 


Fig.  19. 


Fig.  20. 


Fig.  21, 


In  the  one  as  well  as  in  the  other  case,  you  have  to  read  off  at 
the  lower  border  of  the  dark  zone,  this  being  the  most  distinctly 
marked  line.  FR.  MOHR  recommends  the  following  device  for 
reading  off :  Paste  on  a  sheet  of  white  paper  a  broad  strip  of  black 
paper,  and,  when  reading-off,  hold  this  close  behind  the  burette,  in 
a  position  to  place  the  border  line  between  white  and  black  from  2 
to  3  mm.  below  the  lower  border  of  the  dark  zone,  as  shown  in 
Fig.  21 ;  then  read-off  at  the  lower  border  of  the  dark  zone. 

Great  care  must  be  taken  to  hold  the  paper  invariably  in  the 
same  position,  since,  if  it  be  held  lower  down,  the  lower  border  of 
the  black  zone  will  move  higher  up. 

I  prefer  to  read-off  in  a  light  which  causes  the  appearance  rep- 
resented in  Fig.  19. 

By  the  use  of  ERDMANN'S  float*  all  uncertainties  in  reading-off 

*  Journ.  f.  prakt.  Chem.,  LXXI,  194. 


§  21.]  MEASURING    OF   FLUIDS.  47 

may  be  avoided.  Fig.  22  represents  a  burette  thus  provided.  In 
tins  case  we  always  read-off  the  mark  on  the  burette  which  coin- 
cides with  the  circle  in  the  middle  of  the  float.  The  float  must  be 
so  fitted  to  the  width  of  the  burette  that  when  placed  in  the  filled 
burette,  it  will,  on  allowing  the  fluid  to  run  out  gradually,  sink 
down  without  wavering ;  and  when  it  has  been  pressed  down  into 
the  fluid  of  the  closed  burette,  it  will  slowly  rise  again.  The  weight 
of  the  float  must,  if  necessary,  be  so  regulated  by  mercury  that 
when  placed  in  the  filled  tube,  the  surface  of  the  liquid  will  coin- 
cide uniformly  all  around  with  the  upper  shoulder  of  the  float.  A 
further  important  condition  of  the  float  is  that  its  axis  should 
coincide  as  nearly  as  possible  with  that  of  the  burette  tube,  so  that 
the  division-mark  on  the  burette  may  be  always  parallel  with  the 
circular  line  on  the  float. 

The  correctness  of  the  graduation  of  a  burette  is  tested  in  the 
most  simple  way,  as  follows :  Fill  tlie  instrument  up  to  the  highest 
division  with  water  at  17 '5°,  then  let  10  c.  c.  of  the 
liquid  flow  out  into  an  accurately  weighed  flask,  and 
weigh;  then  let  another  quantity  of  10  c.  c.  flow 
out,  and  weigh  again,  and  repeat  the  operation 
until  the  contents  of  the  burette  are  exhausted. 
If  the  instrument  is  correctly  graduated,  every 
10  c.  c.  of  water  at  IT '5°  must  weigh  10  grin. 
Differences  up  to  O'Ol  grm.  may  be  disregarded, 
since  even  with  the  greatest  care  bestowed  on  the 
process  of  reading-off,  deviations  to  that  extent 
will  occur  in  repeated  measurements  of  the  upper- 
most 10  c.  c.  of  one  and  the  same  burette.  With 
the  float-burettes  the  weighings  agree  much  more 
accurately,  and  the  differences  for  10  c.  c.  do  not 
exceed  0*002  grm. 

MOHR'S  burette  is  unquestionably  the  best  and 
most  convenient  instrument  of  the  kind,  and  ought 
to  be  employed  in  the  measurement  of  all  liquids 
which  are  not  injuriously  affected  by  contact  with 
caoutchouc.  Of  the  standard  solutions  used  at 
present  in  volumetric  analysis,  that  of  potassium  Fig.  22. 
permanganate  alone  cannot  bear  contact  with  caoutchouc.  Excel- 


48 


OPERATIONS. 


[§22. 


lent  directions  for  calibrating  MOHR  burettes  have  been  given  by 
SCHEIBLER.  * 

§22. 
II.    Gay-Lussatf 8  Burette. 

Fig.  23  represents  this  instrument  in,  as  I  believe,  its  most 
practical  form. 

I  make  use  of  two  sizes,  one  of  50  c'.  c.  graduated  in  0'5  c.  c., 
the  other  of  30  c.  c.  graduated  in  O'l  c.  c.  The  former  is  about 
33  cm.  long;  the  graduated  portion  occupies  about 
25  cm.  ;  the  internal  diameter  of  the  wide  tube  meas- 
ures 15  mm. ;  that  of  the  narrow  tube  4  mm.,  which 
in  the  upper  bent  end  gradually  decreases  to  2  mm. 
The  graduated  portion  of  the  smaller  burette  is  about 
28  cm.  long,  and'has  accordingly  an  internal  diameter 
of  about  11  mm. 

In  use  this  burette  is  held  in  the  left  hand,  with 
the  lower  end  resting  lightly  against  the  chest.  Held 
in  this  manner,  and  with  an  occasional  turn  of  the  spout 
sideways,  the  outflow  of  liquid  may  be  regulated  at 
will.  The  fluid  is  not  allowed,  as  a  rule,  to  run  back 
into  the  narrow  tube  during  the  course  of  an  opera- 
tion, as  it  is  frequently  difficult  to  renew  the  flow  of  the 
fluid  because  of  the  formation  of  an  air-bubble  be- 
tween the  fluid  and  the  drop  remaining  in  the  mouth 
of  the  spout. 

To  provide  a  substantial  stand  for  the  burette,  a 
solid  disk  of  wood  10  to  12  cm.  in  diameter  and  from 
Fig.  23.  5  to  6  cm.  thick  has  a  suitable  hole  bored  in  its  centre, 
in  which  the  burette  may  be  inserted.  This  seems  to  me  more 
convenient  than  to  cement  the  burette  in  a  wooden  foot. 

To  overcome  the  difficulty  of  renewing  the  flow  of  liquid  when 
an  air-bubble  has  become  enclosed  between  the  fluid  and  the  drop 
remaining  in  the  tip  of  the  spout,  MOHR  has  proposed  closing  the 
wider  tube  with  a  perforated  cork  bearing  a  short  glass  tube  bent 
at  right  angles.  On  slipping  a  piece  of  rubber  tubing  over  this 


40 


50 


*Jour.f.  prakt.  Chemie,  LXXVI,  177. 


§23.]  MEASURING   OF   FLUIDS.  49 

sliort  tube,  and  blowing  into  it  more  or  less  strongly,  the  outflow 
of  liquid  may  be  regulated  at  will.  Instead  of  blowing  with  the" 
mouth,  a  rubber  bulb  may  be  used,  but  the  latter  must  be  provided 
with  a  small  hole  through  which  air  may  enter  after  the  bulb  has 
been  compressed,  and  which  is  closed  by  the  finger  during  compres- 
sion (HERYE-MANGON).* 

The  readings  with  this  burette  are  taken  just  as  with  the  MOHR 
burette.  The  instrument  is  preferably  held  firmly  against  a  per- 
pendicular wall,  a  strongly  illuminated  white  door,  or  a  window 
pane,  to  insure  its  being  held  perfectly  vertical.  When  operating 
with  concentrated,  and  hence  opaque,  solutions  of  potassium  per- 
manganate, the  method  of  reading  requires  modification,  the  upper 
border  of  the  liquid  being  then  observed,  and  the  readings  best 
taken  by  reflected  light  against  a  white  background. 

The  GAY-LUSSAG  burette  is  tested  as  to  its  correctness  just  as  is 
the  MOHR  burette. 

§  23. 
III.    Geissler's  Burette. 

In  this  instrument,  figured  in  Fig.  24,  the  narrow  tube,  which 
is  outside  in  the  GAY-LUSSAC  burette,  is  placed  within  the  wide 
tube.  The  glass  of  that  part  of  the  inner  tube  which  projects  from 
the  wide  tube  is  very  thick,  while  the  part  within  the  wide  tube  is 
very  thin. 

This  burette  is  very  convenient  to  use,  and  is  but  little  liable  to 
fracture.  I  am  very  partial  to  it. 

What  has  been  stated  above  regarding  reading-oil  and  testing 
applies  to  this  burette  as  well. 

*  Sep.  chim.  appliquee,  I,  68. 


50 


OPERATIONS. 


[§24 


U    PRELIMINARY  OPERATIONS. — PREPARATION  OF  SUBSTANCES  FOB 
THE  PROCESSES  OF  QUANTITATIVE  ANALYSIS. 

§24. 
1.  THE  SELECTION  OF  THE  SAMPLE. 

Before  the  analyst  proceeds  to  make  the  quantitative  analysis 
of  a  body,  he  cannot  too  carefully  consider 
whether  the  desired  result  is  fully  attained  if  he 
simply  knows  the  respective  quantity  of  every 
individual  constituent  of  that  body.  This  pri- 
mary point  is  but  too  frequently  disregarded,  and 
thus  false  impressions  are  made,  even  by  the 
most  careful  analysis.  This  remark  applies  both 
to  scientific  and  to  technical  investigations. 

Therefore,  if  the  constitution  of  a  mineral 
is  to  be  determined,  take  the  greatest  pos- 
sible care  to  remove  in  the  first  place  every 
particle  of  gangue,  and  disseminated  imp-un- 
ties ;  remove  any  adherent  matter  by  wiping  or 
washing,  then  wrap  the  substance  up  in  a  sheet 
of  thick  paper,  crush  it  to  pieces  on  a  steel 
anvil,  and  pick  out  with  a  pair  of  small  pincers 
the  cleanest  pieces.  Crystalline  substances, 
prepared  artificially,  ought  to  be  purified  by  re- 
crystallization  ;  precipitates  by  thorough  wash- 
ing, &c.,  &c. 

In  technical  investigations, — when  called  * 
upon,  for  instance,  to  determine  the  amount  of 
peroxide  present  in  a  manganese  ore,  or  the 
amount  of  iron  present  in  an  iron  ore, — the  first 
point  for  consideration  ought  to  be  whether  the 
samples  selected  correspond  as  much  as  possible 
to  the  average  quality  of  the  ore.  What  would 
it  serve,  indeed,  to  the  purchaser  of  a  manganese 
mine  to  know  the  amount  of  peroxide  present 
in  a  select,  possibly  particularly  rich,  sample  ? 

These  few  observations  will  suffice  to  show  that  no  universally 
applicable  and  valid  rules  to  guide  the  analyst  in  the  selection  of 
the  sample  can  be  laid  down ;  he  must  in  every  individual  case, 


Fig.  24. 


§  25.]  MECHANICAL   DIVISION.  51 

on  the  one  hand,  examine  the  substance  carefully,  and  more  par- 
ticularly also  under  the  microscope,  or  through  a  lens  ;  and,  on  the 
other  hand,  keep  clearly  in  view  the  object  of  the  investigation, 
and  then  take  his  measures  accordingly. 

§25. 
2.  MECHANICAL  DIVISION. 

In  order  to  prepare  a  substance  for  analysis,  i.e.,  to  render  it 
accessible  to  the  action  of  solvents  or  fluxes,  it  is  generally  indis- 
pensable, in  the  first  place,  to  divide  it  into  minute  parts,  since 
this  will  create  numerous  points  of  contact  for  the  solvent,  and 
will  counteract,  and,  so  far  as  practicable,  remove  the  adverse 
influences  of  the  power  of  cohesion,  thus  fulfilling  all  the  condi- 
tions necessary  to  effect  a  complete  and  speedy  solution. 

The  means  employed  to  attain  this  object  vary  according  to  the 
nature  of  the  different  bodies  we  have  to  operate  upon.  In  many 
cases,  simple  crushing  or  pounding  is  sufficient ;  in  other  cases  it 
is  necessary  to  reduce  the  powder  to  the  very  highest  degree  of 
fineness,  by  sifting  or  by  elutriation. 

The  operation  of  powdering  is  conducted  in  mortars.  The 
first  and  absolutely  indispensable  condition  is,  that  the  material  of 
the  mortar  be  considerably  harder  than  the  substance  to  be  pulver- 
ized, so  as  to  prevent,  so  far  as  practicable,  the  latter  from  being 
contaminated  with  any  particles  of  the  former.  Thus,  for  crush- 
ing salts  and  other  substances  possessing  no  very  considerable 
degree  of  hardness,  porcelain  mortars  may  be  used,  whilst  the 
trituration  of  harder  substances  (of  most  minerals,  for  instance,) 
requires  vessels  of  agate,  chalcedony,  or  flint.  In  such  cases,  the 
larger  pieces  are  first  reduced  to  a  coarse  powder,  best  effected  by 
wrapping  them  up  in  several  sheets  of  writing-paper,  and  striking 
them  with  a  hammer  upon  a  steel  or  iron  plate ;  the  coarse  powder 
thus  obtained  is  then  pulverized,  in  small  portions  at  a  time,  in  an 
agate  mortar,  until  reduced  to  the  state  of  an  impalpable  powder. 
If  we  have  but  a  small  portion  of  a  mineral  to  operate  upon,  and 
indeed  in  all  cases  where  we  are  desirous  of  avoiding  loss,  it  is 
advisable  to  use  a  steel  mortar  (Fig.  25)  for  the  preparatory  reduc- 
tion of  the  mineral  to  coarse  powder. 

ab  and  cd  represent  the  two  parts  of  the  mortar ;  these  may  be 
readily  taken  asunder.  The  substance  to  be  crushed  (having,  if 


OPERATIONS. 


[§25. 


Fig.  25. 


practicable,  first  been  broken  into  small  pieces),  is  placed  in  the 
cylindrical  chamber  ef  $  the  steel  cylinder,  which  fits  somewhat 
loosely  into  the  chamber,  serves  as  pestle.  The  mortar  is  placed 
upon  a  solid  support,  and  perpendicular  blows  are  repeatedly 

struck  upon  the  pestle  with  a  hammer 
until  the  object  in  view  is  attained. 

Minerals  which  are  very  difficult 
to  pulverize  may  be  strongly  ignited, 
and  then  suddenly  plunged  into  cold 
water,  and  subsequently  again  ignited. 
This  process  is  of  course  applicable  only 
to  minerals  which  lose  no  essential  con- 
stituent on  ignition,  and  are  perfectly 
insoluble  in  water. 

In  the  purchase  of  agate  mortars, 
especial  care  ought  to  be  taken  that  they 
have  no  palpable  cracks  or  indentations  ; 
very  slight  cracks,  however,  that  cannot 
be  felt,  do  not  render  the  mortar  useless,  although  they  impair  its 
durability. 

Minerals  insoluble  in  acids,  and  which  consequently  require 
fusing,  must  especially  be  finely  divided,  otherwise  we  cannot  calcu- 
late upon  complete  decomposition.  This  object  may  be  obtained 
either  by  triturating  the  crushed  mineral  with  water,  or  by  elutri- 
ation,  or  by  sifting ;  the  two  former  processes,  however,  can  be 
resorted  to  only  in  the  case  of  substances  which  are  not  attacked 
by  water.  It  is  quite  clear  that  analysts  must  in  future  be  much 
more  cautious  on  this  point  than  has  hitherto  been  the  case,  since 
we  know  now  that  many  substances  which  are  usually  held  to  be 
insoluble  in  water  are,  when  in  a  state  of  minute  division,  strongly 
affected  by  that  solvent;  thus,  for  instance,  water,  acting  upon 
some  sorts  of  finely  pulverized  glass,  is  found  to  rapidly  dissolve 
from  2  to  3  per  cent,  of  glass  even  in  the  cold.  (PELOUZE.*) 
Thus,  again,  finely  divided  feldspar,  granite,  trachyte  and  porphyry 
give  up  to  water  both  alkali  and  silica.  (H.  LuDwia.f) 

Levigation  (trituration  with  water).  Add  a  little  water  to 
the  crushed  mineral  in  the  mortar,  and  triturate  the  paste  until  all 
crepitation  ceases,  or,  which  is  a  more  expeditious  process,  transfer 


*  Compt.  Rend.,  XLIII,  117-123. 


f  Archiv  d#r  Pharm.,  xci,  147. 


§  25.]  MEASURING   OF   FLUIDS.  53 

tlie  paste  from  the  mortar  to  an  agate  or  flint  slab,  and  triturate 
it  thereon  with  a  nmller.     Rinse  the  paste  off,  with  the  washing- 
bottle,   into    a   smooth    porcelain   basin    of   hemispherical  form, 
evaporate  the  water  011  the  water-bath,  and  mix  the  residue  most 
carefully  with  the  pestle.     (The    paste  may   be  dried  also  in  the 
agate  mortar,  but  at  a  very  gentle  heat,  since  otherwise  the  mortar 
might  crack.) 

To  perform  the  process  of  elutriation,  the  pasty  mass,  having 
first  been  very  finely  triturated  with  water,  is  washed  off  into  a 
beaker,  and  stirred  with  distilled  water;  the  mixture  is  then  allowed 
to  stand  a  minute  or  so,  after  which  the  supernatant  turbid  fluid  is 
poured  off  into  another  beaker.  The  sediment,  which  contains  the 
coarser  parts,  is  then  again  subjected  to  the  process  of  trituration, 
etc.,  and  the  same  operation  repeated  until  the  whole  quantity  is 
elutriated.  The  turbid  fluid  is  allowed  to  stand  at  rest  until  the 
minute  particles  of  the  substance  held  in  suspension  have  subsided, 
which  generally  takes  many  hours.  The  water  is  then  finally 
decanted,  and  the  powder  dried  in  the  beaker. 

The  process  of  sifting  is  conducted  as  follows  :  A  piece  of  fine, 
well-washed,  and  thoroughly  dry  linen  is  placed  over  the  mouth  of 
a  bottle  about  10  cm.  high,  and  pressed  down  a  little  into  the  mouth, 
so  as  to  form  a  kind  of  bag ;  a  portion  of  the  finely  triturated  sub- 
stance is  put  into  the  bag,  and  a  piece  of  soft  leather  stretched  tightly 
over  the  top  by  way  of  cover.  By  drumming  with  the  finger  on  the 
leather  cover,  a  shaking  motion  is  imparted  to  the  bag,  which 
makes  the  finer  particles  of  the  powder  gradually  pass  through  the 
linen.  The  portion  remaining  in  the  bag  is  subjected  again  to 
trituration  in  an  agate  mortar,  and,  together  with  a  fresh  portion 
of  the  powder,  sifted  again;  the  process  is  repeated  until  the 
entire  mass  has  passed  through  the  bag  into  the  glass. 

When  operating  on  substances  consisting  of  different  com- 
pounds it  would  be  a  grave  error  indeed  to  use  for  analysis  the 
powder  resulting  from  the  first  process  of  elutriation  or  sifting, 
since  this  will  contain  the  more  readily  pulverizable  constituents  in 
a  greater  proportion  to  the  more  resisting  ones  than  is  the  ca>e 
with  the  original  substance. 

Great  care  must,  therefore,  also  be  taken  to  avoid  a  loss  of 
substance  in  the  process  of  elutriation  or  sifting,  as  this  loss  is 
likely  to  be  distributed  unequally  among  the  several  component 
parts.  It  is  safer  in  such  cases  to  effect  the  subdivision  by  patiently 
triturating  the  dry  substance,  and  to  avoid  elutriation  or  sifting. 


54  OPERATIONS.  [§  26. 

In  cases  where  it  is  intended  to  ascertain  the  average  composi- 
tion of  a  heterogeneous  substance,  of  an  iron  ore  for  instance,  a 
large  average  sample  is  selected,  and  reduced  to  a  coarse  powder ; 
the  latter  is  thoroughly  intermixed,  a  portion  of  it  powdered  more 
finely,  and  mixed  uniformly,  and  finally  the  quantity  required  for 
analysis  is  reduced  to  the  finest  powder.  The  most  convenient 
instrument  for  the  crushing  and  coarse  powdering  of  large  samples 
of  ore,  &c.,  is  a  steel  anvil  and  hammer.  The  anvil  in  my  own 
laboratory  consists  of  a  wood  pillar,  85  cm.  high  and  26  cm.  in 
diameter,  into  which  a  steel  plate,  3  cm.  thick  and  20  cm.  in 
diameter,  is  let  to  the  depth  of  one-half  of  its  thickness.  A  brass 
ring,  5  cm.  high,  fits  round  the  upper  projecting  part  of  the  steel 
plate.  The  hammer,  which  is  well  steeled,  has  a  striking  surface 
of  5  cm.  diameter.  An  anvil  and  hammer  of  this  kind  afford, 
among  others,  this  advantage,  that  their  steel  surfaces  admit  most 
readily  of  cleaning.  To  convert  the  coarse  powder  into  a  finer,  a 
smooth-turned  steel  mortar  of  about  130  mm.  upper  diameter  and 
74  mm.  deep  is  used — the  final  trituration  is  conducted  in  an  agate 
mortar. 

§26. 

3.  DEYING. 

Bodies  which  it  is  intended  to  analyze  quantitatively  must  be, 
when  weighed,  in  a,  definite  state — in  a  condition  in  which  they 
can  be  always  obtained  again. 

Now,  the  essential  constituents  of  a  substance  are  usually  accom- 
panied by  an  unessential  one,  viz.,  a  greater  or  less  quantity  of 
water,  enclosed  either  within  its  lamellae,  or  adhering  to  it  from 
the  mode  of  its  preparation,  or  absorbed  by  it  from  the  atmosphere. 
It  is  perfectly  obvious  that  to  estimate  correctly  the  quantity  of  a 
substance,  we  must,  in  the  first  place,  remove  this  variable  quantity 
of  water.  Most  solid  bodies,  therefore,  require  to  ~be  dried  before 
they  can  ~be  quantitatively  analyzed. 

The  operation  of  drying  is  of  the  very  highest  importance  for 
the  correctness  of  the  results  ;  indeed  it  may  safely  be  averred  that 
many  of  the  differences  observed  in  analytical  researches  proceed 
entirely  from  the  fact  that  substances  are  analyzed  in  different 
states  of  moisture. 

Many  bodies  contain,  as  is  well  known,  water  which  is  proper 


§  26.]  DESICCATION.  55 

to  them  either  as  inherent  in  their  constitution,  or  as  so-called  water 
of  crystallization.     In  contradistinction  to  this,  we  will  employ  the ' 
term  moisture  to  designate  that  variable  adherent  or  mechanically 
enclosed  water,  with  the  removal  of  which  the  operation  of  drying 
in  the  sense  here  in  view  is  alone  concerned. 

In  the  drying  of  substances  for  quantitative  analysis,  our  object 
is  to  remove  all  moisture,  without  interfering  in  the  slightest  degree 
with  combined  water  or  any  other  constituent  of  the  body.  To 
accomplish  this  object,  it  is  absolutely  requisite  that  we  should 
know  the  properties  which  the  substance  under  examination  mani- 
fests in  the  dry  state,  and  whether  it  loses  water  or  other  constitu- 
ents at  a  red  heat,  or  at  100°,  or  in  dried  air,  or  even  simply  in 
contact  with  the  atmosphere.  These  data  will  serve  to  guide  us  in 
the  selection  of  the  process  of  desiccation  best  suited  to  each  sub- 
1  stance.  The  dried  substance  should  always  at  once  be  transferred 
to  a  well-closed  vessel ;  glass  tubes,  sealed  at  one  end,  and  of  suf- 
ficiently thick  glass  to  bear  the  firm  insertion  of  tight-fitting 
smooth  corks — weighing-tubes — are  usually  employed  for  this 
purpose.  It  is  also  advisable  to  cover  the  corks  with  tinfoil. 

The  following  classification  may  accordingly  be  adopted : — 

a.  Substances  which  lose  water  even  on  simple  contact  with  the 
atmosphere  •  such  as  sodium  sulphate,  crystallized  sodium  carbon- 
ate, etc.  Substances  of  this  kind  turn  dull  and  opaque  when 
exposed  to  the  air,  and  finally  crumble  wholly  or  partially  to  a  white 
powder.  They  are  more  difficult  to  dry  than  many  other  bodies. 
The  process  best  adapted  for  the  purpose,  is  to  press  the  pulverized 
salts  with  some  degree  of  force  betwreen  thick  layers  of  fine  white 
blotting-paper,  repeating  the  operation  with  fresh  paper  until  the 
last  sheets  remain  absolutely  dry. 

It  is  generally  advisable  in  the  course  of  this  operation  to  repow- 
der  the  salt. 

1.  Substances  which  do  not  yield  water  to  the  atmosphere  (unless 
it  is  perfectly  dry),  but  effloresce  in  artificially  dried  air  y  such  as 
magnesium  sulphate,  sodium-potassium  tartrate  (Rochelle  salt),  &c. 
Salts  of  this  kind  are  reduced  to  powder,  which,  if  it  be  very 
moist,  is  pressed  between  sheets  of  blotting-paper,  as  in  a ;  after 
this  operation,  it  must  be  allowed  to  remain  for  some  time  spread 
in  a  thin  layer  upon  a  sheet  of  blotting-paper,  effectually  protected 
against  dust,  and  shielded  from  the  direct  rays  of  the  sun. 


56 


OPERATIONS. 


[§27, 


§  27. 

c.  Substances  which  undergo  no  alteration  in  dried  air,  but 
lose  water  at  100° ;  calcium  tartrate,  for  instance.  These  are  finely 
pulverized ;  the  powder  is  put  in  a  thin  layer  into  a  watch-glass  or 
shallow  dish,  and  the  latter  placed  inside  a  chamber  in  which  the 
air  is  kept  dry  by  means  of  concentrated  sulphuric  acid  or  calcium 
chloride.  This  process  is  usually  conducted  in  one  of  the  follow- 
ing apparatus,  which  are  ternied  desiccators,  and  which  subserve 
still  another  purpose  besides  that  of  drying,  viz.,  that  of  allowing 
hot  crucibles,  dishes,  etc.,  to  cool  in  dry  air. 


Fig.  26.  Fig.  27. 

In  Fig.    26,    a   represents  a  glass  plate    (ground-glass  plates 
answer  the  purpose  best),  #,  a  bell  jar  with  ground  rim,  which  is 

greased  with  tallow;  c  is  a  glass 
basin  with  concentrated  sulphuric 
acid;  d,  a  round  iron  plate,  sup- 
ported on  three  feet,  with  circular 
holes  of  various  sizes,  for  the  recep- 
tion of  the  watch-glasses,  crucibles, 
etc.,  containing  the  substance. 

In  Fig.  27,  a  represents  a  beaker 
with  ground  and  greased  rim,  and 
filled  to  one-fourth  or  one-third 
with  concentrated  sulphuric  acid ; 
1)  is  a  ground-glass  plate;  c  is  a 

bent  wire  of  lead,  which  serves  to  support  the  watch-glass  contain- 
ing the  substance. 

Fig.  28  is  a  similarly  constructed  calcium-chloride  desiccator. 


Fig.  28. 


§.27.] 


DESICCATION. 


57 


Fig.  29  represents  a  readily  portable  desiccator,  used  more  par- 
ticularly to  receive  crucibles  in  course 
of  cooling,  and  carry  them  to  the  bal- 
ance. The  apparatus  consists  of  a 
strong  glass  jar ;  the  lid  must  be  ground 
to  shut  air-tight ;  the  place  on  which  it 
joins  is  greased  with  tallow.  The  outer 
diameter  of  my  jars  is  105  mm. ;  the 
sides  are  6  mm.  thick.  The  aperture 
has  a  diameter  of  80  mm. ;  the  box  up 
to  the  small  part  is  65  mm.  high;  the 
lid  has  the  same  height ;  the  small  part 
itself  is  .15  mm.  high,  and  ground  to  a 
slightly  conical  shape. 
A  brass  ring,  with 
rim,  fits  exactly  into 
the  aperture ;  the  rim 
must  not  project  be- 
yond the  glass.  The  ring  bears  a  triangle  of 
iron  or,  better,  platinum  wire,  intended  for  the 
reception  of  crucibles,  &c.  The  vessel  is  one- 
third  filled  with  calcium  chloride. 

Fig.  30  represents  an  exsiccator  devised  by 
A.  SCHKOTTER;  it  affords  free  egress  to  the  air, 
which  expands  when  a 
hot  crucible  is  placed 
within  the  exsiccator 
and  passes  through  the 
small  tube,  &,  escaping 
through  two  small  holes 
placed  at  the  base  of  &, 
whence  it  rises  through 
sulphuric  acid  contained 
in  c,  and  finally  escapes 
through  the  bulb  d 
filled  with  calcium  chlo- 
ride. When  the  appa-  Fig.  30. 
ratus  is  cooling,  per- 
fectly dry  air  re-enters  by  the  same  way.  The  operation  may  be 


.58  OPERATIONS.  [§  %&. 

•considered  at  an  end  when  no  more  air-bubbles  pass  through  the 
sulphuric  acid.  The  small  tube,  e,  serves  to  catch  any  sulphuric 
acid  that  might  be  carried  down  through  a ;  it  must  not  close  air- 
tight, the  lower  orifice  of  the  apparatus  serving  as  a  stopper  for 
the  bell- jar,  hence  the  cork  carrying  it  must  be  channelled. 
f  serves  as  a  stand  for  the  bell-jar.  This  desiccator  possesses  the 
advantage  that  the  substances  placed  in  it  are  cooled  in  dry  air 
at  the  ordinary  atmospheric  pressure,  and  hence,  when  ^amoved 
from  the  apparatus,  have  no  tendency  to  absorb  air  and,  with  this, 
moisture,  which  can  not  be  said  of  substances  cooled  in  air  slightly 
rarefied  by  heat. 

The  substance  to  be  dried  is  exposed  to  the  action  of  the  dry 
air,  until  it  ceases  to  lose  weight.  Substances  which  are  acted  on 
by  atmospheric  oxygen  are  in  a  similar  manner  dried  under  the 
bell- jar  of  an  air-pump.  Substances  which,  although  they  lose 
no  water,  yet  lose  ammonia,  in  dry  air,  are  dried  over  caustic 
lime  mixed  with  a  little  powdered  ammonium  chloride,  i.e.,  in  an 
anhydrous  ammoniacal  atmosphere. 

§  28. 

d.  Substances  which  'at  100°  completely  lose  their  moisture, 
without  suffering  any  other  alteration,  such  as  hydrogen  potassium 
tartrate,  sugar,  etc.  These  are  dried  in  the  water-bath  ;  in  the  case 
of  slow-drying  substances,  or  where  it  is  wished  to  expedite  the 

operation,  with  the  aid  of  a  current  of 
dry  air. 

Fig.  31  represents  the  water-bath 
most  commonly  used.  It  is  made  of 
sheet  copper.  The  engraving  renders 
a  detailed  description  unnecessary.  The 
inner  chamber,  <?,  is  surrounded  on 
five  sides  by  the  outer  case  or  jacket, 
FiS-  31-  d  e,  without  communicating  with  it. 

The  object  of  the  apertures  g  and  h  is  to  effect  change 
of  air,  which  purpose  they  answer  sufficiently  well.  When 
it  is  intended  to  use  the  apparatus,  the  outer  case  is  filled  to  about 
one- half  with  rain-water,  and  the  aperture  a  is  closed  with  a  perfor- 
ated cork,  into  which  a  glass  tube  is  fitted ;  the  aperture  I  is  entirely 


§  28.]  DESICCATION.  59 

closed.  If  the  apparatus  is  intended  to  be  heated  over  charcoal,  it 
should  have  a  length  of  about  20  cm.  from  d  to/*;  but  if  over  a 
gas-,  alcohol-,  or  oil-lamp,  it  should  be  only  about  13  cm.  long.  In 
the  former  case,  the  inner  chamber  is  17  cm.  deep,  14  cm.  broad, 
and  10  cm.  high ;  in  the  latter  case,  it  ic  10  cm.  deep,  9  cm.  broad, 
and  6  cm.  high.  The  temperature  in  the  inner  chamber  never 
quite  reaches  100° ;  to  bring  it  up  to  100°,  F.  ROCHLEDER  has  sug- 
gested to  close  l>  with  a  double-limbed  tube,  the  outer  longer  limb 
of  which  dips  into  a  cylinder  filled  with  water ;  a  is  in  that  case 
closed  with  a  perforated  cork  bearing  a  sufficiently  tall  funnel 
tube,  which  fits  air-tight  in  the  cork.  The  lower  end  of  this  tube 
reaches  down  to  one  inch  from  the  bottom. 

In  large  analytical  laboratories  water  is  usually  kept  boiling  all 
day  long,  for  the  production  of  distilled  water.  The  boilers  used 
in  my  own  laboratory  have  the  shape  of  somewhat  oblong  square 
boxes,  about  120  cm.  long,  60  cm.  broad,  and  24  cm.  high ;  the 
front  of  the  boiler  has  soldered  into  it,  one  above  the  other,  two 
rows  of  drying  chambers,  of  the  kind  shown  in  Fig.  31.  This 
gives  so  many  ovens  that  almost  every  student  may  have  one  for 
his  special  use.  Most  of  these  ovens  are  from  11  to  12  cm.  deep 
and  broad,  and  8  cm.  high ;  some  of  them,  however,  are  16  cm. 
deep  and  broad,  to  enable  them  to  receive  large-sized  dishes.  The 
substances  to  be  dried  are  usually  put  on  double  watch-glasses, 
laid  one  within  the  other,  which  are  placed  in  the  oven,  and  the 
door  is  then  closed.  In  the  subsequent  process  of  weighing,  the 
upper  glass,  which  contains  the  substance,  is  covered  with  the 
lower  one.  The  glasses  must  be  quite  cold  before  they  are  placed 
on  the  scale.  In  cases 
where  we  have  to  deal  with 
hygroscopic  substances,  the 
reabsorption  of  water  upon 
cooling  is  prevented  by  the 
selection  of  close-fitting 

glasses,    which    are    held  Fig.  32. 

tight  together  by  a  clasp 
(Fig.  32),  and  allowed  to  cool  with  their  contents  under  a  bell- 
glass  over  sulphuric  acid  (see  Fig.  26).  These  latter  instructions 
apply  equally  to  the  process  of  drying  conducted  in  other  appa- 
ratus. 

The  clasp  used  for  pressing  the  watch-glasses  together — and 


60  OPERATIONS.  [§  28. 

which  in  all  cases  where  it  is  intended  to  ascertain  the  loss  of 
weight  which  a  substance  suffers  on  desiccation,  is  to  be  looked  upon 
as  belonging  to  the  glasses,  and  must  accordingly  be  weighed  with 
them — is  constructed  of  two  strips  of  thin  brass  plate, 'about  10  cm. 
long  and  1  cin.  wide,  which  are  laid  the  one  over  the  other,  and 
soldered  together  at  the  ends,  to  the  extent  of  5  to  6  mm.  ;  or, 
they  may  be  made  of  one  piece,  as  our  illustration  shows. 

The  following  apparatus  serves  for  drying  substances  in  a  cur- 
rent of  air : 

In  Fig.  33  the  air-current  is  caused  by  simply  warming  the  air, 
hence  the  apparatus  is  very  convenient  to  use.  a  b  is  a  copper  or 


Fig.  33. 

tinned-iron  box  into  which  the  canal  c  d  is  soldered,  and  communi- 
cates with  the  chimney  ef.  The  latter  is  surrounded  on  three 
sides  by  the  case  g  7^,  which  also  communicates  with  a  1>,  but  has  no- 
opening  at  the  top ;  i  is  a  round  hole  leading  into  -  the  canal,  and 
which  may  be  closed  with  a  cork ;  I  k  is  provided  with  a  well-fitting 
sliding  door  running  in  grooves. 

In  use,  the  aperture  n,  which  serves  as  an  outlet  for  the  water, 
is  closed  with  a  cork,  when  the  outer  case  is  half  filled,  through  the 
opening  m,  with  water,  which  is  then  heated  to  boiling.  The 
watch-glasses  containing  the  substances  to  be  dried  are  then  placed 
on  the  holes  in  the  sliding  shelf  B  (Fig.  33),  which  is  then  intro- 
duced into  the  canal  at  I  &,  and  the  latter  closed.  The  steam  sur- 
rounding the  chimney  soon  causes  an  upward  current  of  the 
warmed  air  within  it,  and  this  causes  cold  air  to  be  drawn  in  at 
the  opening  ^,  and  to  pass  over  the  substance  to  be  dried,  carrying 
-with  it  the  evaporating  moisture. 


§28.] 


DESICCATION. 


61 


The  disadvantage  that  the  substances  are  always  kept  at  a  point 
below  100°  C.  by  the  current  of  cold  air,  is  easily  remedied  by  sol- 
dering a  tube  under  the  bottom  of  the  canal  along  its  entire  length 
and  back  again,  and  conducting  the  air  through  it  into  the  canal. 
The  air  is  thus  heated  to  100°  C.  before  it  comes  into  contact  with 
the  substances.  This  tube  is  not  shown  in  the  illustration,  in  order 
to  avoid  confusion.  It  is  very  practical,  also,  to  omit  the  opening 
m,  and  instead  to  cut  in  the  top  of  the  case  round  holes  of  different 
sizes  (provided  with  suitable  covers)  for  receiving  small  evaporating 
dishes.  According  to  requirements,  .the  apparatus  may  be  made 
20  to  30  cm.  long,  15  cm.  wide,  and  about  10  cm.  high.  The 
chimney  should  be  6  cm.  wide  arid  3  cm.  high.  Should  a  stronger 
current  of  air  be  desired  than  that  afforded  by  the  small  chimney,  a 
current  of  air  previously  passed  through  sulphuric  acid  or  over 
calcium  chloride  may  be  blown  through  the  opening  i  by  means  of 
a  gasometer,  rubber  bulb,  or  other  suitable  contrivance.  Or,  air 
dried  by  passing  through  sulphuric  acid  may  be  drawn  through  the 
apparatus  by  means  of  an  air-pump  (§  47),  or  an  aspirator  (d  in 
Fig.  34)  connected  with  the  small  chimney  by  a  tube-bearing  cork 
inserted  into  a  short  tube  with  which  the  chimney  is  in  this  case 
provided.  If  a  higher  temperature  than  that  of  boiling  water  is 
desired,  the  (copper)  apparatus  is  filled  with  oil,  and  the  tempera- 
ture taken  with  a  thermometer  inserted  in  a  perforated  cork  fixed 
in  the  opening  m. 

In  Fig.  34  the  air-current  is  produced  by  a  stream  of  water. 

Fig.  34. 


Fig.  35. 


a  represents  a  flask  one-third  filled  with  concentrated  sulphuric 
acid;   o  is  a  glass  vessel  (commonly  called  a  LIEBIG'S  drying-tube), 


62  OPERATIONS.  [§  29. 

and  d  a  tin  vessel  (the  aspirator)  provided  with  a  stop-cock  at  e, 
and  arranged  in  other  respects  as  the  cut  shows.  Fig.  35  repre- 
sents a  small  tin  vessel,  containing  water  and  covered  with  a  lid ; 
two  apertures  are  cut  into  the  border  of  the  latter,  to  receive 
the  ascending  limbs  of  c. 

The  tube  c  is  first  weighed  with  the  substance,  then  placed  in 
the  water-bath  (Fig.  35),  which  is  placed  over  an  alcohol-  or  gas- 
lamp;  the  aspirator  d  is  then  filled  with  water,  and  c  connected 
with  the  flask  a  by  the  perforated  cork  g,  and  with  d  by  means  of 
a  caoutchouc  tube  f.  If  the  stop-cock  e  be  now  opened  so  as  to 
allow  the  water  to  drop  from  d,  the  air  will  pass  through  the  tube 
£,  and  after  being  dehydrated  by  the  sulphuric  acid,  will  pass  over 
the  heated  substance  in  c.  After  the  operation  has  been  continued 
for  some  time,  it  is  interrupted  for  the  purpose  of  weighing  the 
tube  c  and  its  contents,  and  then  resumed  again,  and  continued 
until  the  weight  of  c  (and  its  contents)  remains  stationary.  The 
current  of  cold  air  exercising  its  constant  cooling  action  upon  the 
substance,  the  latter  never  really  reaches  100°.  It  is,  therefore, 
sometimes  advisable  to  substitute  for  the  water  in  the  bath  a  satu- 
rated solution  of  common  salt. 

"With  this  substitution,  the  apparatus  will  be  found  to  effect  its 
purpose  most  expeditiously.  It  is  not  adapted,  however,  for  dry- 
ing such  substances  as  have  a  tendency  to  fuse  or  agglutinate  at 
100°.  It  is,  moreover,  less  adapted  for  determining  the  moisture 
in  substances  than  for  simply  drying  them,  because  the  glass  is  some- 
what attacked  by  the  prolonged  action  of  the  boiling  water,  hence 
causing  a  slight  loss  in  weight  of  the  drying-tube  in  the  course  of 
the  operation.  This  loss,  too,  varies  with  different  kinds  of  glass. 


§29. 

e.  Substances  which  persistently  retain  moisture  at  100°,  or 
become  completely  dry  only  after  a  very  long  time,  but  which  are 
decomposed  by  a  red  heat. 

The  desiccation  of  such  substances  is  affected  by  means  of  air-, 
oil-,  paraffin-,  or  mercury-baths,  or  on  drying-disks  (Fig.  42),  at  a 
temperature  of  100°-120°  or  even  still  higher;  and,  according  to 
circumstances,  with  or  without  the  aid  of  a  current  of  air,  some- 
times in  a  partial  vacuum,  and  sometimes  in  diluted  carbon  dioxide. 


§29.] 


DESICCATION. 


Figs.  36  and  37  represent  two  air-baths  of  simple  construction. 
The  latter  is  adapted  for  the  desiccation  of  a  single  substance ;  the 
former  is  suited  for  the  simultaneous  drying  of  several  substances. 

In  Fig.  36,  a  5%is  a  case  of  stout  sheet  copper  soldered  with 
brass,  15  to  20  cm.  wide  and  deep,  and  of  suitable  height.  In  the 


Fig.  36. 


Fig.  37. 


aperture  c  there  is  fixed  a  cork  carrying  a  thermometer,  d,  which 
extends  into  the  interior  of  the  case,  e  is  a  wire  stand  on  which  the 
watch-glasses  containing  the  substances  to  be  dried  are  placed. 
The  case  is  heated  by  means  of  a  gas-,  alcohol-,  or  oil-lamp.  When 
the  temperature  has  reached  the  point  desired,  it  is  easily  main- 
tained at  this  point  by  regulating  the  flame.*  In  order  to  limit  as 
much  as  possible  the  cooling  from  without,  it  is  advisable  to  cover 
the  apparatus  with  a  pasteboard  hood  having  a  movable  front. 

In  Fig  37,  A  is  a  box  of  strong  sheet  copper,  about  11  cm. 

*  When  using  gas,  BUNSEN'S  improved  KEMP  regulator  (made  by  DESAGA,  of 
Heidelberg)  may  be  advantageously  employed  in  order  to  obtain  constant  tem- 
peratures. A  modification  has  been  recommended  by  TH.  SCHORER  (Zeitschr.f. 
analyt.  Chem.,  ix,  213).  SCHEIBLER'S  regulator  (ibid.,  vn,  88)  is  more  certain  in 
action,  even  also  under  sudden  changes  in  gas  pressure,  but,  as  its  action  depends* 
on  an  electromagnet,  its  construction  is  more  complicated. 


64 


OPEKATIONS. 


[§29. 


high  and  9  cm.  in  diameter.  Tlie  box  is  closed  with  the  loosely- 
fitting  cover  B,  which  is  provided  with  a  narrow  rim,  and  has  two 
apertures,  0  and  E  ;  C  is  intended  to  receive  the  thermometer  Z>, 
which  is  fitted  into  it  by  a  perforated  cork ;  .#  affords  an  exit  to  the 

aqueous  vapors,  and  is,  according 
to  circumstances,  either  left  open 
or  loosely  closed.  Within  the 
box,  about  half-way  up,  are  fixed 
three  pins,  for  the  support  of  a 
triangle  of  moderately  stout  wire, 
upon  which  the  crucible  with  the 
substance  is  placed.  The  ther- 
mometer bulb  should  be  as  near 
the  crucible  as  possible,  but  must 
not  touch  the  triangle.  Heating 
is  effected  by  means  of  a  gas-  or 
alcohol-lamp.  When  the  appa- 
ratus has  cooled  to  the  extent  that 
it  may  be  conveniently  grasped, 
the  cover  is  removed,  and  the  still 
warm  crucible  taken  out,  covered, 
and  allowed  to  become  cold  in  an 
exsiccator,  when  it  is  weighed. 

The  air-bath  shown  in  Fig.  38 
serves  for  drying  substances  in  a 
bulb-tube  with  the  simultaneous 
employment  of  a  current  of  dry 
air.  The  apparatus  consists  of  a 
sheet-iron  box  having  the  follow- 
ing dimensions  in  cm.  :  a  ~b  =  20 ; 
ac  =  13;  ad  =  12;  ef=  11; 
e  g  —  6.  The  diameter  of  the 
aperture  on  each  side  is  16  mm. 
The  thermometer  is  thrust  so  far 

down  until  its  bulb  is  on  a  level  with  and  touches  the  side  of  the 
bulb-tube.  To  this  end  the  opening  h  is  not  placed  exactly  in  the 
middle  line,  but  1  cm.  behind  it.  In  this  apparatus  a  temperature 
of  200°  to  260°  may  be  easily  attained.  To  produce  the  current  of 
dry  air,  one  of  the  projecting  ends  of  the  bulb-tube  is  connected 


§29.] 


DESICCATION. 


with  a  hydraulic  air-pump  (§  47)  or  an  aspirator,  as  in  Fig.  34 ; 
the  other  end  is  connected  with  a  calcium -chloride  tube  or  a  flask 
containing  concentrated  sulphuric  acid  (Fig.  34,  a) ;  the  current 
should  be  somewhat  rapid  at  first,  slower  afterwards.  If  the  tube 
with  the  dried  substance  is  to  be  weighed,  it  must  be  allowed  to 
cool,  with  a  current  of  dry  air  still  passing  through  it. 

In  the  air-bath  shown  in  Fig.  39,  the  drying  is  promoted  by 
alternate  exhaustion  and  readmission  of  air.     a  is  a  vessel  of  stout 


Fig.  39. 

sheet  copper,  provided  with  two  apertures,  and  soldered  with  brass ; 
b  is  a  glass  tube  in  which  the  substance  is  dried ;  c  is  a  thermom- 
eter; d  is  a  calcium-chloride  tube;  e  is  a  small  hand  air-pump, 
which  may,  of  course,  be  replaced  by  a  hydraulic  or  mercurial 
air-pump. 

In  use  a  is  heated  to  the  desired  degree ;  then  5  and  d  are  ex- 
hausted. After  a  few  minutes  the  stop- cock  f  is  opened  and  air  is 
allowed  to  re-enter,  first  becoming  perfectly  dry  by  passing  over 
the  calcium  chloride.  The  exhaustion  and  readmission  of  air  are 
then  repeated  until  not  the  slightest  trace  of  moisture  is  visible  in 
the  tube  g  when  the  latter  is  cooled  by  surrounding  it  with  cotton 
saturated  with  ether. 


66 


OPERATIONS. 


[§30. 


§  30. 

As  an  oil-bath,  the  copper  drying  closet  figured  in  Fig.  31  is 
employed  as  a  rule,  being  then  filled  two-thirds  with  refined  rape- 
oil.  The  temperature  is  ascertained  by  means  of  a  thermometer 
borne  by  a  perforated  cork  inserted  in  the  aperture  a.  The  ther- 
mometer bulb  must  reach  nearly  to  the  bottom,  or  must  at  least  be 
entirely  immersed  in  the  oil.  As  the  oil  emits  a  disagreeable  and 
most  annoying  odor  when  heated,  it  is  preferable  to  use  paraffin 
instead.  The  air-bath  shown  in  Fig.  39  may  also  serve  as  an  oil- 
bath.  If  it  is  intended  to  weigh  the  substance,  after  drying,  in 
the  tube,  a  shorter  tube  should  be  selected  which  may  be  readily 
inserted  into  the  tube  standing  in  the  oil. 

Some  organic  substances,  when  dried  at  high  temperatures, 
suffer  alteration  from  the  action  on  them  of  atmospheric  oxygen 
(see  FR.  ROCHLEDER,  Jour,  fur  prakt.  Chemie,  LXVI,  20'8). 
When  drying  such  substances,  hence,  contact  with  oxygen  must  be 
avoided. 

Figs.  40  and  41  show  apparatus  devised  by  ROCHLEDER  for  this 
purpose.  The  former  may  also  be  advantageously  employed  for 
drying  with  an  air- current ;  in  the  latter,  the  drying  is  effected  in 
a  rarefied  gas.  J?,  Fig.  40,  is  a  sheet-copper 
"*  cylinder  18  cm.  high  and  9  cm.  in  diameter, 
containing  a  suitable  quantity  of  oil  or  paraffin 
in  which  is  suspended  and  suitably  fixed  an 
iron  or  glass  vessel,  A,  containing  mercury. 
In  the  mercury  there  dip  a  thermometer,  and 
the  glass  tube,  (7,  containing  the  substance 
to  be  dried.  The  dried  gas  (hydrogen,  car- 
bonic acid,  air,  etc.)  enters  at  5,  and  escapes 
at  a  —  if  necessary,  through  a  weighed  cal- 
cium-chloride tube.  To  prevent  any  possible 
air-currents  acting  on  the  substance,  the  end 
of  &  is  bent  upwards.  The  advantage  of  hav- 
ing mercury  in  a  is  that,  on  removing  the 
tube  (7,  it  is  perfectly  clean. 
In  Fig.  41  the  cock  If  is  screwed  on  to  the  air-pump  at  a ; 
b  is  connected  by  means  of  rubber  tubing  with  a  rubber  bag  or 
bladder  filled  with  carbon  dioxide.  B  is  an  oil-bath,  the  tern- 


Fig.  40. 


31.] 


DESICCATION. 


67 


perature  of  which  is  ascertained  by  a  thermometer.     In  the  oil- 
bath  is  suspended  a  wide-mouthed,  stout  glass  vessel,  $,  in  which 


Fig.  41. 


is  placed  the  substance  to  be  dried,  and  contained  in  a  glass  tube, 
as  wide  as  practicable,  and  sealed  at  one  end.  On  pumping,  while 
the  cock  II  is  open  and  the  cock  Hr  is  closed,  the  air  in  S  is  rare- 
fied; on  now  closing  JTand  opening  II' ,  the  vessel  becomes  filled 
with  carbon  dioxide,  previously  dried  by  passing  through  the 
calcium-chloride  tube  C'.  By  repeating  this  procedure  the  appa- 
ratus is  entirely  filled  with  dried  carbon  dioxide.  H'  is  then 
closed,  and  the  pump  operated.  The  oil-bath  is  then  heated  to  the 
desired  temperature,  carbon  dioxide  being  admitted  from  time  to 
time  by  opening  II' .  On  closing  H'  and  pumping  again,  the 
moisture  taken  up  by  the  carbon  dioxide  is  removed  with  the  lat- 
ter, and  is  retained  in  the  calcium- chloride  tube  C.  Within  half 
an  hour  the  drying  is  complete. 


§31. 

In  technical  and  agricultural  chemical  investigations,  in  which 
a  number  of  specimens  are  to  be  simultaneously  dried  at  a  high 
temperature,  the  drying-disk  illustrated  in  Fig.  42  and  devised 
by  me  is  recommended. 

The  apparatus  consists  of  a  turned  cast-iron  plate  21  cm.  in 
diameter  and  37  mm.  thick,  supported  by  a  tripod,  and  weighing 
about  8  kilos.  This  weight  enables  the  plate  to  be  uniformly 
heated,  and  permits  the  desired  temperature  to  be  readily  main- 
tained. At  equal  distances  from  the  centre  of  the  plate  six  smooth, 


68 


OPERATIONS. 


cylindrical  cavities  are  turned.     Each  cavity  is  fitted  with  a  turned 

brass  pan  55  mm.  in  diameter  and 
18  mm.  deep,  and  fitting  rather 
loosely,  so  as  to  be  readily  removable 
after  heating.  Every  pan  is  provided 
with  a  small  handle  pointing  toward 
the  periphery  of  the  plate,  and  resting 
in  appropriate  grooves  made  in  the 
latter.  Each  handle,  moreover,  bears 
stamped  upon  it  a  number,  from  1 
to  6,  corresponding  to  a  similar  num- 
ber stamped  in  the  plate  behind  the 
cavities — so  that  each  pan  has  its  own 
proper  cavity.  The  centre  of  each 
pan  is  6.5  cm.  distant  from  the  centre 
of  the  plate ;  and  the  rims  of  the 
pans  are  level  with  the  surface  of  the 
plate.  Five  of  the  pans  are  intended 
for  the  samples  (ores,  parts  of  plants, 
etc.),  and  the  sixth  for  the  thermom- 
eter. The  sixth  cavity  bears  fitted 
into  it  a  brass  rim  extending  3  cm- 
above  the  surface  of  the  plate;  the 
pan  so  heightened  is  filled  with  brass 
or  copper  filings,  and  the  thermom- 
eter bulb  is  embedded  in  these  down 
to  the  bottom.  Heat  is  applied  under 
the  centre  of  the  plate. 

f.  Substances  which  suffer  no  alteration  at  a  red  heat,  such  as 
barium  sulphate,  pearlash,  etc.,  are  very  readily  freed  from  mois- 
ture. They  need  simply  be  heated  in  a  platinum  or  porcelain 
crucible  over  a  gas-  or  spirit-lamp  until  the  desired  end  is  attained. 
The  crucible,  having  first  been  allowed  to  cool  a  little,  is  put,  still 
hot,  under  a  desiccator,  and  finally  weighed  when  cold. 


§  32.]  DESICCATION.  69 

III.  GENERAL  PROCEDURE  IN  QUANTITATIVE  ANALYSES. 

§32. 

It  is  important,  in  the  first  place,  to  observe  that  we  embrace 
in  the  following  general  analytical  method  only  the  separation  and 
determination  of  the  metals  and  their  combinations  with  the 
metalloids,  and  of  the  inorganic  acids  and  salts.  With  respect  to 
the  quantitative  analysis  of  other  compounds,  it  is  not  easy  to  lay 
down  a  universally  applicable  method,  except  that  their  constitu- 
ents usually  require  to  be  converted  first  into  acids  or  bases,  before 
their  separation  and  estimation  can  be  attempted ;  this  is  the  case, 
for  instance,  with  phosphorus  sulphide,  sulphur  chloride,  iodine 
chloride,  nitrogen  sulphide,  &c. 

The  quantitative  analysis  of  a  substance  presupposes  an 
accurate  knowledge  of  its  properties,  and  of  the  nature  of  its 
several  constituents.  These  data  will  enable  the  operator  at  once 
to  decide  whether  the  direct  estimation  of  each  individual  constitu- 
ent is  necessary ;  whether  he  need  operate  only  on  one  portion 
of  the  substance,  or  whether  it  would  be  advantageous  to  deter- 
mine each  constituent  in  different  portions.  -  Let  us  suppose,  for 
instance,  we  have  a  mixture  of  sodium  chloride  and  anhydrous 
sodium  sulphate,  and  wish  to  ascertain  the  proportion  in  which 
these  two  substances  are  mixed.  Here  it  would  be  superfluous  to 
determine  each  constituent  directly,  since  the  determination  either 
of  the  quantity  of  the  chlorine,  or  of  the  sulphuric  acid,  is  quite 
sufficient  to  answer  the  purpose;  still  the  estimation  of  both  the 
chlorine  and  the  sulphur  trioxide  will  afford  us  an  infallible  con- 
trol for  the  correctness  of  our  analysis ;  since  the  united  weights 
of  these  two  substances,  added  to  the  sodium  and  soda  respectively 
equivalent  to  them,  must  be  equal  to  the  weight  of  the  substance 
taken. 

These  estimations  may  be  made,  either  in  one  and  the  same 
portion  of  the  mixture,  by  first  precipitating  the  sulphuric  acid 
with  barium  nitrate,  and  subsequently  the  hydrochloric  acid  from 
the  filtrate  with  solution  of  silver  nitrate  ;  or  a  separate  portion  of 
the  mixture  may  be  appropriated  to  each  of  these  two  operations. 
Unless  there  is  some  objection  to  its  use  (e.g.,  deficiency  or  hetero- 
geneousness  of  substance),  the  latter  method  is  more  convenient, 


70  OPERATIONS.  [§  33. 

and  generally  yields  more  accurate  results;  since,  in  the  former 
method,  the  unavoidable  washing  of  the  first  precipitate  swells  the 
amount  of  liquid  so  considerably  that  the  analysis  is  thereby 
delayed,  and,  moreover,  loss  of  substance  less  easily  guarded 
against. 

Before  beginning  all  analyses,  at  least  those  of  a  more  complex 
nature,  the  student  should  write  out  an  exact  plan,  and  accurately 
note  on  paper,  during  the  entire  process,  everything  that  lie  does. 
It  is  in  the  highest  degree  unwise  to  rely  on  the  memory  in  a  com- 
plicated analysis.  When  students,  wTho  imagine  they  can  do  so, 
come,  a  week  or  a  fortnight  after  they  have  begun  their  analysis, 
to  work  out  the  results,  they  find  generally  too  late  that  they  have 
forgotten  much,  which  now  appears  to  them  of  importance  to, 
know.  The  intelligent  pursuit  of  chemical  analysis  consists  in  the 
projecting  and  accurate  testing  of  the  plan  ;  acuteness  and  the 
power  of  passing  in  review  all  the  influencing  chemical  relations 
must  here  support  each  other.  He  who  works  without  a  thor- 
oughly thought-out  plan,  has  no  right  to  say  he  is  practising  chem- 
istry ;  for  a  mere  unthinking  stringing  together  of  a  series  of  filtra- 
tions,  evaporations,  ignitions,  and  weighings,  howsoever  well  these 
several  operations  may  be  performed,  is  not  chemistry. 

We  will  now  proceed  to  describe  the  various  operations  consti- 
tuting the  process  of  quantitative  analysis. 

§  33. 
1.  WEIGHING  THE  SUBSTANCE. 

The  amount  of  matter  required  for  the  quantitative  analysis  of  a 
substance  depends  upon  the  nature  of  its  constituents ;  it  is,  there- 
fore, impossible  to  lay  down  rules  for  guidance  on  this  point. 
Half  a  gramme  of  sodium  chloride,  and  even  less,  is  sufficient  to 
effect  the  estimation  of  the  chlorine.  For  the  quantitative  analy- 
sis of  a  mixture  of  common  salt  and  anhydrous  sodium  sulphate,  1 
gramme  will  suffice  ;  whereas,  in  the  case  of  ashes  of  plants,  com- 
plex minerals,  &c.,  3  or  4  grammes,  and  even  more,  are  required. 
1  to  3  grm.  can  therefore  be  indicated  as  the  average  quantity 
suitable  in  most  cases.  For  the  estimation  of  constituents  present 
in  very  minute  proportions  only,  as,  for  instance,  sodium  and 
potassium  in  limestones,  phosphorus  or  sulphur  in  cast-iron,  &c., 
much  greater  quantities  are  often  required — 10,  20,  or  50  grammes. 


§  33.]  WEIGHING   THE    SUBSTANCE.  71 

The  greater  the  amount  of  substance  taken  the  more  accurate 
will  be  the  analysis  ;  the  smaller  the  quantity,  the  sooner,  as  a  rule, 
will  the  analysis  be  finished.  We  would  advise  the  student  to 
endeavor  to  combine  accuracy  with  economy  of  time.  The  less 
substance  lie  takes  to  operate  upon,  the  more  carefully  he  ought  to 
weigh ;  the  larger  the  amount  of  substance,  the  less  harm  can 
result  from  slight  inaccuracies  in  weighing.  Somewhat  large 
quantities  of  substance  are  generally  weighed  to  1  milligramme  ; 
minute  quantities,  to  Ol  milligramme. 

If  one  portion  of  a  substance  is  to  be  weighed  off,  w^e  first 
weigh  twTo  watch-glasses  which  fit  on  each  other,  or  else  an  empty 
platinum  crucible  with  lid,  then  we  put  some  substance  in,  and 
weigh  again ;  the  difference  between  the  two  weighings  gives  the 
weight  of  the  substance  taken. 

This  mode  of  weighing  off,  however,  is  advisable  only  when 
the  substance  is  to  be  further  treated  in  the  watch-glasses  or 
crucible,  or  when  the  substance  is  not  of  an  adherent  nature,  or 
when  the  adherent  particles  may  be  washed  away  with 
water.  If  the  substance  is  to  be  transferred  to  a  flask 
or  beaker,  and  treated  with  a  concentrated  solvent,  the 
weighing  is  best  done  in  a  small  tube  sealed  at  one  end. 
In  this  case  the  approximate  weight  of  the  tube  should 
be  known.  After  receiving  the  substance,  the  tube  and 
its  contents  are  carefully  weighed ;  then  nearly  the 
whole,  or  a  suitable  quantity,  of  the  substance  is  shaken 
out  into  the  flask  or  beaker,  the  weight  again  taken,  and 
the  difference  in  weight,  showing  the  quantity  of  sub- 
stance taken,  noted.  When  the  substances  handled  are 
hygroscopic,  the  tube  must  be  closed ;  this  is  easily  ac-  •^1^*  ***' 
complished  by  inserting  the  tube  into  one  slightly  larger,  as  shown 
in  Fig.  43. 

If  several  quantities  of  a  substance  are  to  be  operated  upon, 
the  best  way  is  to  weigh  off  the  several  portions •  successively ; 
which  may  be  accomplished  most  readily  by  weighing  in  a  glass 
tube,  or  other  appropriate  vessel,  the  whole  amount  of  substance, 
and  then  shaking  out  of  the  tube  the  quantities  required  one 
after  another  into  appropriate  vessels,  weighing  the  tube  after  each 
time. 

The  wrork  may  often  also  be  materially  lightened,  by  weighing 
off  a  larger  portion  of  the  substance,  dissolving  this  to  £,  i  or  1 


72  OPERATIONS.  [§§  34,  35, 

litre,  and  taking  out  for  the  several  estimations  aliquot  parts,  with 
the  50-  or  100-c.  c.  pipette.  The  first  and  most  essential  condition 
of  this  proceeding,  of  course,  is  that  the  pipettes  must  accurately 
correspond  with  the  measuring  flasks  (§§18  and  20). 

§34. 
2.   ESTIMATION  OF  THE  WATER. 

If  the  substance  to  be  examined — after  having  been  freed  from 
moisture  by  a  suitable  drying  process  (§§  26-32) — contains  water, 
it  is  usual  to  begin  by  determining  the  amount  of  this  Water.  Thi& 
operation  is  generally  simple  ;  in  some  instances,  however,  it  has 
its  difficulties.  This  depends  upon  various  circumstances,  viz., 
whether  the  compounds  intended  for  analysis  yield  their  water 
readily  or  not ;  whether  they  can  bear  a  red  heat  without  suffering 
decomposition ;  or  whether,  on  the  contrary,  they  give  off  other 
volatile  substances,  besides  water,  even  at  a  lower  temperature. 

The  correct  knowledge  of  the  constitution  of  a  compound 
depends  frequently  upon  the  accurate  estimation  of  the  water  con- 
tained in  it ;  in  many  cases — for  instance,  in  the  analysis  of  the 
salts  of  known  acids — the  estimation  of  the  water  contained  in  the 
analyzed  compound  suffices  to  enable  us  to  deduce  the  formula. 
The  estimation  of  the  water  contained  in  a  substance  is,  therefore, 
one  of  the  most  important,  as  well  as  most  frequently  occurring 
operations  of  quantitative  analysis.  The  proportion  of  water  con- 
tained in  a  substance  may  be  determined  in  two  ways,  viz.,  «,  from 
the  diminution  of  weight  consequent  upon  the  expulsion  of  the 
water ;  J,  by  weighing  the  amount  of  water  expelled. 

§  35. 
a.  ESTIMATION  OF  THE  WATER  FROM  THE  Loss  OF  WEIGHT. 

This  method,  on  account  of  its  simplicity,  is  most  frequently 
employed.  The  modus  operandi  depends  upon  the  nature  of  the 
substance  under  examination. 

a.  The  substance  hears  ignition  without  losing  other  Constituents 
hesides  Water,  and  without  absorhing  Oxygen. 

The  substance  is  weighed  in  a  platinum  or  porcelain  crucible, 
and  placed  over  the  gas-  or  spirit-lamp  ;  the  heat  should  be  very 


$  35.]  ESTIMATION    OF    WATER.  73 

gentle  at  first,  and  gradually  increased.  When  the  crucible  has 
been  maintained  some  time  at  a  red  heat,  it  is  allowed  to  cool  a 
little,  put  still  warm  under  the  desiccator,  and  finally  weighed  when 
cold.  The  ignition  is  then  repeated,  and  the  weight  again  ascer- 
tained. If  no  further  diminution  of  weight  has  taken  place,  the 
process  is  at  at  end,  the  desired  object  being  fully  attained.  But 
if  the  weight  is  less  than  after  the  first  heating,  the  operation  must 
be  repeated  until  the  weight  remains  constant. 

In  the  case  of  silicates,  the  heat  must  be  raised  to  a  very  high 
degree,  since  many  of  them  (e.g.,  talc,  steatite,  nephrite)  only  begin 
at  a  red  heat  to  give  off  water,  and  require  a  yellow  heat  for  the 
complete  expulsion  of  that  constituent.  (Tn.  SCHEERER.*)  Such 
bodies  are  therefore  ignited  over  a  blast-lamp.  The  flame  should 
be  observed ;  if  it  be  colored,  it  indicates  some  volatilization  of 
alkali. 

hi  the  case  of  substances  that  have  a  tendency  to  puff  off,  or  to 
spirt,  a  small  flask  or  retort  may  sometimes  be  advantageously  sub- 
stituted for  the  crucible.  Care  must  be  taken  to  remove  the  last 
traces  of  aqueous  vapor  from  the  vessel,  by  suction  through  a  glass 
tube. 

Decrepitating  salts  (sodium  chloride,  for  instance)  are  put — 
finely  pulverized,  if  possible — in  a  small  covered  platinum  crucible,. 
whi;'h  is  then  placed  in  a  large  one,  also  covered ;  the  whole  is 
weighed,  then  heated,  gently  at  first  for  some  time,  then  more 
strongly  ;  finally,  after  cooling,  weighed  again. 

ft.   The    substance   loses  on    ignition    other    Constituents   besides 
Water  (Boric  Acid,  Sulphuric  Acid,  Silicon  Fluoride,  t£v?.). 

Here  the  analyst  has  to  consider,  in  the  first  place,  whether  the 
water  may  not  be  expelled  at  a  lower  degree  of  heat,  which  does 
not  involve  the  loss  of  other  constituents.  If  this  may  be  done, 
the  substance  is  heated  either  in  the  water-bath,  or  where  a  higher 
temperature  is  required,  in  the  air-bath  or  oil-bath,  the  tempera- 
ture being  regulated  by  the  thermometer.  The  expulsion  of  the 
water  may  be  promoted  by  the  co-operation  of  a  current  of  air 
(compare  §§  29  and  J30)  ;  or  by  the  addition  of  pure  dry  sand  to 
the  substance,  to  keep  it  porous,  f  The  process  must  be  continued 
under  these  circumstances  also,  until  the  weight  remains  constant. 

*  Jahresber.  von  LIEBIG  u.  KOPP,  1851,  610. 
f  Ann.  d.  Chem.  u.  Pkarm.,  LIII,  233. 


74  OPERATIONS.  [§  35. 

In  cases  where,  for  some  reason  or  other,  such  gentle  heating 
is  insufficient,  the  analyst  has  to  consider  whether  the  desired  end 
may  not  be  attained  at  a  red  heat,  by  adding  some  substance  that 
will  retain  the  volatile  constituent  whose  loss  is  apprehended. 
Thus,  for  instance,  the  crystallized  aluminium  sulphate  loses  at  a 
red  heat,  besides  water,  also  sulphuric  acid ;  now,  the  loss  of  the 
latter  constituent  may  be  guarded  against  by  adding  to  the  sul- 
phate an  excess  (about  six  times  the  quantity)  of  finely  pulverized, 
recently  ignited,  pure  lead  oxide.  But  the  addition  of  this  sub- 
stance will  not  prevent  the  escape  of  silicon  fluoride  from  silicates 
when  exposed  to  a  red  heat  (LIST  *). 

Thus,  again,  the  amount  of  water  in  commercial  iodine  may 
be  determined  by  triturating  the  iodine  together  with  eight  times 
the  quantity  of  mercury,  and  drying  the  mixture  at  100°  (BoL- 
LEY,  DINGLER'S  Polytech.  Journ.,  cxxvi,  39). 

For  the  determination  of  water  in  silicofluorides  magnesia  is 
added  to  the  substance.  For  this  purpose  about  twice  as  much 
magnesia  as  is  required  for  decomposing  the  silicofluoride  is  ig- 
nited in  a  platinum  crucible,  weighed,  stirred  with  warm  water 
to  a  thick  paste,  the  weighed  silicofluoride  added,  the  whole 
stirred  with  a  platinum  wire  of  known  weight,  more  water  added 
if  necessary  to  effect  solution,  and  the  mixture  then  carefully 
dried  and  ignited.  The  loss  of  weight  represents  the  water  con- 
tained in  the  silicofluoride,  since  the  decomposition  products — 
magnesium  fluoride,  silicic  acid,  and  metallic  oxide — weigh  as 
much  as  the  anhydrous  silicofluoride  plus  the  magnesia.  A  cor- 
rection in  this  case  would  be  necessary  only  when  the  separated 
metallic  oxide,  e.g.,  ferrous  oxide,  takes  up  atmospheric  oxygen 
on  ignition  (F.  STOLBA  f ). 

y.  The  substance  contains  several  differently  combined  quantities 
of  Water  which  require  different  Degrees  of  Temperature 
for  Expulsion. 

Substances  of  this  nature  are  heated  first  in  the  water-bath, 
until  their  weight  remains  constant;  they  are  then  exposed  in  the 
oil-  or  air-bath  to  150°,  200°,  or  250°,  &c.,  and  finally,  when 

*  Ann.  d.  Chem.  u.  Pharm.,  LXXXI,  189. 
\  Zeitschr.f.  analyt.  Chem.,  vn,  93. 


§  36.]  ESTIMATION    OF    WATER.  75 

practicable,  ignited  over  a  gas-  or  spirit-lamp.  In  such  opera- 
tions I  prefer  to  use  the  apparatus  illustrated  in  Fig.  38. 

The  bulb-tube  may  also  be  replaced,  if  desired,  by  a  tube  of 
uniform  width  in  which  is  slid  a  small  porcelain  boat  containing 
the  substance.  In  order  to  prevent  a  desiccated  substance  from 
attracting  water  during  the  weighing,  the  boat  is  weighed  in  a 
cork-stoppered  glass  tube. 

In  this  manner  differently  combined  quantities  of  water  may 
be  distinguished,  and  their  respective  amounts  correctly  esti- 
mated. Thus,  for  instance,  crystallized  copper  sulphate  contains 
28 "87  per  cent,  of  water,  which  escapes  at  a  temperature  below 
140°,  and  7*22  per  cent.,  which  escapes  only  at  a  temperature 
between  220°  and  260°. 

It  is  frequently  advisable  to  assist  the  action  of  heat  by  the 
aid  of  a  partial  vacuum.  Thus  magnesium  sulphate  in  vacua 
over  sulphuric  acid  at  100°  loses  5  equivalents  of  water;  dried 
in  the  air  at  132°  it  loses  6,  and  at  alow  red  heat  7,  equivalents. 

tf.  When  the  substance  has  a  tendency  to  absorb  oxygen  (from 
the  presence  of  ferrous  compounds,  for  instance)  the  water  is  bet- 
ter determined  in  the  direct  way  than  by  the  loss.  (§  36.) 

§36. 
5.  ESTIMATION  .OF  WATER  BY  DIRECT  WEIGHING. 

This  method  is  resorted  to  by  way  of  control,  or  in  the  case  of 
substances  which,  upon  ignition,  lose,  besides  water,  other  con- 
stituents, which  cannot  be  retained  even  by  the  addition  of  some 
other  substance  (e.g.,  carbon  dioxide,  oxygen),  or  in  the  case  of 
substances  containing  bodies  inclined  to  oxidation  (e.g.,  ferrous 
compounds).  The  principle  of  the  method  is  to  expel  the  water 
by  the  application  of  a  red  heat,  so  as  to  admit  of  the  condensa- 
tion of  the  aqueous  vapor,  and  the  collection  of  the  condensed 
water  in  an  appropriate  apparatus,  partly  physically,  partly  by 
the  agency  of  some  hygroscopic  substance.  The  increase  in  the 
weight  of  this  apparatus  represents  the  quantity  of  the  water  ex- 
pelled. 

The  operation  may  be  conducted  in  various  ways ;  the  follow- 
ing apparatus,  Fig.  44-,  is  one  of  the  most  appropriate : — 


76 


OPERATIONS. 


[§36, 


.Z?,  Fig.  44,  represents  a  gasometer  filled  with  air ;  b  a  flask 
half-filled  with  concentrated  sulphuric  acid  ;  c  and  a  o  are  calcium- 
chloride  tubes  (§  66,  7);  d  is  a  bulb-tube  of  highly  infusible  glass. 


Fig.  44. 

The  substance  intended  for  examination  is  weighed  in  the 
perfectly  dry  tube  d,  which  is  then  connected  with  c  and  the 
weighed  calcium  chloride  tube  ao,  by  means  of  sound  and  well- 
dried  perforated  corks. 

The  operation  is  commenced  by  opening  the  stop-cock  of  the 
gasometer  a  little,  to  allow  the  air,  which  loses  all  its  moisture  in  A 
and  <?,  to  pass  slowly  through  d ;  the  tube  d  is  then  neated  to  be- 
yond the  boiling-point  of  water,  by  holding  a  lamp  towards/', 
taking  care  not  to  burn  the  cork ;  and  finally,  the  bulb  which  con- 
tains the  substance  is  exposed  to  a  low  red  heat,  the  temperature 
at /"being  maintained  all  the  while  at  the  point  indicated.  When 
the  expulsion  of  the  water  has  been  accomplished,  a  slow  current 
of  air  is  still  kept  up  till  the  bulb-tube  is  cold ;  the  apparatus  is 
then  disconnected,  and  the  calcium  chloride  tube  ao,  weighed. 
The  increase  in  the  weight  of  this  tube  represents  the  quantity  of 
water  originally  present  in  the  substance  examined. 

The  empty  bulb  $,  in  which  the  greater  portion  of  the  water 
collects,  has  not  only  for  its  object  to  prevent  the  liquefaction  <  f 
the  calcium  chloride,  but  enables  the  analyst  also  to  test  the  COL 
densed  water  as  to  its  reaction  arid  purity. 


§  36.] 


ESTIMATION    OF    WATER. 


77 


TLe  apparatus  may,  of  course,  be  modified  in  various  ways; 
thus,  the  chloride  of  calcium  tubes  may  be  U-shaped ;  a  U-tube, 
filled  with  pieces  of  pumice-stone  saturated  with  sulphuric  acid, 
may  be  substituted  for  the  flask  with  sulphuric  acid;  and  the  gas- 
ometer maybe  replaced  by  an  aspirator  (Fig.  34)  joined  to  o.  The 
calcium-chloride  tube  c  is,  however,  absolutely  indispensable,  be- 
cause I  have  found  *  that  air  dried  by  sulphuric  acid  takes  up  a 
slight  quantity  of  moisture  from  well-dried  calcium  chloride,  i.e.) 
the  sulphuric- acid  dried  air  is  converted  into  a  calcium- chloride 
dried  air.  Were  the  tube  c  omitted,  the  water-content  of  the 
substance  would  hence  be  found  to  be  too  low,  and  by  just  the 
quantity  of  moisture  taken  up  by  the  sulphuric  acid  from  the 
dried  calcium-chloride  tube.  On  interposing  <?,  however,  the  air 
is  dried  by  calcium  chloride  both  before  and  after  passing  over 
the  substance,  hence  the  increase  in  weight  of  ao  will  give  the 
exact  water-content  of  the  substance. 


Fig.  45. 

The  expulsion  of  the  aqueous  vapor  from  the  tube  containing 
the  substance  under  examination,  into  the  calcium  chloride  tube, 
may  be  effected  also  by  other  means  than  a  current  of  air  sup-r 
plied  by  a  gasometer  or  aspirator ;  viz.,  the  substance  under  ex- 
amination may  be  heated  to  redness  in  a  perfectly  dry  tube,  to- 
gether with  lead  carbonate,  since  the  carbon  dioxide  escaping 
from  the  latter  at  a  red  heat,  serves  here  the  same  purpose  as  a 
stream  of  air.  This  method  is  principally  applied  in  cases  where 
it  is  desirable  to  retain  an  acid  which  otherwise  would  volatilize 
together  with  the  -water ;  thus,  it  is  applied,  for  instance,  for 
the  direct  estimation  of  the  water  contained  in  acid  potassium 
sulphate. 


*  Zeitschr.f.  analyt.  Chem.,  \\,  177. 


78  OPERATIONS.  [§  36. 

Fig.  45  represents  the  disposition  of  the  apparatus. 

a  l>  is  a  common  combustion  furnace  ;  cf  a  tube  filled  as  fol- 
lows : — from  c  to  d  with  lead  carbonate, *  from  d  to  e  the  substance 
intimately  mixed  with  lead  carbonate,  and  from  e  to  f  pure  lead  car- 
bonate. The  calcium-chloride  tube  </,  being  accurately  weighed, 
is  connected  with  the  tube  cf,  by  means  of  a  well-dried  perfo- 
rated cork,^. 

The  operation  is  commenced  by  surrounding  the  tube  with  red- 
hot  charcoal,  advancing  from  f  toward  c ;  the  fore  part  of  the 
tube  which  protrudes  from  the  furnace  should  be  maintained  at  a 
degree  of  heat  which  barely  permits  the  operator  to  lay  hold  of  it 
with  his  fingers.  All  further  particulars  of  this  operation  will  be 
found  in  the  chapter  on  organic  elementary  analysis.  The  mix- 
ing is  performed  best  in  the  tube  with  a  wire.  The  tube  cf  may 
be  short  and  moderately  narrow.  Of  course,  the  charcoal  furnace 
may  be  replaced  by  a  gas  combustion  furnace. 

The  volatilization  of  an  acid  cannot  in  all  cases  be  prevented 
by  lead  oxide ;  thus,  for  instance,  we  could  not  determine  the 
water  in  crystallized  boric  acid  by  the  above  process.  This  could 
readily  be  done,  however,  by  igniting  the  acid  mixed  with  excess 
of  dry  sodium  carbonate  in  a  glass  tube  drawn  out  behind  in  the 
form  of  a  beak,  receiving  the  water  in  a  calcium -chloride  tube, 
and  transferring  the  final  residue  of  aqueous  vapor  into  the  Ca  Cla 
tube  by  suction,  after  the  point  of  the  beak  has  been  broken  off. 
(See  Organic  Analysis.) 

The  foregoing  methods  for  the  direct  estimation  of  water  do 
not,  however,  yet  embrace  all  cases  in  which  those  described 
in  §  35  are  inapplicable ;  since  they  can  be  employed  only  if  the 
substances  escaping  along  with  the  water  are  such  as  will  not 
wholly  or  partly  condense  in  the  calcium-chloride  tube  (or  in  a 
tube  containing  fused  potassa,  or  one  filled  with  pumice-stone  satu- 
rated with  sulphuric  acid,  which  might  be  used  instead).  Thus- 
they  are  perfectly  well  adapted  for  determining  the  water  in  the 
basic  zinc  carbonate,  but  they  cannot  be  applied  to  determine  the 
water  in  sodium- ammonium  sulphate.  With  substances  like  the 
latter,  we  must  either  have  recourse  to  the  processes  of  organic 
elementary  analysis,  or  we  must  rest  satisfied  with  the  indirect 
estimation  of  the  water. 

*  The  lead  carbonate  must  have  been  previously  ignited  to  incipient  decom- 
position, and  cooled  in  a  closed  tube. 


§§37,  38.]  SOLUTION.  79 

§37. 
3.  SOLUTION.  OF  SUBSTANCES. 

Before  pursuing  the  analytical  process  further,  it  is  in  most 
cases  .necessary  to  obtain  a  solution  of  the  substance.  This  opera- 
tion  is  simple  where  the  body  may  be  dissolved  by  direct  treat- 
ment with  water,  or  acids,  or  alkalies,  &c. ;  but  it  is  more  compli- 
cated in  cases  where  the  body  requires  fluxing  as  an  indispensable 
preliminary  to  solution. 

When  we  have  mixed  substances  to  operate  upon,  the  compo- 
nent parts  of  which  behave  differently  with  solvents,  it  is  not  by 
any  means  necessary  to  dissolve  the  whole  substance  at  flrst ;  on 
the  contrary,  the  separation  may,  in  such  cases,  be  often  effected, 
in  the  most  simple  and  expeditious  manner,  by  the  solvents  them- 
selves. Thus,  for  instance,  a  mixture  of  potassium  nitrate,  calcium 
carbonate,  and  barium  sulphate  may  be  readily  and  accurately 
analyzed  by  dissolving  out,  in  the  first  place,  the  potassium  nitrate 
with  water,  and  subsequently  the  calcium  carbonate  by  hydrochloric 
acid,  leaving  the  insoluble  barium  sulphate. 

§  38. 
a.  DIRECT  SOLUTION. 

The  direct  solution  of  substances  is  effected,  according  to  cir- 
cumstances, in  beakers,  flasks,  or  dishes,  and  may,  if  necessary,  be 
promoted  by  the  application  of  heat ;  for  which  purpose  the  water- 
bath  will  be  found  most  convenient.  In  cases  where  an  open  fire, 
or  the  sand-bath,  or  an.  iron-plate  is  resorted  to,  the  analyst  must 
take  care  to  guard  against  actual  ebullition  of  the  fluid,  since  this 
would  render  a  loss  of  substance  from  spirting  almost  unavoidable, 
especially  in  cases  where  the  process  is  conducted  in  a  dish.  Fluids 
containing  a  sediment,  either  insoluble,  or,  at  least,  not  yet  dissolved, 
will,  when  heated  over  the  lamp,  often  bump  and  spirt  even  at 
temperatures  far  short  of  the  boiling-point. 

In  cases  where  the  solution  of  a  substance  is  attended  with 
evolution  of  gas,  the  process  is  conducted  in  a  flask,  placed  in  a 
sloping  position,  so  that  the  spirting  drops  may  be  thrown  against 
the  walls  of  the  vessel,  and  thus  secured  from  being  carried  off 
with  the  stream  of  the  evolved  gas  ;  or  it  may  be  conducted  in  a 


80  OPERATIONS.  [§  39. 

beaker,  covered  with,  a  large-sized  watch-glass,  which,  after  the 
solution  is  effected,  and  the  gas  expelled  by  heating  on  the  water- 
bath,  must  be  thoroughly  rinsed  with  the  washing-bottle. 

In  cases  where  the  solution  has  to  be  effected  by  means  of  con- 
centrated volatile  acids  (hydrochloric  acid,  nitric  acid,  aqua  regia), 
the  operation  should  never  be  conducted  in  a  dish,  but  always  in  a 
flask  covered  with  a  watch-glass,  or  placed  in  a  slanting  position, 
and  the  application  of  too  high  a  temperature  must  be  avoided. 
The  operation  should  always  be  conducted  also  under  a  hood,  with 
proper  draught,  to  carry  off  the  escaping  acid  vapors.  All  such 
arrangements  for  carrying  off  vapors  as  require  the  flasks  to  be 
closed  are  not  to  be  recommended ;  at  times  they  are  even  inad- 
missible, because  the  vapors  of  nitric  acid,  nitrohydrochloric  acid, 
etc.,  powerfully  attack  cork  and  caoutchouc,  and  hence  solutions 
may  become  contaminated  with  organic  substances,  sulphuric  acid 
(from  oxidation  of  the  sulphur  contained  in  the  vulcanized  caout- 
chouc), and  other  substances. 

It  is  often  necessary,  in  conducting  a  process  of  solution,  to 
guard  against  the  action  of  the  atmospheric  oxygen ;  in  such  cases, 
a  slow  stream  of  carbon  dioxide  is  transmitted  through  the  solu- 
tion-flask ;  in  some  cases  it  is  sufficient  to  expel  the  air,  by  simply 
first  putting  a  little  hydrogen -sodium  carbonate  into  the  flask,  con- 
taining an  excess  of  acid,  before  introducing  the  substance. 

In  selecting  vessels  in  which  solutions  are  to  be  effected,  some 
care  must  be  exercised  that  the  substance  of  the  vessels  be  as  little 
liable  to  be  attacked  as  possible  by  the  solvent.  As  a  general 
rule,  glass  vessels  are  but  slightly  attacked  by  acids,  but  are 
strongly  acted  on  by  alkalies  (see  §  41). 

§  39. 

-** 

b.   SOLUTION,  PRECEDED  BY  FLUXING. 

Substances  insoluble  in  water,  acids,  x>r  aqueous  alkalies 
usually  require  decomposition  by  fluxing,  to  prepare  them  for 
analysis.  Substances  of  this  kind  are  often  met  with  in  the  min- 
eral kingdom ;  most  silicates,  the  sulphates  of  the  alkali-earth 
metals,  chrome  ironstone,  &c.,  belong  to  this  class. 

The  object  and  general  features  of  the  process  of  fluxing  have 
already  been  treated  of  in  the  qualitative  part  of  the  present  work. 


§§40,41.]  SOLUTION. — EVAPORATION.  81 

The  special  methods  of  conducting  this  important  operation  will 
be  described  hereafter  under  "The  analysis  of  silicates,"  and  in 
the  proper  places;  as  a  satisfactory  description  of  the  process, .with 
its  various  modifications,  cannot  well  be  given  without  entering 
'more  minutely  into  the  particular  circumstances  of  the  several 
special  cases. 

Decomposition  by  fluxing  often  requires  a  higher  temperature 
than  is  attainable  with  a  spirit-lamp  with  double  draught,  or  with 
a  common  gas-lamp.  In  such  cases,  the  glass-blower's  lamp,  fed 
with  gas,  is  used  with  advantage.* 

§40. 
4.  CONVERSION  OF  DISSOLVED  SUBSTANCES  INTO  WEIGHABLE  FORMS. 

The  conversion  of  a  substance  in  a  state  of  solution  into  a  form 
adapted  for  weighing  may  be  effected  either  by  evaporation  or  by 
•precipitation.  The  former  of  these  operations  is  applicable  only 
in  cases  where  the  substance,  the  weight  of  which  we  are  desirous 
to  ascertain,  either  exists  already  in  the  solution  in  the  form  suit- 
able for  the  determination  of  its  weight,  or  may  be  converted  into 
such  form  by  evaporation  in  conjunction  with  some  reagent.  The 
solution  must,  moreover,  contain  the  substance  unmixed,  or,  at 
least,  mixed  only  with  such  bodies  as  are  expelled  by  evaporation 
or  at  a  red  heat.  Thus,  for  instance,  the  amount  of  sodium 
sulphate  present  in  an  aqueous  solution  of  that  substance  may  be 
ascertained  by  simple  evaporation ;  whilst  the  potassium  carbonate 
contained  in  a  solution  would  better  be  converted  into  potassium 
chloride,  by  evaporating  with  solution  of  ammonium  chloride. 

Precipitation  may  always  be  resorted  to,  whenever  the  substance 
in  solution  admits  of  being  converted  into  a  combination  which  is 
insoluble  in  the  menstruum  present,  provided  that  the  precipitate 
is  fit  for  determination,  which  can  never  be  the  case  unless  it  can 
be  washed  and  is  of  constant  composition. 

§41. 
a.  EVAPORATION. 

In  processes  of  evaporation  for  pharmaceutical  or  technico- 
chemical  purposes  the  principal  object  to  be  considered  is  saving 

*  Excellent  lamps  of  this  kind  are  made  by  DESAGA,  of  Heidelberg. 


82 


OPERATIONS. 


$  41. 


of  time  and  fuel;  but  in  evaporating  processes  in  quantitative 
analytical  researches  this  is  merely  a  subordinate  point,  and  the 
analyst  has  to  direct  his  principal  care  and  attention  to  the  means 
of  guarding  against  loss  or  contamination  of  the  substance  operated 
upon. 

The  simplest  case  of  evaporation  is  when  we  have  to  concentrate 
a  clear  fluid,  without  carrying  the  process  to  dryness.  To  effect 
this  object,  the  fluid  is  poured  into  a  basin,  which  should  not  be 
filled  to  more  than  two-thirds.  Heat  is  then  applied  by  placing 
the  basin  either  on  a  water-bath,  sand-bath,  common  stove,  or 
heated  iron  plate,  or  over  the  flame  of  a  gas-  or  spirit-lamp,  care 
being  taken  always  to  guard  against  actual  ebullition,  as- this  in- 
variably and  unavoidably  leads  to  loss  from  small  drops  of  fluid 
spirting  out.  Evaporation  over  a  gas-  or  spirit-lamp,  when  con- 
ducted with  proper  care,  is  an  expeditious  and  cleanly  process, 
BUNSEN'S  gas-lamp,  Fig.  46,  may  be  used  most  advantageously 
in  operations  of  this  kind ;  a  little  wire-gauze  cap,  loosely  fitted 
upon  the  tube  of  the  lamp,  is  a  material  improvement.  By  means 


Fig.  46.  Fig.  47. 

of  this  simple  arrangement  it  is  easy  to  produce  even  the  smallest 
flame,  without  the  least  apprehension  of  the  flame  striking  back. 
The  lamp  recently  introduced  by  the  MASTE  Brothers,  of 
Iserlohn,  and  illustrated  in  Fig.  47,  affords  excellent  service  both 
for  evaporation  and  ignition.  In  it  the  burner  is  very  similar  to 


§41.] 


EVAPORATION. 


83 


Fig.  48. 


that  of  the  BERZELIUS  alcohol  lamp,  and  affords  a  very  small  as  well 
as  a  very  powerful  large  flame,  and  it  has  given  me  excellent 
results  during  long-continued  use.  Five  different  sizes  are  made. 

The  gas  furnace 
shown  in  Fig.  48  is 
also  excellently  adapt- 
ed for  evaporations 
carried  en  in  evapo- 
rating-dishes.  In  this 
furnace  the  mixture 
of  gas  and  air  issues 
from  many  small  ori- 
fices, and  the  construc- 
tion enables  the  flames 
to  be  made  so  small 
that  the  contents  of 
the  dish  can  be  quietly 
evaporated  without 
ebullition.* 

If  the  evaporation  is  to  be  effected  on  the  water-bath,  and  the 
operator  happens  to  possess  a  BEINDOKF,  or  other  similarly  con- 
structed, steam  apparatus,  the  evaporating- 
dish  may  be  placed  simply  into  an  opening 
corresponding  in  size.  Otherwise  recourse 
must  be  had  to  the  water-bath  illustrated  by 
Fig.  49. 

It  is  made  of  strong  sheet  copper,  and 
when  used  is  half  filled  with  water,  which  is  kept  boiling  over  a 
gas-,  spirit-,  or  oil-lamp.  The  breadth  from  a  to  b  should  be  from 
12  to  18  cm.  Various  flat  rings  of  the  same  outside  diameter  as' 
the  top  of  the  bath,  and  adapted  to  receive  dishes  and  crucibles  of 
difierent  sizes,  are  essential  adjuncts  to  the  bath.  These  rings 
when  required  are  simply  laid  on  the  bath. 

It  is  very  inconvenient  to  have  the  water  in  the  bath  com- 
pletely evaporate  unnoticed,  because  frequently  residues  become 
heated  to  a  higher  degree  than  is  desirable,  or  concentrated  solu- 
tions spirt,  etc.  To  avoid  such  inconveniences,  I  make  use  of  a 
water-bath  with  constant  level,  as  shown  in  Fig.  50.  This  ap- 
paratus consists  of  a  zinc  vessel,  abed,  10  cm.  high  and  12  cm. 
*  My  furnaces  are  made  by  KILIAN,  of  Wiesbaden. 


-  49> 


84  OPERATIONS.  [§  41. 

in  diameter,  and  connected  with  the  water-bath,  g,  by  means  of 
the  short  rubber  tube.  0,  and  the  copper  tube,  f.     A  sheet-zinc 


Fig.  50. 

bottle,  h  i  k  Z,  the  cylindrical  part  of  which  is  17  cm.  high  and 
the  neck  3  cm.  in  diameter,  is  filled  with  water,  and,  inverted, 
placed  in  the  vessel,  abed.  The  orifice  of  the  bottle  at  the  neck  is 
15  mm.  wide,  and,  in  the  inverted  position,  is  closed  by  a  valve,  m. 
On  inserting  the  bottle  into  the  vessel,  the  wire  carrying  the 
valve  strikes  the  bottom  of  the  vessel  and  opens  the  valve. 
The  level  of  the  water  in  g  is  readily  regulated  by  raising  or  low- 
ering the  pillar  support,  0,  and  remains  constant  so  long  as  any 
water  remains  in  the  bottle.  The  tube/"  is  bent  downward  in  the 
water-bath  and  reaches  nearly  to  the  bottom. 

A  simple  arrangement  for  extinguishing  the  flame  when  all  the 
water  in  the  water-bath  has  evaporated  has  been  described  by 
K.  REUSS*;  the  construction  of  BUNSEN'S  constant  water-bath 
has  been  detailed  by  W.  H.  WAHL.f 

If  the  operator  can  conduct  his  processes  of  evaporation  in  a 

room  set  apart  for  the  purpose,  where  he  may  easily  guard  against 

any  occurrence  tending  to  suspend  dust  in  the  air,  he  will  find  it 

no  very  difficult  task  to  keep  the  evaporating  fluid  clean ;   in  this 

*  Zeitschr.f.  analyt.  Chem.,  ix,  336.  f  Ibid.,  x,  88. 


§  41.]  EVAPORATION.  85 

case  it  is  best  to  leave  the  dishes  uncovered.*  But  in  a  large 
laboratory,  frequented  by  many  people,  or  in  a  room  exposed  to 
draughts  of  air,  or  in  which  coal  fires  are  burning,  the  greatest 
caution  is  required  to  protect  the  evaporating  fluid  from  contami- 
nation by  dust  or  ashes. 

For  this  purpose  the  evaporating  dish  is  either  covered  with  a 
sheet  of  filtering-paper  turned  down  over  the  edges,  or  a  glass  rod 
twisted  into  a  triangular  shape,  Fig.  51,  is  laid' 
upon  it,  and  a  sheet  of  filtering-paper  spread 
over  it,  which  is  kept  in  position  by  a  glass  rod 
laid  across,  the  latter  again  b.eing  kept  from 
rolling  down  by  the  slightly  turned  up  ends, 
a  and  £>,  of  the  triangle. 

The  best  way,  however,  is  the  following : — Take  two  small 
thin  wooden  hoops,  Fig.  52,  one  of  which  fits  loosely  in  the  other ; 
spread  a  sheet  of  blotting-paper  over  the  smaller  ^^ 
one,  and  push  the  other  over  it.  This  forms  a 
cover  admirably  adapted  to  the  purpose  ;  and 
whilst  in  no  way  interfering  with  the  operation, 
it  completely  protects  the  evaporating  fluid 
from  dust,  and  may  be  readily  taken  off ;  the  paper  cannot  dip 
into  the  fluid  ;  the  cover  lasts  a  long  time,  and  may,  moreover,  at 
any  time  be  easily  renewed. 

It  must  be  borne  in  mind,  however,  that  the  common  filtering- 
paper  contains  always  certain  substances  soluble  in  acids,  such  as 
lime,  ferric  oxide,  &c.,  which,  were  covers  of  the  kind  just 
described  used  over  evaporating  dishes  containing  a  fluid  evolving 
acid  vapors,  would  infallibly  dissolve  in  these  vapors,  and  the  solu- 
tion dripping  down  into  the  evaporating  fluid,  would  speedily  con- 
taminate it.  Care  must  be  taken,  therefore,  in  such  cases,  to  use 
only  such  filtering-paper  as  has  been  freed  by  washing  from  sub- 
stances soluble  in  acids. 

Evaporation  for  the  purpose  of  concentration  may  be  effected 
also  in  flasks ;  these  are  only  half  filled,  and  placed  in  a  slanting 

*  In  my  own  laboratory  separate  closets  are  set  apart  for  evaporations  in 
quantitative  analyses.  It  is  best  to' have  the  floor  and  roof  of  sandstone,  and 
the  walls  of  brick  lined  with  glazed  tiles  or  finished  with  plaster  of  Paris.  At 
the  top  of  the  back  wall  is  a  horizontal  channel  of  suitable  width,  and  leading 
into  a  Russian  chimney.  No  fire  must  be  made  under  this  chimney,  but  it  is 
very  desirable  to  place  this  chimney  close  to  another  chimney  kept  constantly 
warm  (by  the  fire  used  for  the  steam  apparatus,  for  instance).  The  front  wall 
of  the  evaporating  chamber  may  be  of  sandstone  pillars  18  decimeters  high,  be- 
tween which  are  fitted  wooden  frames  wherein  balanced  windows  may  slide  up 
and  down. 


OPERATIONS. 


[§41. 


position.  The  process  may  be  conducted  on  the  sand-bath,  or  over 
a  gas-  or  spirit-lamp,  or  even,  and  with  equal  propriety,  over  a  char- 
coal fire.  In  cases  where  the  operation  is  conducted  over  a  lamp 
or  a  charcoal  fire,  it  is  the  safest  way  to  place  the  flasks  on  wire 
gauze.  Gentle  ebullition  of  the  fluid  can  do  no  harm,  here,  since 
the  slanting  position  of  the  flask  guards  effectively  against  risk  of 
loss  from  the  spirting  of  the  liquid.  Still  better  than  in  flasks,  the 
object  may  be  attained  by  evaporating  in  tubulated  retorts  with 
open  tubulure  and  neck  directed  obliquely  upwards.  The  latter 
acts  as  a  chimney,  and  the  constant  change  of  air  thus  effected  is 
extremely  favorable  to  evaporation. 

The  evaporation  of  fluids  containing  a  precipitate  is  best  con- 
ducted on  the  water-bath  ;  since  on  the  sand-bath,  or  over  the  lamp, 
it  is  next  to  impossible  to  guard  against  loss  from  bumping.  This 


Fig.  53. 

bumping  is  occasioned  by  slight  explosions  of  steam,  arising  from 
the  sediment  impeding  the  uniform  diffusion  of  the  heat.  Still 
there  remains  another,  though  less  safe  way,  viz.,  to  conduct  the 
evaporation  in  a  crucible  placed  in  a  slanting  position,  as  illus- 
trated in  fig.  53.  In  this  process,  the  flame  is  made  to  play  upon 
the  crucible  above  the  level  of  the  fluid. 

Where  a  fluid  has  to  be  evaporated  to  dry  ness,  as  is  so  often 
the  case,  the  operation  should  always,  if  possible,  be  terminated  on 
the  water-bath.  In  cases  where  the  nature  of  the  dissolved  sub- 
stance precludes  the  application  of  the  water-bath,  the  object  in 
view  mav  often  be  most  readily  attained  by  heating  the  contents 


§  41.]  EVAPORATION.  87 

of  the  dish  from  the  top,  which  is  effected  by  placing  the  dish  in  a 
proper  position  in  a  drying  closet,  the  upper  plate  of  which  is  heated 
by  a  flame  (that  of  the  water-  or  sand-bath)  passing  over  it.  If  the 
substance  is  in  a  covered  platinum  dish  or  crucible,  place  the  gas- 
lamp  in  such  a  position  that  the  flame  may  act  on  the  cover  from 
above. 

In  cases  where  the  heat  has  to  be  applied  from  the  bottom,  a 
method  must  be  chosen  which  admits  of  an  equal  diffusion  and 
ready  regulation  of  the  heat. 

An  air-bath  is  well  adapted  for  this  purpose,  i.e..  a  dish  of  iron 
plate,  in  which  the  porcelain  or  platinum  dish  is  to  be  placed  on  a 
wire  triangle,  so  that  the  two  vessels  may  be  at  all  points  J  to  J 
inch  distant  from  each  other.  The  copper  apparatus,  fig.  49  may 
also  serve  as  an  air-bath,  although  I  must  not  omit  to  mention  that 
this  mode  of  application  will  in  the  end  seriously  injure  it.  If  the 
operation  has  to  be  conducted  over  a  lamp,  the  dish  should  be 
placed  high  above  the  flame  ;  best  on  wire  gauze,  since  this  will 
greatly  contribute  to  an  equal  diffusion  of  the  heat.  The  use  of 
the  sand-bath  is  objectionable  here,  because  with  that  apparatus  we 
cannot  reduce  the  heat  so  speedily  as  may  be  desirable.  An  iron 
plate  heated  by  gas  may  perhaps  be  used  with  advantage.  But  no 
matter  which  method  be  employed,  this  rule  applies  equally  to  all  of 
them ;  that  the  operator  must  watch  the  process,  from  the  moment 
that  the  residue  begins  to  thicken,  in  order  to  prevent  spirting,  by 
reducing  the  heat,  and  breaking  the  pellicles  which  form  on  the 
surface,  with  a  glass  rod,  or  a  platinum  wire  or  spatula, 

Saline  solutions  that  have  a  tendency,  upon  their  evaporation,  to 
creep  up  the  sides  of  the  vessel,  and  may  thus  finally  pass  over  the 
brim  of  the  latter,  thereby  involving  the  risk  of  a  loss  of  substance, 
should  be  heated  from  the  top,  in  the  way  just  indicated ;  since  by 
that  means  the  sides  of  the  vessel  will  get  heated  sufficiently  to 
cause  the  instantaneous  evaporation  of  the  ascending  liquid,  pre- 
venting thus  its  overflowing  the  brim.  The  inconvenience  just 
alluded  to  may,  however,  be  obviated  also,  in  most  cases,  by  cover- 
ing  the  brim,  and  the  uppermost  part  of  the  inner  side  of  the  ves- 
sel, with  a  very  thin  coat  of  tallow,  thus  diminishing  the  adhesion 
between  the  fluid  and  the  vessel. 

In  the  case  of  liquids  evolving  gas-bubbles  upon  evaporating^ 
particular  caution  is  required  to  guard  against  loss  from  spirting. 
The  safest  way  is  to  heat  such  liquids  in  an  obliquely-placed 
flask,  or  in  a  beaker  covered  with  a  large  watch-glass ;  the  latter  i? 


88  OPERATIONS.  [§  41. 

removed  as  soon  as  the  evolution  of  gas-bubbles  has  ceased,  and  the 
fluid  that  may  have  spirted  up  against  it  is  carefully  rinsed  into 
the  glass,  by  means  of  a  washing-bottle.  If  the  evaporation  has  to 
be  conducted  in  a  dish,  a  rather  capacious  one  should  be  selected, 
and  a  very  moderate  degree  of  heat  applied  at  first,  and  until  the 
evolution  of  gas  has  nearly  ceased. 

If  a  fluid  has  to  be  evaporated  with  exclusion  of  air,  the  best 
way  is  to  place  the  dish  under  the  bell  of  an  air-pump,  over  a  ves- 
sel with  sulphuric  acid,  and  to  exhaust;  or  a  tubulated  retort  may 
be  used  through  whose  tubulure  hydrogen  or  carbon  dioxide  is 
passed  by  the  acid  of  a  tube  not  quite  reaching  to  the  surface  of 
the  fluid. 

The  material  of  the  evaporating  vessels  may  exercise  a  much 
greater  influence  on  the  results  of  an  analysis  than  is  generally 
believed.  Many  rather  startling  phenomena  that  are  observed  in 
analytical  processes  may  arise  simply  from  a  contamination  of  the 
evaporated  liquid  by  the  material  of  the  vessel ;  great  errors  may 
also  spring  from  the  same  source.* 

The  importance  of  this  point  has  induced  me  to  subject  it  to 
a  searching  investigation  (see  Appendix,  Analytical  Experiments, 
1-4)  ;  more  recently  A.  EMMERLING  has  also  exhaustively  inves- 
tigated the  subject,  and  fully  confirms  the  results,  which  I  will 
here  briefly  intimate. 

Distilled  water  kept  boiling  for  some  length  of  time  in  glass 
(flasks  of  Bohemian  glass)  dissolves  very  appreciable  traces  of  that 
material.  This  is  owing  to  the  formation  of  soluble  silicates ;  the 
particles  dissolved  consist  chiefly  of  potassa,  or  soda  and  lime,  in 
combination  with  silicic  acid.  A  much  larger  proportion  of  the 
glass  is  dissolved  by  water  containing  caustic  or  carbonated  alkali ; 
boiling  solution  of  ammonium  chloride  also  strongly  attacks  glass 
vessels.  Boiling  dilute  acids,  with  the  exception,  of  course,  of 
hydrofluoric  and  hydrofluosilicilic  acids,  exercise  a  less  powerful 
solvent  action  on  glass  than  pure  water.  Porcelain  (Berlin  dishes) 
is  much  less  affected  by  water  than  glass ;  alkaline  liquids  also 
exercise  a  less  powerful  solvent  action  on  porcelain  than  on  glass ; 
the  quantity  dissolved  is,  however,  still  notable.  Solution  of 
ammonium  chloride  acts  on  porcelain  as  strongly  as  on  glass ; 
dilute  acids,  though  exercising  no  very  powerful  solvent  action  on 
porcelain,  yet  attack  that  material  more  strongly  than  glass.  It 
results  from  these  data,  that  in  analyses  pretending  to  a  high 
*  Compare  A.  SOUCHAY,  Zeitschr.f.  analyt.  Chem.,  iv,  66. 


§  42.]  EVAPORATION.  89 

degree  of  accuracy,  platinum  or  platinum- iridium  or  silver  dishes 
should  always  be  preferred.  The  former  may  be  used  in  all  casei 
where  no  free  chlorine,  bromine,  or  iodine  is  present  in  the  fluid, 
or  can  be  formed  during  evaporation.  Fluids  containing  caustic 
alkalies  may  safely  be  evaporated  in  platinum,  but  not  to  the  point 
of  fusion  of  the  residue.  Silver  vessels  should  never  be  used  to 
evaporate  acid  fluids  nor  liquids  containing  alkaline  sulphides ; 
but  they  are  admirably  suited  for  solutions  of  alkali  hydroxides 
and  carbonates,  as  well  as  of  most  normal  salts.  If  the  use  of 
porcelain  or  glass  vessels  for  evaporating  large  volumes  of  fluid 
cannot  be  avoided,  then  porcelain  dishes  are  to  be  preferred  ;  with 
alkaline  fluids  glass  vessels  are  totally  inadmissible,  at  least  in 
accurate  analyses. 

§  42. 

We  come  now  to  weighing  the  residues  remaining  upon  the 
evaporation  of  fluids.  We  allude  here  simply  to  such  as  are 
soluble  in  water;  those  which  are  separated  by  filtration  will  be 
treated  of  afterwards.  Residues  are  generally  weighed  in  the 
same  vessel  in  which  the  evaporation  has  been  completed,  for 
which  purpose  platinum  dishes,  from  4  to  8  cm.  in  diameter,  pro- 
vided with  light  covers,  or  large  platinum  cruci- 
bles, are  best  adapted,  since  they  are  lighter  than 
porcelain  vessels  of  the  same  capacity. 

However,  in  most  cases,  the  amount  of  liquid 
to  be  evaporated  is  too  large  for  so  small  a  vessel, 
and  its  evaporation  in  portions  would  occupy  too 
much  time.     The  best  way,  in  cases  of  this  kind, 
is  to  concentrate  the  liquid  first  in  a  larger  vessel, 
and  to  terminate  the  operation  afterwards  in  the 
smaller  weighing  vessel.     In  transferring  the  fluid  from  the  larger 
to  the  smaller  vessel,  the  lip  of  the  former  is  slightly  greased,  and 
the  liquid  made  to  run  down  a  glass  rod  (Fig.  54). 

Finally  the  large  vessel  is  carefully  rinsed  with  a  washing- 
bottle,  until  a  drop  of  the  last  rinsing  leaves  no  longer  a  residue 
upon  evaporation  on  a  platinum  knife.  When  the  fluid  has  thus 
been  transferred  to  the  weighing-vessel,  the  evaporation  is  com- 
pleted on  the  water-bath  and  the  residuary  substance  finally  ignited, 
provided,  of  course,  it  will  admit  of  this  process.  For  this  pur- 


90  OPERATIONS.  [§  42. 

pose  the  dish  is  covered  with  a  lid  of  thin  platinum  (or  a  thin  glass 
plate),  and  then  placed  high  over  the  flame  of  a  lamp,  and  heated 
very  gently  until  all  the  water  which  may  still  adhere  to  the  sub- 
stance is  expelled ;  the  dish  is  now  exposed  to  a  stronger,  and  finally 
to  a  red  heat.  (Where  a  glass  plate  is  used,  this  must,  of  course,  be 
removed  before  igniting.)  In  this  case  it  is  also  well  to  make  the 
flame  play  obliquely  on  the  cover  from  above,  so  as  to  run  as 
little  risk  as  possible  of  loss  by  spirting.  After  cooling  in  a  desic- 
cator, the  covered  dish  is  weighed  with  its  contents.  When  oper- 
ating upon  substances  which  decrepitate,  such  as  sodium  chloride, 
for  instance,  it  is  advisable  to  expose  them — after  their  removal 
from  the  water-bath,  and  previously  to  the  application  of  a  naked 
flame — to  a  temperature  somewhat  above  100°,  either  in  the  air- 
bath,  or  on  a  sand-bath,  or  on  a  common  stove. 

If  the  residue  does  not  admit  of  ignition,  as  is  the  case,  for 
instance,  with  organic  substances,  ammonium  salts,  &c.,  it  is  dried 
.at  a  temperature  suited  to  its  nature.  In  many  cases,  the  tempera- 
ture of  the  water-bath  is  sufficiently  high  for  this  purpose,  for  the 
drying  of  ammonium  chloride,  for  instance ;  in  others,  the  air  or 
oil-bath  must  be  resorted  to.  (See  §§  29  and  30.)  Under  any  cir- 
cumstances, the  desiccation  must  be  continued  until  the  substance 
ceases  to  suffer  the  slightest  diminution  in  weight,  after  renewed 
exposure  to  heat  for  half  an  hour.  The  dish  should  invariably  be 
covered  during  the  process  of  weighing. 

Since  saline  residues  obtained  on  evaporation  are  frequently 
prone  to  attract  moisture  after  drying  or  ignition,  the  first  weigh- 
ing, which  always  requires  some  time,  may  give  results  which  are 
too  high.  To  avoid  this,  the  dish  is  reheated  after  the  first 
weighing,  then  allowed  to  cool  in  the  exsiccator ;  the  weight  ob- 
tained in  the  first  weighing  is  then  placed  on  one  scale-pan  and 
the  dish  placed  on  the  other,  when  the  second  weighing  is  ac- 
complished with  as  little  loss  of  time  as  possible. 

If,  as  will  frequently  happen,  we  have  to  deal  with  a  fluid  con- 
taining a  small  quantity  of  a  potassium  or  sodium  salt,  the  weight 
of  which  we  want  to  ascertain,  in  presence  of  a  comparatively  large 
amount  of  an  ammonium  salt,  which  has  been  mixed  with  it  in  the 
course  of  the  analytical  process,  I  prefer  the  following  method : 
The  saline  mass  is  thoroughly  dried,  in  a  large  dish,  on  the  water- 
bath,  or,  towards  the  end  of  the  process,  at  a  temperature  some- 
what exceeding  100°.  The  dry  mass  is  then,  with  the  aid  of  a 


§  43.]  EVAPORATION.  91 

platinum  spatula,  transferred  to  a  small  glass  dish,  which  is  put 
aside  for  a  time  in  a  desiccator.  The  last  traces  of  the  salt  left 
adhering  to  the  sides  and  bottom  of  the  large  dish  are  rinsed  off 
with  a  little  water  into  the  small  dish,  or  the  large  crucible,  in 
which  it  is  intended  to  weigh  the  salt ;  the  water  is  then  evaporated, 
and  the  dry  contents  of  the  glass  dish  are  added  to  the  residue : 
the  ammonium  salts  are  now  expelled  by  ignition,  and  the  residu- 
ary fixed  salts  finally  weighed.  Should  some  traces  of  the  saline 
mass  adhere  to  the  smaller  glass  dish,  they  ought  to  be  removed 
and  transferred  to  the  weighing  vessel,  with  the  aid  of  a  little 
pounded  ammonium  chloride,  or  some  other  ammonium  salt,  as  the 
moistening  again  with  water  would  involve  an  almost  certain  loss 
of  substance. 

§43 
1).  PRECIPITATION. 

Precipitation  is  resorted  to  in  quantitative  analysis  far  more 
frequently  than  evaporation,  since  it  serves  not  merely  to  convert 
substances  into  forms  adapted  for  weighing,  but  also,  and  more 
especially,  to  separate  them  from  one  another.  The  principal  in- 
tention in  precipitation,  for  the  purpose  of  quantitative  estimations, 
is  to  convert  the  substance  in  solution  into  a  form  in  which  it  is 
insoluble  in  the  menstruum  present.  The  result  will,  therefore, 
cceteris  paribus,  be  the  more  accurate,  the  more  the  precipitated 
body  deserves  the  epithet  insoluble,  and  in  cases  where  precipi- 
tates are  of  the  same  degree  of  solubility,  that  one  will  suffer  the 
least  loss  which  comes  in  contact  with  the  smallest  amount  of 
solvent. 

Hence  it  follows,  first,  that  in  all  cases  where  other  circum- 
stances do  not  interfere,  it  is  preferable  to  precipitate  su-bstances 
in  their  most  insoluble  form ;  thus,  for  instance,  barium  had  better 
be  precipitated  as  sulphate  than  as  carbonate  ;  secondly,  that  when 
we  have  to  deal  with  precipitates  that  are  not  quite  insoluble  in 
the  menstruum  present,  we  must  endeavor  to  remove  that  men- 
struum, as  far  as  practicable,  by  evaporation  ;  thus  a  dilute  solution 
of  strontium  should  be  concentrated,  before  proceeding  to  precipi- 
tate the  strontium  with  sulphuric  acid ;  and,  thirdly,  that  when  we 
have  to  deal  with  precipitates  slightly  soluble  in  the  liquid  present, 
but  insoluble  in  another  menstruum,  into  which  the  former  may 


92  OPEEATIONS.  [§  43. 

be  converted  by  the  addition  of  some  substance  or  other,  we  ought 
to  endeavor  to  bring  about  this  modification  of  the  menstruum. 
Thus,  for  instance,  alcohol  may  be  added  to  water,  to  induce  com- 
plete precipitation  of  ammonium  platinic  chloride,  lead  chloride, 
calcium  sulphate,  &c.;  thus  again,  ammonium  magnesium  phosphate 
may  be  rendered  insoluble  in  an  aqueous  menstruum  by  adding 
ammonia  to  the  latter,  &c. 

Precipitation  is  generally  effected  in  beakers.  In  cases,  how- 
ever, where  we  have  to  precipitate  from  fluids  in  a  state  of  ebulli- 
tion, or  where  the  precipitate  requires  to  be  kept  boiling  for  some 
time  with  the  fluid,  flasks  or  dishes  are  substituted  for  beakers, 
with  due  regard  always  to  the  material  of  which  they  are  made 
(see  Evaporation,  §  41,  at  the  end). 

The  separation  of  precipitates  from  the  fluid  in  which  they  are 
suspended,  is  effected  either  by  decantation  or  filtration,  or  by 
both  these  processes  jointly.  But,  before  proceeding  to  the  sepa- 
ration of  the  precipitate  by  any  of  these  methods,  the  operator 
must  know  whether  the*  precipitant  has  been  added  in  sufficient 
quantity,  and  whether  the  precipitate  is  completely  formed.  To 
determine  the  latter  point,  an  accurate  knowledge  of  the  properties 
of  the  various  precipitates  must  be  attained,  which  we  shall  en- 
deavor to  supply  in  the  third  section.  To  decide  the  former  ques- 
tion, it  is  usually  sufficient  to  add  to  the  fluid  (after  the  precipitate 
has  settled)  cautiously  a  fresh  portion  of  the  precipitant,  arid 
to  note  if  a  further  turbidity  ensues.  This  test,  however,  is  not 
infallible,  when  the  precipitate  has  not  the  property  of  forming 
immediately ;  as,  for  instance,  is  the  case  with  ammonium  phos- 
pho-molybdate.  When  this  is  apprehended,  pour  out  (or  transfer 
with  a  pipette)  a  small  quantity  of  the  clear  supernatant  fluid  into 
another  vessel,  add  some  of  the  precipitant,  warm  if  necessary ; 
and  after  some  time  look  and  see  whether  a  fresh  precipitate  has 
formed.  As  a  general  rule,  the  precipitated  liquid  should  be 
allowed  to  stand  at  rest  for  several  hours,  before  proceeding  to  the 
separation  of  the  precipitate.  This  rule  applies  more  particularly 
to  crystalline,  pulverulent,  and  gelatinous  precipitates,  whilst  curdy 
and  flocculent  precipitates,  more  particularly  when  the  precipitation 
was  effected  at  a  boiling  temperature,  may  often  be  filtered  off  im- 
mediately. However,  we  must  observe  here,  that  all  general  rules, 
in  this  respect,  are  of  limited  application. 


§  44.]  DECANTATION.  93 

§44. 
a.  SEPARATION  OF  PRECIPITATES  BY  DECANTATION. 

When  a  precipitate  subsides  so  completely  and  speedily  in  a 
fluid  that  the  latter  may  be  decanted  off  perfectly  clear,  or  drawn 
off  with  a  syphon,  or  removed  by  means  of  a  pipette,  and  that 
the  washing  of  the  precipitate  does  not  require  a  very  long  time, 
decantation  is  often  resorted  to  for  its  separation  and  washing  ; 
this  is  the  case,  for  instance,  with  silver  chloride,  metallic  mer- 
cury, &c. 

Decantation  will  always  be  found  a  very  expeditious  and  accu- 
rate method  of  separation,  if  the  process  be  conducted  with  due 
care  ;  it  is  necessary,  however,  in  most  cases,  to  promote  the  speedy 
and  complete  subsidence  of  the  precipitate ;  and  it  may  be  laid  down 
as  a  general  rule,  that  heating  the  precipitate  with  the  fluid  will 
produce  the  desired  effect.  Nevertheless,  there  are  instances  in 
which  the  simple  application  of  heat  will  not  suffice ;  in  some  cases, 
as  with  silver  chloride,  for  instance,  agitation  must  be  resorted  to ; 
in  other  cases,  some  reagent  or  other  is  to  be  added — hydrochloric 
acid,  for  instance,  in  the  precipitation  of  mercury,  &c.  We  shall 
have  occasion,  subsequently,  in  the  fourth  section,  to  discuss  this 
point  more  fully,  when  we  shall  also  mention  the  vessels  best 
adapted  for  the  application  of  this  process  to  the  various  precipitates. 

After  having  washed  the  precipitate  repeatedly  with  fresh 
quantities  of  the  proper  fluid,  until  there  is  no  trace  of  a  dissolved 
substance  to  be  detected  in  the  last  rinsings,  it  is  placed  in  a 
crucible  or  dish,  if  not  already  in  a  vessel  of  that  description ;  the 
fluid  still  adhering  to  it  is  poured  off  as  far  as  practicable,  and  the 
precipitate  is  then,  according  to  its  nature,  either  simply  dried,  or 
heated  to  redness. 

A  far  larger  amount  of  water  being  required  for  washing  pre- 
cipitates by  decantation  than  on  filters,  the  former  process  can  be 
3xpected  to  yield  accurate  results  only  where  the  precipitates  are 
absolutely  insoluble.  For  the  same  reason,  decantation  is  not  ordi- 
narily resorted  to  in  cases  where  we  have  to  determine  other  con^ 
stituents  in  the  decanted  fluid. 

The  decanted  fluid  must  be  allowed  to  stand  at  rest  from 
twelve  to  twenty-four  hours,  to  make  quite  sure  that  it  contains 
no  particles  of  the  precipitate ;  if,  after  the  lapse  of  this  time,  no 


94  OPERATIONS.  [§  45. 

precipitate  is  visible,  the  fluid  may  be  thrown  away ;  but  if  a  pre- 
cipitate has  subsided,  this  had  better  be  estimated  by  itself,  and  the 
weight  added  to  the  main  amount ;  the  precipitate  may,  in  such 
cases,  be  separated  from  the  supernatant  fluid  by  decantation,  or 
by  filtration. 

§45. 
/3.  SEPARATION  OF  PRECIPITATES  BY  FILTRATION. 

This  operation  is  resorted  to  whenever  decantation  is  imprac- 
ticable, and,  consequently,  in  the  great  majority  of  cases  ;  provided 
always  the  precipitate  is  of  a  nature  to  admit  of  its  being  com- 
pletely freed,  by  mere  washing  on  the  filter,  from  all  foreign 
substances.  Where  this  is  not  the  case,  more  particularly,  there- 
fore, with  gelatinous  precipitates,  aluminium  hydroxide  for  in- 
stance, a  combination  of  decantation  and  filtration  is  resorted  to 
(§  48).  Filtration  is  effected  either  with  or  without  exhaustion  of 
the  liquid ;  in  the  latter  case,  however,  it  is  greatly  accelerated. 

§    4:5. 

aa.  ORDINARY  FILTRATION. 
aa.  FILTERING  APPARATUS. 

Filtration,  as  a  process  of  quantitative  analysis,  is  almost 
exclusively  effected  by  means  of  paper. 

Plain  circular  filters  are  most  generally  employed ;  plaited  fil- 
ters are  only  occasionally  used.  Much  depends  upon  the  quality 
of  the  paper.  Good  filtering  paper  must  possess  the  three  follow- 
ing properties: — 1.  It  must  completely  retain  the  finest  precipi- 
tates ;  2.  It  must  filter  rapidly ;  and  3.  It  must  be  as  free  as 
possible  from  any  admixture  of  inorganic  bodies,  but  more  espe- 
cially from  such  as  are  soluble  in  acid  or  alkaline  fluids. 

It  is  a  matter  of  some  difficulty,  however,  to  procure  paper 
fully  answering  these  conditions.  The  Swedish  filtering  paper, 
with  the  water-mark  J.  H.  MUNKTELL,  is  considered  the  best,  and, 
consequently,  fetches  the  highest  price ;  but  even  this  answers  only 
the  first  two  conditions,  being  by  no  means  sufficiently  pure  for 
very  accurate  analyses,  since,  it  leaves  upon  incineration  about  O3> 


45.] 


FILTRATION. 


95 


per  cent,  of  ash,*  and  yields  to  acids  perceptible  traces  of  lime,  mag- 
nesia, and  ferric  oxide.  For  exact  experiments  it  is,  consequently, 
necessary  first  to  extract  the  paper  with  dilute  hydrochloric  acid, 
then  to  wash  the  acid  completely  out  with  water,  and  finally  to 
dry  the  paper.  In  the  case  of  very  fine  filtering  paper,  the  best 
way  to  perform  this  operation  is  to  place  the  ready-cut  filters, 
several  together,  in  a  funnel,  exactly  the  same  way  as  if  intended 
for  immediate  filtration  ;  they  are  then  moistened  with  a  mixture 
of  one  part  of  ordinary  pure  hydrochloric  acid  with  two  parts  of 
water,  which  is  allowed  to  act  on  them  for  about  ten  minutes ; 
after  this  all  traces  of  the  acid  are  carefully  removed  by  washing 
the  filters  in  the  funnel  repeatedly  with  warm  water.  The  funnel 
being  then  covered  with  a  piece  of  paper,  turned  over  the  edges, 
is  put  in  a  warm  place  until  the  filters  are  dry.  Compare  the 
instruction  given  in  the  "Qual.  Anal.,"  Am.  Ed.,  p.  8,  on  the 
preparation  of  washed  filters.  Filter  paper  containing  lead,  and 
which  is  consequently  blackened  by  sulphuretted  hydrogen,  should 
be  rejected,  f 

Iteady-cut  filters  of  various  sizes  should  always  be  kept  on  hand. 
Filters  are  either  cut  by  circular  patterns  of  pasteboard  or  tin,  or, 

still  better,  by  Moire's  filter- 
patterns,  fig.  55.  This  little 
apparatus"  is  made  of  tin-plate, 
and  consists  of  two  parts.  B  is 
a  quadrant  fitting  in  A,  whose 
straight  edges  are  turned  up, 
and  which  is  slightly  smaller 
than  B.  The  sheets  of  filter- 
paper  are  first  cut  up  into  squares,  which  are  folded  in  quarters, 
and  placed  in  A,  then  B  is  placed  on  the  top,  and  the  free  edge  of 
the  paper  is  cut  off  with  scissors.  Filters  cut  in  this  way  are  per- 
fectly circular,  and  of  equal  size. 

Several  pairs  of  these  patterns  of  various  sizes  (3,  4,  5,  6,  6*5,, 
and  8  cm.  radius)  should  be  procured.  In  taking  a  filter  for  a 
given  operation,  you  should  always  choose  one  which,  after  the 
fluid  has  run  through,  will  not  be  more  than  half  filled  with  the 
precipitate, 

*  Plantamour  found  the  ash  of  Swedish  filtering  paper  to  consist  of  63*23 
silicic  acid,  12 '83  lime,  6*21  magnesia,  2*94  alumina,  and  13*92  ferric  oxide,  ill 
100  parts. 

|  WICKE,  Annal.  d.  Chem.  u.  Fharm.,  cxn,  127. 


Fig.  55. 


96 


OPERATIONS. 


[§45. 


As  to  the  funnels,  they  should  be  inclined  at  the  angle  of  60°, 
and  not  bulge  at  the  sides.  Glass  is  the  most  suitable  material  for 
them. 


Fig.  56.  Fig.  57. 

The  filter  should  never  protrude  beyond  the  funnel.  It  should 
come  up  to  one  or  two  lines  from  the  edge  of  the  latter. 

The  filter  is  firmly  pressed  into  the  funnel,  to  make  the  paper 
fit  closely  to  the  side  of  the  latter ;  it  is  then  moistened  with 
water ;  any  extra  water  is  not  poured  out,  but  allowed  to  drop 
through. 

The  stands  shown  in  figs.  56  and  57  complete  the  apparatus  for 
filtering. 

The  stands  are  made  of  hard  wood.  The  arm  holding  the 
funnel  or  funnels  must  slide  easily  up  and  down,  and  be  fixable  by 
the  screw.  The  holes  for  the  funnels  must  be  cut  conically,  to 
keep  the  funnels  steadily  in  their  place. 

These  stands  are  very  convenient,  and  may  be  readily  moved 
about  without  interfering  with  the  operation. 

fifi.   RULES    TO    BE    OBSERVED    IN    THE    PROCESS    OF    FlLTRATION. 

In  the  case  of  curdy,  flocculent,  gelatinous,  or  crystalline  pre- 
cipitates there  is  no  danger  of  the  fluid  passing  turbid  through  the 
filter.  But  with  fine  pulverulent  precipitates  it  is  generally  neces- 
sary, and  always  advisable,  to  let  the  precipitate  subside,  and  then 
filter  the  supernatant  liquid,  before  proceeding  to  place  the  precipi- 


§  45.]  FILTRATION.  97 

tate  upon  the  filter.  We  generally  proceed  in  this  way  also  with 
other  kinds  of  precipitates,  especially  with  those  that  require  to 
stand  long  before  they  completely  separate.  Precipitates  which 
have  been  thrown  down  hot,  are  most  properly  filtered  off  before 
cooling  (provided  always  there  be  no  objections  to  this  course), 
since  hot  fluids  run  through  the  filter  more  speedily  than  cold  ones. 
Some  precipitates  have  a  tendency  to  be  carried  through  the  filter 
along  with  the  fluid  ;  this  may  be  prevented  in  some  instances  by 
modifying  the  latter.  Thus  barium  sulphate,  when  filtered  from 
an  aqueous  solution,  passes  rather  easily  through  the  filter — the 
addition  of  hydrochloric  acid  or  ammonium  chloride  prevents  this 
in  a  great  measure. 

If  the  operator  finds,  during  a  filtration,  that  the  filter  would 
be  much  more  than  half  filled  by  the  precipitate,  he  would  better 
use  an  additional  filter,  and  thus  distribute  the  precipitate  over  the 
two  ;  for,  if  the  first  were  too  full,  the  precipitate  could  not  be 
properly  washed. 

The  fluid  ought  never  to  be  poured  directly  upon  the  filter, 
but  always  down  a  glass  rod  (see  Fig.  54),  and  the  lip  or  rim  of  the 
vessel  from  which  the  fluid  is  poured  should  always  be  slightly 
greased  with  tallow.*  The  stream  ought  invariably  to  be  directed 
towards  the  sides  of  the  filter,  never  to  the  centre,  since  this 
might  occasion  loss  by  splashing.  In  cases  where  the  fluid  has  to 
be  filtered  off,  with  the  least  possible  disturbance  of  the  precipitate, 
the  glass  rod  must  not  be  placed,  during  the  intervals,  in  the 
vessel  containing  the  precipitate;  but  it  may  conveniently  be  put 
into  a  clean  glass,  which  is  finally  rinsed  with  the  wash- water. 

The  filtrate  is  received  either  in  flasks,  beakers,  or  dishes, 
according  to  the  various  purposes  for  which  it  may  be  intended. 
Strict  care  should  be  taken  that  the  drops  of  fluid  filtering  through 
glide  down  the  side  of  the  receiving  vessel ;  they  should  never  be 
allowed  to  fall  into  the  centre  of  the  filtrate,  since  this  again 
might  occasion  loss  by  splashing.  The  best  rnethocl  is  that  shown 
in  Fig. 56, viz.,  to  rest  the  point  -of  the  funnel  against  the  upper 
part  of  the  inside  of  the  receiving  vessel. 

If  the  process  of  filtration  is  conducted  in  a  place  perfectly 
free  from  dust,  there  is  no  necessity  to  cover  the  funnel,  nor  the 

*  The  tallow  may  be  kept  under  the  edge  of  the  work-table  at  a  conv3nient 
point,  where  it  will  adhere  by  a  little  pressure.  The  best  way  of  applying  the 
tallow  to  the  lip  of  a  vessel  i>  with  the  greased  ringer.' 


98  OPERATIONS.  [§  46. 

vessel  receiving  the  filtrate  ;  however,  as  this  is  but  rarely  the  case, 
it  is  generally  indispensable  to  cover  both.  This  is  best  effected 
with  round  plates  of  sheet-glass.  The  plate  used  for  covering  the 
receiving  vessel  should  have  a  small  U-shaped  piece  cut  out  of  its 
edge,  large  enough  for  the  funnel-tube  to  go  through.  The  effect 
desired  may  be  produced  by  cautiously  chipping  out  the  glass  bit 
by  bit  with  the  aid  of  a  key.  Plates  perforated  in  the  centre  are 
worthless  as  regards  the  object  in  view. 

After  the  fluid  and  precipitate  have  been  transferred  to  the 
filter,  and  the  vessel  which  originally  contained  them  has  been 
rinsed  repeatedly  with  water,  it  happens  generally  that  small  par- 
ticles of  the  precipitate  remain  adhering  to  the  vessel,  which  can- 
not be  removed  with  the  glass  rod.  From  beakers  or  dishes  these 
particles  may  be  readily  removed  by  means  of  a  feather  prepared 
for  the  purpose  by  tearing  off  nearly  the  whole  of  the  plumules, 
leaving  only  a  small  piece  at  the  end  which  should  be  cut  per- 
fectly straight.  From  flasks,  minute  portions  of  heavy  precipitates 
which  are  not  adherent,  are  readily  removed  by  blowing  a  jet  of 
water  into  the  flask,  held  inverted  over  the  funnel ;  this  is  effected 
by  means  of  the  washing-bottle  shown  in  Fig.  60,  after  the  tube 
5  has  been  properly  directed.  If  the  minute  adhering  particles  of 
a  precipitate  cannot  be  removed  by  mechanical  means,  solution  in 
an  appropriate  menstruum  must  be  resorted  to,  followed  by  re-pre- 
cipitation. Bodies  for  which  we  possess  no  solvent,  such  as  barium 
sulphate,  for  instance,  must  not  be  precipitated  in  flasks. 

§46. 
yy.   WASHING  OF  PRECIPITATES. 

After  having  transferred  the  precipitate  completely  to  the  filter, 
we  have  next  to  perform  the  operation  of  washing ;  this  is  effected 
by  means  of  owe.  of  the  well-known  washing-bottles,  Figs.  58,  59, 
and  60.*  The  doubly  perforated  stoppers  are  of  vulcanized  rubber. 

By  the  arrangement  shown  in  Fig.  60,  in  which  a  short  piece 
of  wide  glass  tubing  a  is  connected  by  means  of  pieces  of  rubber 
tubing  with  the  tip  J,  the  jet  of  water  may  be  turned  in  any  direc- 

*  A  wash-bottle  for  odorous  liquids  has  been  devised  by  JACOB,  Zeitschr.f. 
analyt.  Chem.,  v,  168. 


§46.] 


FILTRATION. 


tion,   and  even  upwards,   by   simply  turning   b.     Care  must  be 
taken  that  no  loss  is  occasioned  by  too  violent  a  stream  of  water. 


Fig.  58. 


Fig.  59. 


Fig.  60. 


Where  great  caution  is  required  in   washing   a   precipitate,    the 
arrangement    shown    in  Fig.    61  can  be  used  with  good  results; 
its    construction  requires    110    explanation. 
The  point  of  a  is  drawn  out  and  broken  off. 
On   inverting  the  flask  it  delivers  a  fine, 
continuous  stream  of  water. 

Precipitates  requiring  washing  with 
water  are  washed  most  expeditiously  with 
hot  water,  provided  always  there  be  no 
special  reason  against  its  use.  The  wash- 
ing-bottle shown  in  Fig.  59  is  particularly 
well  adapted  for  boiling  water.  The 
wooden  handle  which  is  fastened  to  the 
neck  of  the  flask  with  wire  serves  to  facil- 
itate holding  it.  If  this  is  not  desired,  the 
neck  of  the  bottle  may  be  wound  with  cord 
of  suitable  thickness. 

It  is  a  rule  in  washing  precipitates  not  to  add  fresh  wash- water 
to  the  filter  till  the  old  has  quite  run  through.  In  applying  the 
jet  of  water  you  have  to  take  care  on  the  one  hand  that  the  upper 
edge  of  the  filter  is  properly  washed,  and  on  the  other  hand  that 


Fig.  01. 


ICO  OPERATIONS.  [§  47. 

no  canals  are  formed  in  the  precipitate,  through  whicli  the  fluid 
runs  off,  without  coming  in  contact  with  the  whole  of  the  precipi- 
tate. If  such  canals  have  formed  and  cannot  be  broken  up  by  the 
jet,  the  precipitate  must  be  stirred  cautiously  with  a  small  platinum 
knife  or  glass  rod. 

The  washing  may  be  considered  completed  when  all  soluble 
matter  that  is  to  be  removed  has  been  got  rid  of.  The  beginner 
wrho  devotes  proper  attention  to  the  completion  of  this  operation 
shuns  one  of  the  rocks  which  he  is  most  likely  to  encounter. 
Whether  the  precipitate  has  been  completely  washed  may  generally 
be  ascertained  by  slowly  evaporating  a  drop  of  the  last  washings 
upon  a  platinum  knife,  and  observing  if  a  residue  is  left.  But  in 
cases  where  the  precipitate  is  not  altogether  insoluble  in  water 
(strontium  sulphate,  for  instance),  recourse  must  be  had  to  more 
special  tests,  which  we  shall  have  occasion  to  point  out  in  the 
course  of  the  work.  The  student  should  never  discontinue  the 
washing  of  a  precipitate  because  he  simply  imagines  it  is  finished 
— he  must  be  certain. 

Formerly  continuous  wash-bottles  were  employed  for  pro- 
tracted washings.  They  have,  however,  fallen  into  disuse  because 
in  their  employment  canals  readily  form  in  the  precipitates,  a 
large  quantity  of  water  is  required,  and  the  use  of  hot  water 
is  excluded.  Hence  it  is  now  customary  to  treat  precipitates  as 
described  in  §  48.  Those  interested  in  the  construction  of  con- 
tinuous wash-bottles  will  find  them  described  and  illustrated  in  the 
Handwdrterbuch  der  Chemie,  2d  edit.,  n,  584—586. 

§47. 
J5.    Filtration  by  Suction. 

Filtration  being  a  frequent  and  tedious  operation,  many  at- 
tempts to  facilitate  the  operation  by  employing  suction  have  been 
made  for  a  long  time.  BUNSEN  *  has  more  recently  studied  the 
subject  exhaustively.  In  order  to  avoid  the  danger  of  breaking 
the  filter,  the  fear  of  which  has  prevented  chemists  from  generally 
employing  this  method,  care  must  be  taken  that  the  filter  lies  close 
to  the  funnel  down  to  the  point.  Hence  funnels  should  be  chosen 

*  Ann.  d.    Chem.  u.  Pharm.,  CXLVIII,  269;  also  Zeitschr.  f.   analyt.   Ckem., 
viii,  174 


§  47.]  FILTRATION.  101 

the  sides  of  which  are  inclined  at  an  angle  of  60°,  arid  free  from 
inequalities  of  surface.  In  the  funnel  should  be  placed  a  vary 
thin,  exactly  fitting  platinum  cone,  and  in  the  latter  is  placed  the 
filter  so  that,  after  being  moistened,  it  will  be  in  contact  at  all 
points,  and  without  any  intervening  air-bubbles. 

The  preparation  of  the  platinum  cone  is  thus  given  by  BUNSEN  : 
A  sheet  of  writing-paper  is  formed  into  a  filter,  and  accurately 
adjusted  to  the  sides  of  a  carefully  selected  funnel,  when  it  is  fixed 
in  place  in  the  funnel  by  a  few  drops  of  sealing-wax  applied  to  the 
upper  margin.  The  filter  is  then  impregnated  with  oil,  and  filled 
with  plaster  of  Paris,  in  which  a  handle  is  inserted  before  the 
plaster  has  set.  After  a  few  hours  the  plaster  cast,  with  its  paper 
again  oiled,  is  imbedded  in  a  crucible  4  or  5  cm.  high  and  filled 
with  plaster  of  Paris.  After  this  has  set  the  plaster  cone  is  re- 
moved and  freed  from  its  oiled-paper  covering.  There  are  thus 
obtained  a  solid  cone  and  a  conical  hollow  which  fit  each  other 
perfectly  and  exactly  correspond  to  the  funnel.  The  platinum 
cone  is  now  prepared  by  cutting  a  piece  of  sheet  platinum  (weigh- 
ing about  0*1-54  grm.  per  square  cm.),  of 
suitable  size,  to  the  shape  shown  in  Fig.  62. 
With  a  scissors  a  slit,  «5,  is  cut  from  the 
centre  of  the  piece  to  a  point  midway  on  the 
line  cd.  The  foil  is  next  softened  by  igni- 
tion, after  which  it  is  laid  against  the  solid 
cone  with  the  point  of  the  latter  at  #,  and 
abd  pressed  against  the  cone  arid  the  foil 
wrapped  around  as  closely  as  possible.  The  platinum  cone  is  now 
again  ignited,  after  which  it  is  molded  to  the  plaster  cone  by  hand, 
then  inserted  into  the  hollow  cone  in  which  it  is  tightly  pressed. 
The  platinum  cone,  when  finished,  should  let  no  light  pass  through 
its  apex,  and  even  without  being  soldered  is  sufficiently  firm  for  all 
uses. 

The  glass  funnel,  carrying  its  platinum  cone  and  filter,  is  now 
inserted  air-tight  into  one  hole  of  a  doubly  perforated  rubber  stopper, 
so  that  the  stern  projects  from  5  to  8  cm.  from  the  stopper;  the 
other  perforation  carries  a  short  tube  bent  at  right  angles,  the  lower 
end  of  which  should  not  project  below  the  stopper.  On  now 
inserting  the  stopper  in  the  neck  of  a  bottle,  arid  applying  suction 
to  the  tube,  the  filtration  of  any  liquid  in  the  filter  is  effected  the 


102 


OPERATIONS. 


[§47. 


Fig.  63. 


§  47.]  FILTRATION.  103 

more  rapidly  the  greater  the  difference  in  pressure  between  the 
external  air  and  that  within  the  flask.  If  it  is  intended  to  filter 
under  a  great  difference  of  pressure,  an  ordinary  flask  will  not 
suffice,  as  it  may  be  shattered  by  the  pressure  of  the  external  air. 
For  such  a  purpose,  therefore,  a  stout  glass  flask  must  be  used, 
placed  conveniently  within  a  tin  vessel,  <?,  Fig.  64,  down  the  inner 
conical  sides  of  which  three  strips  of  thick  cloth  or  india  rubber  are 
glued.  This  arrangement  is  advantageous  in  affording  a  firm  stand 
for  flasks  of  various  sizes,  and  in  that  any  danger  from  a  possible 
explosion  may  be  avoided  by  simply  covering  it  with  a  cloth.  As 
an  exhauster,  any  aspirator  capable  of  effecting  a  difference  in 
pressure  of  one  quarter  of  an  atmosphere  will  answer,  as  a  rule. 
A  very  simple  arrangement  is  shown  in  Fig.  63,  in  which  the 
thick-walled  rubber  tube  b  connects  C  withal,  while  a  similar  tube 
connects  A  and  B.  On  opening  the  cocks  in  A  and  J9,  water 
runs  from  the  former  into  the  latter,  creating  a  partial  vacuum  in 
A,  and  drawing  in  air  through  the  filter,  thus  effecting  filtration 
under  a  pressure  which  is  the  greater  the  difference  in  height 
between  the  water  levels  in  the  two  bottles.  These  may  have  a 
capacity  of  2  to  4  litres,  and  they  should  be  similar,  so  that  when 
the  upper  one  is  emptied  it  may  be  replaced  by  the  filled  one. 

The  most  convenient  of  all  aspirators,  however,  is  the  hydraulic 
air-pump,  more  or  less  perfect  forms  of  which  had  been  known 
for  a  long  time  *  before  BUNSEN  brought  it  to  a  high  degree  of 
perfection.  Fig.  64  represents  his  apparatus,  connected  with  a 
flask,  and  as  now  furnished  by  P.  DESAGA,  of  Heidelberg.  The 
following  is  BUNSEN'S  description  of  it: 

On  opening  the  pinch-cock  #,  water  flows  through  the  supply 
pipe  iv  into  the  enlarged  glass  tube  c,  and  passes  off  through  the 
leaden  pipe  d  which  has  a  diameter  of  8  mm.  This  pipe  leads  to 
the  bottom  of  a  drain  30  to  40  feet  below. f  The  tube  <?,  fused 
into  <?,  reaches  to  the  lower  end  of  c,  and  its  lower  aperture  is  very 
small.  Its  upper  continuation  is  connected  aty,  by  means  of  a  side 


*  Compare  Zeitschr.f.  analyt.  Chem.,  n,  359,  and  iv,  46. 

f  If  the  hydraulic  air-pump  is  in  the  upper  part  of  the  house,  it  will  suffice 
to  conduct  the  lead  pipe  to  the  bottom  of  a  cistern  placed  in  the  cellar  and 
provided  with  a  lateral  tube  inserted  a  little  more  than  half-way  from  the  bottom. 
On  connecting  this  tube  with  a  deep  drain,  the  apparatus  performs  its  functions 
without  requiring  any  attention 


104 


OPERATIONS. 


[§47. 


tube,  with  a  mercurial  manometer ;  at  g  it  is  connected  with  the 
vessel  A,  which  is  intended  to  retain  the  steam  given  off  on  wash- 
ing with  hot  water.  A  side  tube  in  A  connects  the  latter,  by 


Fig.  64. 

means  of  a  rubber  tube,  with  the  filtering  flask.  All  the  rubber 
tubing  used  should  be  thick-walled  and  of  2  to  3  mm.  bore.  The 
entire  apparatus  should  be  so  screwed  to  a  board,  fixed  to  the  wall, 
as  to  avoid  any  tendency  on  the  part  of  the  glass  tubes  to  break  by 
reason  of  the  board  warping. 

On  opening  «,  the  water  runs  down  d,  and  the  thirty-foot 
column  of  water  in  this  draws  with  it  the  air,  which  escapes 
through  the  opening  in  the  tube  e  in  the  form  of  a  stream  of  bub- 
bles. Even  if  the  water  runs  at  its  fastest,  however,  and  even  with 


§  47.]  FILTRATION.  105 

a  column  40  feet  in  length,  it  is  impossible  to  create  any  considerable 
exhaustion,  because  of  the  friction,  increasing  in  rapid  progression, 
between  the  water  and  the  lead  pipe.  A  second  pinch-cock,  5,  is 
hence  applied,  which  so  regulates  the  flow  of  water  once  and  for 
all  as  to  minimize  the  friction  by  reducing  the  flow,  thus  enabling 
a  maximum  of  exhaustion  to  be  obtained  when  a  is  completely 
open.  Such  an  apparatus,  properly  regulated,  exhausts  in  a  com- 
paratively short  time  quite  large  vessels  to  a  point  where  the 
pressure  is  exactly  equal  to  the  tension  of  aqueous  vapor  at  the 
temperature  of  the  water,  i.e.,  7  to  11  mm.  of  mercury. 

After  connecting  i  with  the  pump  at  ~k  by  means  of  a  rubber 
tube,  the  filtration  is  effected  by  first  pouring  the  supernatant 
liquid  into  the  filter,  and  afterwards  the  precipitate.  The  liquid 
at  first  runs  off  in  a  rapid  stream,  then  in  rapidly  flowing  drops. 
It  is  advisable  to  keep  the  filter  filled  to  the  edge.  The  precipi- 
tate will  be  compressed  by  the  pressure  to  a  thin  layer,  permeated 
by  channels.  As  soon  as  the  fluid  has  ceased  to  drop,  and  the  first 
channels  are  visible,  the  precipitate  will  adhere  so  firmly  to  the 
paper  that  it  will  not  be  disturbed  on  adding  water  carefully. 
The  washing  is  completed  by  filling  the  funnel  1  cm.  above  the 
edge  of  the  filter,  not  using  a  wash-bottle,  however,  but  pouring 
the  water  carefully  down  the  side  from  an  open  vessel.  When  the 
filtration  has  been  completed,  which  is  usually  the  case  in  from  one 
to  four  fillings  of  the  funnel,  the  filter  is  allowed  to  drain  com- 
pletely, when  it  will  be  obtained  half-dry  so  that  it  may  be  fre- 
quently transferred  without  further  manipulation,  together  with  the 
precipitate,  to  the  crucible,  and  ignited  (see  §52).  It  will  be 
readily  seen  that,  by  this  method  of  filtering,  the  operation  is 
greatly  shortened.  In  this  process,  the  filters  employed  may  be 
smaller  than  usual,  because  the  precipitates  obtained  under  pres- 
sure occupy  less  space  than  is  otherwise  the  case ;  besides,  precipi- 
tates which  are  ordinarily  very  difficult  to  wash  can  be  completely 
washed  with  a  relatively  small  quantity  of  water.  Finally,  the 
half-dried  condition  in  which  precipitates  are  obtained  after  com- 
plete  withdrawal  of  the  water  from  them,  permits  them  to  be  easily 
and  completely  separated  from  the  filter,  arid  free  from  fibres. 

We  have  described  above  the  hydraulic  air-pump  in  its  most 
perfect  form,  in  which  it  is  applicable  for  use  not  only  in  filter- 
ing, but  also  for  all  other  purposes  (exhausting  desiccator  jars, 


106 


OPERATIONS. 


[§47. 


etc.) ;  for  pumps,  however,  which  are  intended  only  for  filtering 
and  drawing  air  through  drying- closets  (§  28),  a  fall  of  10  to  15 
feet  of  water  will  suffice.  Such  a  fall  may  be  readily  obtained 


Fig.  65. 

even  on  the  ground  floor  and  without  a  deep  drain,  by  affixing  the 
pump  to  the  upper  part  of  the  wall.  The  cock  on  the  water  sup- 
ply pipe  must  then,  of  course,  be  placed  where  it  can  be  manipu- 
lated from  the  floor ;  and  the  air  exhaust  pipe  should  be  connected 
with  a  thin  lead  pipe  which  is  conducted  down  to  the  work-table, 
and  fastened  as  shown  in  Fig.  65.  In  this  a  is  the  lead  pipe  lead- 
ing to  the  air-pipe ;  ~b  is  the  rubber  tube  connecting  the  flask  a  by 
means  of  the  glass  tube  c ;  d  is  a  glass  tube  of  suitable  height,  dip- 
ping into  mercury  at  £,  and  serving  as  a  manometer ;  f  is  a  glass 
tube  to  which  is  connected  the  rubber  tube  g  closed  by  the  pinch- 
cock  h.  On  connecting  this  tube  with  the  suction- tube  of  the 
filtering-flask,  starting  the  pump,  and  opening  A,  filtration  begins. 
Slighter  but  still  quite  effective  suction  for  filtering  purposes  is 
also  obtainable  without  the  aid  of  air-pumps  or  aspirators.  Such 
apparatus  is  shown  in  Figs.  66  and  67.  The  apparatus  illustrated 
in  Fig.  66  is  recommended  by  WEIL.*  On  applying  suction  at  <?, 
the  liquid  is  raised  in  A  to  any  desired  height,  thereby  effecting 

*  Zettsclir.  /.  analyt.  Cltem. ,  n,  359. 


47.] 


FILTRATION. 


107 


filtration  through  the  suction  caused  by  the  raised  column  of  liquid. 
The  filter  may  be  strengthened  by  first  inserting  a  small  filter,  #, 


Fig.  67. 


Fig.  66. 

into  the  funnel,  and  then  the  filter-paper,  5.  The  filters  should  lie 
close  to  the  funnel,  and  be  free  from  folds. 

The  apparatus  shown  in  Fig.  67  was  devised  by  PICCARD.* 
If  the  column  of  water  is  not  given  a  greater  length  than  30  cm. 
the  filter  need  not  be  reinforced.  However,  it  is  always  advisable 
even  in  this  case  to  use  the  small  extra  filter.  If  the  filter  lies 
close  to  the  funnel  and  is  free  from  folds,  filtration  is  much  more 
rapid  (10  to  12  times,  according  to  PICCARD)  than  when  the  bent 
tube  is  used. 

The  use  of  exhaust  apparatus  renders  it  possible  to  dispense 
entirely  with  paper  filters,  and  to  effect  filtration  through  asbestos 
or  glass  powder;  in  cases  where  the  precipitates  must  be  dried  at  a 
certain  temperature  and  then  weighed,  such  arrangements  for  filter- 
ing are  frequently  very  useful. 


*  Zeitschr.  /.  analyt.  Chem. ,  iv,  47. 


108 


OPERATIONS. 


Pig.  68. 


68  is  a  filter-tube  which  is  recommended  for  the 
weighing  of  small  quantities  of  anti- 
mony sulphide.*  The  tube  is  charged 
at  a  with  long-fibred  asbestos  which 
is i  then  washed  with  water  to  free  it 
from  all  the  finer  particles,  next  fixed  per- 
pendicularly in  a  support,  suction  applied 
at  5,  and  finally  heated  by  suitable  means 
until  the  asbestos  has  been  completely 
dried.  It  is  then  weighed,  after  which  it 
is  fixed  at  5  in  the  perforated  stopper  of 
the  filtering-flask,  when  a  small  funnel  is 
inserted  at  *',  and  slight  suction  applied, 
the  fluid  to  be  filtered  being  carefully^ 
poured  in. 

Fig.  69  is  an  apparatus  recommended 
by  W.  GIBBS  and  TAYLOR  f  for  a 
similar  purpose.  The  tube  contains  at  a- 
first  fragment  of  glass,  then  glass  or 
sand  in  coarse  powder  and  finally  in  fine 
Fig"  69.  powder. 


48. 


y.  SEPARATION  OF  PRECIPITATES  BY  DECANTATION  AND  FILTRATION 

COMBINED. 

In  the  case  of  precipitates  which,  from  their  gelatinous  nature, 
or  from  the  firm  adhesion  of  certain  coprecipitated  salts,  oppose 
insuperable,  or,  at  all  events,  considerable  obstacles  to  perfect  wash- 
ing on  the  filter,  the  following  method  is  resorted  to :  Let  the 
precipitate  subside  as  far  as  practicable,  pour  the  nearly  clear  super- 
natant liquid  on  the  filter,  stir  the.  precipitate  up  with  the  washing 
fluid  (in  certain  cases,  where  such  a  course  is  indicated,  heat  to 
boiling),  let  it  subside  again,  and  repeat  this  operation  until  the 
precipitate  is  almost  thoroughly  washed.  Transfer  it  now  to  the 
filter,  and  complete  the  operation  with  the  washing-bottle  (see 
§  46).  This  method  is  highly  to  be  recommended ;  there  are 
many  precipitates  that  can  be  thoroughly  washed  only  by-  its- 
application. 

*  ZeitscTir.  /.  analyt.  Chem.,  vin,  154. 
f  Silliman's  Amer.  Jour.,  (n)  XLIV,  215. 


§  49.]  FILTRATION.  109 

In  cases  where  it  is  not  intended  to  weigh  the  precipitate 
washed  by  decantation,  but  to  dissolve  it  again,  the  operation  of 
washing  is  entirely  completed  by  decantation,  and  the  precipitate 
not  even  transferred  to  the  filter.  The  re-solution  of  the  bulk  of 
the  precipitate  being  effected  in  the  vessel  containing  it,  the  filter 
is  placed  over  the  latter,  and  the  solvent  passed  through  it. 
Although  the  termination  of  the  operation  of  washing  may  be 
usually  ascertained  by  testing  a  sample  of  the  washings  for  one  of 
the  substances  originally  present  in  the  solution  which  has  to  be 
removed  (for  hydrochloric  acid,  for  instance,  with  silver 
nitrate),  still  there  are  cases  in  which  this  mode  of  proceeding  is 
inapplicable.  In  such  cases,  and  indeed  in  processes  of  washing  by 
decantation  generally,  BITNSEN'S  method  will  be  found  convenient 
—viz.,  to  continue  the  process  of  washing  until  the  fluid  which 
had  remained  in  the  beaker,  after  the  first  decantation,  has  under- 
gone a  ten  thousand- fold  dilution.  To  effect  this,  measure  with  a 
slip  of  paper  the  height  from  the  bottom  of  this  beaker  to  the 
surface  of  the  fluid  remaining  in  it,  together  with  the  precipitate, 
after  the  first  decantation ;  then  fill  the  beaker  with  water,  if 
possible,  boiling,  and  measure  the  entire  height  of  the  fluid ; 
divide  the  length  of  the  second  column  by  that  of  the  first.  Go 
through  the  same  process  each  time  you  add  fresh  water,  and 
always  multiply  the  quotient  found  with  the  number  obtained  in 
the  preceding  calculation,  until  you  reach  10000. 

§49. 
FURTHER  TREATMENT  OF  PRECIPITATES. 

Before  proceeding  to  weigh  a  precipitate,  it  still  remains  to 
convert  it  into  a  form  of  accurately  known  composition.  This  is 
done  either  by  igniting  or  by  drying.  The  latter  proceeding  is 
more  protracted  and  tedious  than  the  former,  and  is,  moreover,  apt 
to  give  less  accurate  results.  The  process  of  drying  is,  therefore, 
as  ;i  general  rule,  applied  only  to  precipitates  which  cannot  bear 
exposure  to  a  red  heat  without  undergoing  total  or  partial  volatili- 
zation; or  which  leave  upon  ignition  residues  having  wo  constant 
composition ;  thus,  for  instance,  drying  is  resorted  to  in  the  case 
of  mercuric  sulphide,  arsenous  sulphide,  and  other  metallic  sul- 
phides; and  also  in  the  case  of  silver  cyanide,  potassium-platinic 
chloride,  etc. 


110  OPERATIONS.  [§  50. 

But  whenever  the  nature  of  the  precipitate  (e.g.,  barium  sul- 
phate, lead  sulphate,  and  many  other  compounds)  leaves  the 
operator  at  liberty  to  choose  between  drying  and  heating  to  red- 
ness, ignition  is  almost  invariably  preferred. 

§50. 
aa.  Drying  of  Precipitates. 

"When  a  precipitate  lias  been  collected,  washed,  and  dried  on  a 
filter,  minute  particles  of  it  adhere  so  firmly  to  the  paper  that  it  is 
found  impossible  to  remove  them.  The  weighing  of  dried  precipi- 
tates involves,  therefore,  in  all  accurate  analyses,  the  drying  and 
weighing  of  the  filter  also.  Formerly  the  collection  of  precipi- 
tates to  be  dried  was  frequently  done  in  two  filters  of  equal  size, 
one  within  the  other,  the  outer  one  being  removed  and  used  as  a 
counterweight  to  the  filter  containing  the  precipitate.  It  was 
assumed  that  filters  of  equal  size  were  also  of  equal  weight. 
This  assumption  is,  however,  inadmissible  in  accurate  analyses, 
as  every  experiment  proves  that  two  even  very  small  filters  of 
equal  size  differ  in  weight  *by  as  much  as  20,  30,  or  even 
more  milligrammes.  To  obtain  accurate  results,  it  is  neces- 
sary to  dry  and  weigh  the  filter  before  using  it;  the  temperature  at 
which  the  filter  is  dried  must  be  the  same  as  that  to  which  it  is 
intended  subsequently  to  expose  the  precipitate.  Another  condi- 
tion is  that  the  filtering-paper  must  not  contain  any  substance 
liable  to  be  dissolved  by  the  fluid  passing  through  it. 

The  drying  is  conducted  either  in  the  water-,  air-,  or  oil-bath, 
according  to  the  degree  of  heat  required.  The  weighing  is  per- 
formed in  a  closed  vessel,  mostly  between  two  clasped  watch- 
glasses  (Fig.  TO),  in  a  platinum  crucible,  or  in  two  glass  tubes, 
each  sealed  at  one  end,  and  placed  one  within  the  other,  as 
shown  in  Fig.  71.  When  the  filter  appears  dry,  it  is  placed 
between  the  warm  watch-glasses,  or  in  a  warm  crucible,  allowed 
to  cool  under  a  bell-glass  over  sulphuric  acid,  and  weighed. 
The  reopened  crucible  or  watch-glasses,  together  with  the  filter,  are 
then  again  exposed  for  some  time  to  the  required  degree  of  heat, 
and,  after  cooling,  weighed  once  more.  If  the  weight  does  not 
differ  from  that  found  at  first,  the  filter  may  be  considered  dry, 
and  we  have  simply  to  note  the  collective  weight  of  the  watch- 
glasses,  clasp,  and  filter,  or  of  the  crucible  and  filter. 


§  60.] 


DRYING   OF   PRECIPITATES. 


Ill 


After  the  washing  of  the  precipitate  has  been  concluded  and 
the  water  allowed  to  run  off  so  far  as  possible,  the  filter  with  the 


Fig.  70. 


Fig.  71. 


precipitate  is  taken  off  the  funnel,  folded  up,  and  placed  upon 
blotting-paper,  which  is  then  kept  for  some  time  in  a  moderately 
warm  place  protected  from  dust ;  it  is  finally  put  into  one  of 
the  watch-glasses,  or  into  the  uncovered  platinum  crucible,  with 
which  it  was  first  weighed,  and  exposed  to  the  appropriate  degree 
of  heat,  either  in  the  water-,  air-,  or  oil-bath.  When  it  is  judged 
that  the  precipitate  is  dry,  the  second  watch-glass,  or  the  lid  of  the 
crucible  is  put  on  (with  the  clasp  pushed  over  the  two  in  the  former 
case),  and  the  whole,  after  cooling  in  the  desiccator,  is  weighed. 
The  filter  and  the  precipitate  are  then  again  exposed,  in  the  same 
way,  to  the  proper  drying  temperature,  allowed  to  cool,  and 
weighed  again,  the  same  process  being  repeated  until  the  weight 
remains  constant  or  varies  only  to  the  extent  of  a  few  deci-milli- 
grammes.  By  subtracting  from  the  weight 
found  the  tare  of  the  crucible  or  watch-glasses 
and  filter,  we  obtain  the  weight  of  the  dry 
precipitate.  [The  filter  must  not  be  dried  too 
long,  as  it  slowly  loses  weight,  and  even  be- 
comes brown  from  decomposition  when  heated 
to  100°  for  days  together.] 

It  happens  sometimes  that  the  precipitate 
nearly  fills  the  filter,  or  retains  a  considerable 
amount  of  water;  or  sometimes  the  paper  is  so  thin  that  its  re- 


Fig-.  72. 


OPERATIONS.  [§  51. 

moval  from  the  funnel  cannot  well  be  effected  without  tearing. 
In  all  such  cases,  the  best  way  is  to  let  the 
filter  and  precipitate  get  nearly  dry  in  the 
funnel,  which  may  be  effected  readily  by 
covering  the  latter  with  a  piece  of  blotting 
paper  *  to  keep  out  the  dust,  and  placing 
it,  supported  on  a  broken  beaker  (fig.  72), 
or  some  other  vessel  of  the  kind,  on  the 
steam-apparatus  or  sand-bath,  or  stove,  or 
on  a  heated  iron  plate.  For  support  to  a 

funnel  while  drying  a  hollow  frustum  of  a  cone  open  both  ends, 
made  of  stoneware  or  tinned  iron  (Fig.  73),  is  convenient.  Two 
sizes  may  be  used,  10  cm.  and  12  cm.  high  respectively.  The 
lower  diameter  should  be  from  7  to  8,  the  upper  from  4  to  6  cm. 

§51. 
JJ.  Ignition  of  Precipitates. 

It  was  formerly  customary  to  first  dry  the  precipitate 
together  with  the  filter,  then  to  scrape  it  clean  from  the  latter 
into  the  crucible,  and  ignite  it.  In  spite  of  the  most  careful 
scraping,  however,  some  of  the  precipitate  was  inevitably  lost  by 
adherence  to  the  filter.  Experience  has  shown  that  more  accurate 
results  are  obtained  by  igniting  the  filter  with  the  precipitate  and 
deducting  the  weight  of  the  filter-ash  from  the  weight  found. 

If  care  be  taken  to  make  the  filters  always  of  the  same  paper, 
and  to  cut  every  size  by  a  pattern,  as  advised  in  §  45,  act,  the 
quantity  of  ash  which  each  size  yields  upon  incineration  may  be 
readily  determined.  It  is  necessary,  however,  to  determine  sep- 
arately the  quantity  of  ash  left  by  ordinary  filters,  and  that  left  by 
filters  which  have  been  washed  with  hydrochloric  acid  and  water ; 
on  an  average  the  latter  leave  about  half  as  much  ash  as  the  former. 
To  determine  the  filter  ash  take  ten  filters  (or  an  equal  weight  of 
cuttings  from  the  same  paper),  burn  them  in  an  obliquely  placed 
platinum  crucible,  and  ignite  until  every  trace  of  carbon  is  con- 

*  Turned  down  over  the  rim.  Or  more  neatly  as  follows  :  "Wet  a  com- 
mon cut  filter,  stretch  it  over  the  ground  top  of  the  funnel,  and  then  gently 
tear  off  the  superfluous  paper.  The  cover  thus  formed  continues  to  adhere 
after  drying  with  some  force. 


§  51.]  IGNITION   OF   PRECIPITATES.  113 

su med ;  then  weigh  the  ash,  and  divide  the  amount  found  by  ten ; 
the  quotient  expresses,  with  sufficient  precision,  the  average  quan- 
tity of  ash  which  every  individual  filter  leaves  upon  incineration. 
In  the  ignition  of  precipitates,  the  following  four  points  have 
to  be  more  particularly  regarded: 

1.  No  loss  of  substance  must  be  incurred ; 

2.  The  ignited  precipitates  must  really  be  the  bodies  they  are 
represented  to  be  in  the  calculation  of  the  results ; 

3.  The  incineration  of  the  filters  must  be  complete ; 

4.  The  crucibles  must  not  be  attacked. 

The  following  two  methods  seem  to  me  the  simplest  and  most 
appropriate  of  all  that  have  as  yet  been  proposed.  The  selection 
of  either  depends  upon  certain  circumstances,  which  I  shall  imme- 
diately have  occasion  to  point  out.  But  no  matter  which  method 
is  resorted  to,  the  precipitate  must  always  be  thoroughly  dried, 
before  it  can  properly  be  exposed  to  a  red  heat.  The  application 
of  a  red  heat  to  moist  precipitates,  more  particularly  to  such  as  are 
very  light  and  loose  in  the  dry  state  (silicic  acid,  for  instance), 
involves  always  a  risk  of  loss  from  the  impetuously  escaping 
aqueous  vapors  carrying  away  with  them  minute  particles  of  the 
substance.  Some  other  substances,  as  aluminium  hydroxide  or 
ferric  hydroxide,  for  instance,  form  small  hard  lumps  ;  if  such 
lumps  are  ignited  while  still  moist  within  they  are  liable  to  fly 
about  with  great  violence.  The  best  method  of  drying  precipitates 
as  a  preliminary  to  ignition  is  as  described  in  §  50,  the  last 
paragraph.  These  methods  may  also,  according  to  circumstances, 
be  modified ;  for  instance,  BUNSEN  *  has  pointed  out  that  when 
a  precipitate  has  been  sufficiently  washed  and  freed  from  its 
excess  of  water  by  means  of  the  hydraulic  air-pump,  it  may  fre- 
quently be  at  once  ignited  together  with  the  filter,  and  without 
further  drying.  This  is  more  particularly  described  in  §  52. 

Respecting  the  ignition,  the  degree  of  heat  to  be  applied  and 
the  duration  of  the  process  must,  of  course,  depend  upon  the 
nature  of  the  precipitate  and  upon  its  deportment  at  a  red  heat. 
As  a  general  rule,  a  moderate  red  heat,  applied  for  about  five 
minutes,  is  found  sufficient  to  effect  the  purpose ;  there  are,  how- 


*  Annal.  d.  Chem.  u.  Phar.,  CXLVIII,  285;  Zeitschr.  f.  analyt.  Chem.  viu, 
186. 


114  OPERATIONS.  [§  51. 

ever,  many  exceptions  to  this  rule  which  will  be  indicated  where- 
ever  they  occur. 

Whenever  the  choice  is  permitted  between  porcelain  and 
platinum  crucibles,  the  latter  are  always  preferred,  on  account  of 
their  comparative  lightness  and  infrangibility,  and  because  they 
are  more  readily  heated  to  redness.  The  crucible  selected  should 
always  be  of  sufficient  capacity,  as  the  use  of  crucibles  deficient  in 
size  involves  the  risk  of  loss  of  substance.  The  proper  size,  in 
most  cases,  is  4  cm.  in  height,  and  3*5  cm.  in  diameter.  That  the 
crucible  must  be  perfectly  clean,  both  inside  and  outside,  need 
hardly  be  mentioned.  The  analyst  should  acquire  the  habit  of 
cleaning  and  polishing  the  platinum  crucible  always  after  using  it. 
This  should  be  done,  as  recommended  by  BUNSEN,  and  more 
lately  by  EKDMANN,  by  friction  with  moist  sea-sand,  the  grains  of 
which  are  all  round  and  do  not  scratch.  The  sand  is  rubbed 
on  with  the  finger,  and  the  desired  effect  is  produced  in 
a  few  minutes.  The  adoption  of  this  habit  is  attended  with 
the  pleasure  of  always  working  with  a  bright  crucible  and 
the  profit  of  prolonging  its  existence.  This  mode  of  cleaning 
is  all  the  more  necessary,  when  one  ignites  over  gas-lamps,  since 
at  this  high  temperature  crucibles  soon  acquire  a  gray  coat- 
ing, which  arises  from  a  superficial  loosening  of  the  platinum. 
A  little  burnishing  with  sea-sand  readily  removes  the  appear- 
ance in  question,  without  causing  any  notable  diminution  of  the 
weight  of  the  crucible  (EKDMANN).*  The  foregoing  remarks 
on  platinum  crucibles  refer  equally  to  those  of  indium-platinum — 
which,  by  the  by,  are  now  much  used,  and  very  highly  to  be  recom- 
mended— only  the  restoration  of  the  polish  is  somewhat  more  diffi- 
cult with  the  latter,  on  account  of  the  greater  hardness  of  the  alloy. 
If  there  are  spots  on  the  platinum  or  iridium-platinum  crucibles, 
which  cannot  be  removed  by  the  sand  without  wearing  away  too 
much  of  the  metal,  a  little  potassium  disulphate  is  fused  in  the 
crucible,  the  fluid  mass  shaken  about  inside,  allowed  to  cool,  and 
the  crucible  finally  boiled  with  water.  There  are  two  ways  of 
cleaning  crucibles  soiled  outside  ;  either  the  crucible  is  placed  in  a 
larger  one,  and  the  interspace  filled  with  potassium  disulphate, 
which  is  then  heated  to  fusion ;  or  the  crucible  is  placed  on  a 
platinum-wire  triangle,  heated  to  redness,  and  then  sprinkled  over 

*  Jour.f.  prakt.  Chem.,  LXXIX,  117. 


§  51.]  IGNITION    OF    PRECIPITATES.  115 

with  powdered  potassium  disulphate.  Instead  of  the  disulphate 
you  may  use  borax.  Never  forget  at  last  to  polish  the  crucible 
with  sea-sand  again. 

When  the  crucible  is  clean,  it  is  placed  upon  a  clean  platinum- 
wire  triangle  (Fig.  74),  ignited,  allowed  to  cool  in  the  desiccator, 
and  weighed.  This  operation,  though  not  indispensable,  is  still 
always  advisable,  so  that  the  weighing  of  the  empty  and  filled  cru- 
cible may  be'  performed  under  as  nearly  as  possible  the  same  cir- 
cumstances. The  empty  crucible  may  of  course  be  weighed  after 
the  ignition  of  the  precipitate ;  however,  it  is 
preferable  in  most  cases  to  weigh  it  before. 
The  weighing  of  the  crucible  after  ignition 
of  the  precipitate  is  only  then  necessary 
when  this  requires  to  be  subjected  to  the 
action  of  the  gas  blowpipe  for  some  time, 
since  experience  has  shown  that  in  this  case 
the  platinum  crucible  often  loses  weight.*  £' 

If  a  platinum  triangle  is  wanting,  one  of  iron  wire  may  be  used, 
only  that  part  coming  in  contact  with  the  platinum  crucible 
should  be  covered  with  platinum  foil  or  wound  with  platinum 
wire,  or  pieces  of  clay  tobacco  pipes  may  be  slipped  over  the 
iron  wire. 

The  ignition  is  effected  with  a  BEKZELIUS  spirit-lamp  or  a  gas- 
lamp,  or  else  in  a  muffle.  In  igniting  reducible  substances  over 
lamps,  the  analyst  must  always  be  on  his  guard  against  the  con- 
tact of  unconsumed  hydrocarbons  even  in  covered  crucibles. 
When  gas-lamps  are  used  there  is  especial  need  of  caution  in  this 
respect.  Reduction  will  be  avoided  if  the  flame  is  made  no  larger 
than  necessary,  if  the  crucible  is  supported  in  the  upper  part  of 
the  flame,  and  if,  when  the  crucible  is  in  a  slanting  position,  it  is 
heated  from  behind. 

We  pass  on  now  to  the  description  of  the  special  methods. 


*  According  to  WITTSTEIN  platinum  crucibles  lose  weight  on  ignition  only 
when  they  still  contain  sraal!  quantities  ct  osmium  (ZeildcJir.f  anal.  Chem., 
v,  98).  STOLBA  ascribes  the  loss  of  weight  to  formation  of  platinum  carbide, 
and  states  that  the  rougher  the  surface,  the  greater  wili  be  the  loss  (Polyt.  Jour., 
cxcvm,  177). 


116 


OPERATIONS. 


FIKST  METHOD.     (Ignition  of  the  Precipitate  with  the  Filter?) 

This  method  is  resorted  to  in  cases  where  there  is  no  danger  of 
a  reduction  of  the  precipitate  by  the  action  of  the  carbon  of  the 
filter.  The  mode  of  proceeding  is  as  follows : — 

The  perfectly  dry  filter,  with  the  precipitate,  is  removed  from 
the  funnel,  and  its  sides  are  gathered  together  at  the  top,  so  that 
the  precipitate  lies  enclosed  as  in  a  small  bag.  The  filter  is  now 
put  into  the  crucible,  which  is  then  covered  and  heated  over  a 
spirit-lamp  with  double  draught,  or  over  gas  very  gently,  to  effect 
the  slow  charring  of  the  filter;  the  cover  is  now  removed,  the 
crucible  placed  obliquely,  and  a  stronger  degree  of  heat  applied, 
until  complete  incineration  of  the  filter  is  effected ;  the  lid,  which 
had  in  the  meantime  best  be  kept  on  a  porcelain  plate,  or  in  a  por- 
celain crucible,  is  put  on  again,  and  a  red  heat  applied  for  some 
time  longer,  if  needed ;  the  crucible  is  now  allowed  to  cool  a  little, 
and  is  then,  while  still  hot,  though  no  longer  red  hot,*  taken  off 


Fig.  75. 


Fig.  76. 


Fig.  77. 


with  a  pair  of  tongs  of  brass  or  polished  iron  (Figs.  75  and  76), 
and  put  in  the  desiccator,  where  it  is  left  to  cool;  it  is  finally 
weighed. 


*  Taking  hold  of  a  red-hot  crucible  with  brass  tongs  might  cause  the  forma- 
tion of  black  rings  round  it. 


§  52.]  IGNITION    OF  PRECIPITATES.  117 

The  combustion  of  the  carbon  of  the  filter  may  be  promoted, 
in  cases  where  it  proceeds  too  slowly,  by  pushing  the  non-consumed 
particles,  with  a  smooth  and  rather  stout  platinum  wire,  within  the 
focus  of  the  strongest  action  of  the  heat  and  air.  And  the  oper- 
ator may  also  increase  the  draught  of  air  by  leaning  the  lid  of  the 
crucible  against  the  latter  in  the  manner  illustrated  in  Fig.  77. 

It  will  occasionally  happen  that  particles  of  the  carbon  of  the 
filter  obstinately  resist  incineration.  In  such  cases  the  operation 
may  be  promoted  by  putting  a  small  lump  of  fused,  dry  ammonium 
nitrate  into  the  crucible,  placing  on  the  lid  and  applying  a  gentle 
heat  at  first,  which  is  gradually  increased.  However,  as  this  way 
of  proceeding  is  apt  to  involve  some  loss  of  substance,  its  applica- 
tion should  not  be  made  a  general  rule. 

In  cases  where  the  bulk  of  the  precipitate  is  easily  detached 
from  the  filter,  the  preceding  method  is  occasionally  modified  in 
this,  that  the  precipitate  is  put  into  the  crucible,  and  the  filter, 
with  the  still  adhering  particles,  folded  loosely  together,  and  laid 
over  the  precipitate.  In  other  respects,  the  operation  is  conducted 
in  the  manner  above  described. 

As  above  stated,  precipitates  quite  thoroughly  freed  from 
water  by  aspiration  may,  according  to  BUNSEN,*  be  at  once  ignited 
without  further  drying,  but,  of  course,  only  when  the  precipitates 
are  not  reducible  by  the  filter-carbon.  The  process  is  carried 
out  as  follows:  The  portion  of  filter-paper  free  from  precip- 
itate is  tightly  wrapped  round  the  remainder  of  the  filter  in  such 
a  manner  that  the  precipitate  is  enveloped  in  from  four  to  six  folds 
of  clean  paper.  The  whole  is  then  dropped  into  the  platinum  or 
porcelain  crucible  lying  obliquely  upon  a  triangle  over  the  lamp, 
and  pushed  down  against  its  sides  with  the  finger.  The  cover  is 
then  supported  against  the  mouth  of  the  crucible,  as  shown  in  Fig. 
77,  and  the  ignition  commenced  by  heating  the  portion  of  the  cru- 
cible in  contact  with  the  cover.  When  the  flame  has  the  proper 
size  and  position,  the  filter  carbonizes  quietly  without  any  appear- 
ance of  flame  or  considerable  amount  of  smoke.  When  the  car- 
bonization proceeds  too  slowly,  the  flame  is  moved  a  little  toward 
the  bottom  of  the  crucible.  After  some  time  the  precipitate  ap- 

*  Ann.  d.  Chem.  u.  Pharm.,  CXLVIII,  285;  also  Zeitschr.  f.  analyt.  Chem.. 
viu,  186.  In  the  case  of  alumina  AL.  MITSCHEKLICH  bad  previously  highly  rec- 
ommended the  ignition  of  the  moist  precipitate  (Zeitschr.  f.  analyt.  Chem.,  I,  67). 


J18  OPERATIONS.  [§  53. 

pears  to  be  surrounded  only  by  an  extremely  thin  envelope  of 
carbon,  possessing  exactly  the  form  (of  course  diminished  in  size) 
of  the  original  filter  ;  the  flame  is  then  increased,  and  the  crucible 
maintained  at  a  bright-red  heat  until  the  carbon  contained  in  this 
envelope  is  consumed.  The  combustion  proceeds  so  quietly  that 
the  resulting  ash  surrounding  the  precipitate  possesses,  even  to  the 
smallest  fold,  the  exact  form,  of"  the  original  filter.  If  the  ash 
shows  here  and  there  a  dark  color,  it  is  simply  necessary  to  heat 
the  crucible  over  a  blast-lamp  for  a  few  minutes  to  effect  the  com- 
plete removal  of  the  trace  of  carbon.  This  method  of  burning  a 
filter  is  extremely  convenient  and  accurate  ;  it  is  only  necessary  to 
give  a  little  attention  at  first  to  the  slow  carbonization  of  the  paper, 
after  which  the  further  progress  of  the  operation  may  be  left  to 
itself. 

G-elatinous,  finely  divided,  granular,  and  crystalline  precipi- 
tates, such  as  alumina,  calcium  oxalate,  barium  sulphate,  silica, 
etc.,  may  with  equal  facility  be  treated  in  this  manner. 


SECOND  METHOD.     (Ignition  of  the  Precipitate  apart  from  the 

Filter.) 

This  method  is  resorted  to  in  cases  where  a  reduction  of  the 
precipitate  from  the  action  of  the  carbon  of  the  filter  is  appre- 
hended; and  also  where  the  ignited  precipitate  is  required  for 
further  examination,  which  the  presence  of  the  filter  ash  might 
embarrass.  It  may  be  employed  also,  instead  of  the  first  method, 
in  all  cases  where  the  precipitate  is  easily  detached  from  the  filter. 
The  mode  of  proceeding  is  as  follows:  — 

The  crucible  intended  to  receive  the  precipitate  is  placed  upon 
a  sheet  of  glazed  paper  ;  the  perfectly  dry  filter  with  the  precipitate 
is  taken  out  of  the  funnel,  and  gently  pressed  together  over  the 
paper,  to  detach  the  precipitate  from  the  filter  ;  the  precipitate  is 
now  shaken  into  the  crucible,  and  the  particles  still  adhering  to  the 
filter  are  removed  from  it,  so  far  as  practicable,  by  further  pressing 
or  gentle  rubbing  together  of  the  folded  filter,  and  are  then  also 
transferred  to  the  crucible.  The  filter  is  then  cut  up,  using  a  clean 
pair  of  scissors,  into  8  or  10  pieces,  over  a  sheet  of  glazed  paper, 


§  53.]  IGNITION   OF   PRECIPITATES.  119 

and  one  by  one  placed  by  means  of  the  tongs  on  the  crucible  lid 
heated  to  redness.  After  all  are  burned,  the  lid  is  ignited  until 
the  last  trace  of  carbon  is  consumed.  If  the  crucible  lid  be  large, 
and  the  filter  small,  the  cutting  up  of  the  filter  may  be  neg- 
lected; it  need  only  be  folded,  and  then  ignited.  The  lid  is  then 
placed  on  a  crucible  and  covered  with  a  beaker.  Finally  the  cru- 
cible with  the  precipitate  is  ignited  (another  lid  being  placed  on 
it,  if  necessary),  then  the  lid  containing  the  filter-ash  is  put  on 
towards  the  end  of  the  ignition,  and  the  whole  then  allowed  to  cool 
a  little,  when  it  is  placed  in  a  desiccator,  and  finally  weighed  when 
perfectly  cold. 

Where  the  precipitates  are  not  absolutely  insoluble  in  water,  as, 
for  instance,  ammonium-magnesium  phosphate,  in  which  case  the 
filter  is  impregnated  with  a  solution,  however  dilute,  of  the  salt, 
complete  ignition  frequently  requires  considerable  time.  It  may, 
however,  be  hastened  by  pressing  the  blackened  filter  against  the 
red-hot  lid  by  means  of  a  smooth  platinum  wire  or  knife.  A  cer- 
tain amount  of  patience  is,  however,  always  required  in  this  opera- 
tion. 

In  the  case  of  precipitates  which,  when  reduced,  yield  no  sub- 
stances- that  combine  with  platinum,  the  following  method  of  in- 
cineration, devised  by  BUNSEN,  may  also  be  used :  The  filter,  freed 
so  far  as  possible  from  the  precipitate,  is  spread  open  upon  the 
sheet  of  glazed  paper,  and  then  folded  in  form  of  a  little  square  box, 
enclosed  on  all  sides  by  the  parts  turned  up ;  any  minute  particles 
of  the  precipitate  that  may  have  dropped  on  the  glazed  paper  are 
brushed  into  this  little  box,  with  the  aid  of  a'  small  feather ;  the 
box  is  closed  again,  rolled  up,  and  one  end  of  a  long  platinum  wire 
spirally  wound  round  it.  The  crucible  being  placed  on  or  above 
a  porcelain  plate,  the  little  roll  is  lighted,  and,  during  its  com- 
bustion, held  over  the  crucible,  so  that  the  falling  articles  of  the 
precipitate  or  filter-ash  may  drop  into  it,  or,  at  least,  into  the  por- 
celain plate.  In  this  way,  and  by  occasionally  holding  the  little 
roll  again  in  or  against  the  flame,  the  incineration  of  the  filter  is 
readily  and  safely  effected.  "When  the  operation  is  terminated,  a 
slight  tap  will  suffice  to  drop  the  ash  and  the  remaining  particles  of 
the  precipitate  into  the  crucible,  which  is  then  covered,  and  the 
ignition  completed  as  in  §  52.  Where  it  is  intended' to  keep  the 


120  OPERATIONS.  [§  53,  a. 

ash  separate  from  the  precipitate,  it  is  made  to  drop  into  the  lid 
of  the  crucible,  in  which  case  it  is  better  to  ignite  the  crucible  with 
the  principal  portion  of  the  precipitate  first.  Were  this  method  of 
incinerating  adopted  with  such  precipitates  as  silver  chloride,  lead 
carbonate,  etc.,  it  would  entail  a  certain  amount  of  loss,  because 
through  reduction  there  would  be  formed  a  small  quantity  of  me- 
tallic silver  or  lead,  which  would  form  an  alloy  with  the  platinum 
wire.  JSTo  matter  which  method  of  incineration  is  resorted  to,  the 
operation  must  always  be  conducted  in  a  spot  entirely  protected 
from  draughts. 

Certain  precipitates  suffer  some  essential  modification  in  their 
properties,  in  their  solubility,  for  instance,  from  ignition.  In  cases 
where  a  portion  of  a  substance  of  the  kind  is  required,  after  the 
weighing,  for  some  other  purpose  with  which  the  effects  of  a  red 
heat  would  interfere,  the  two  operations  of  drying  and  igniting  t 
may  be  combined  in  the  following  way  : — The  precipitate  is  col- 
lected on  a  filter  dried  at  100°;  it  is  then  also  dried,  at  100°,  arid 
weighed  (§  50).  A  portion  of  the  dry  precipitate  is  put  into  a 
tared  crucible,  and  its  exact  weight  ascertained  ;  it  is  then  exposed 
to  a  red  heat,  allowed  to  cool  in  the  usual  way,  and  weighed  again  ; 
the  diminution  of  weight  which  it  has  undergone  is  calculated  oil 
the  whole  amount  of  the  precipitate. 

§  53,  a. 
USE  OF  ASBESTOS  FILTERS  WITH  BUNSEN'S  FILTERING  APPARATUS. 

A  method  of  filtering,  washing,  and  igniting  precipitates  with 
out  the  use  of  paper  filters,  which  in  many  cases  possesses  great 
advantages,  has  been  devised  by  F.  A.  GOOCH, 
and  is  described  as  follows.*  First.  White, 
silky,  anhydrous  asbestos  is  scraped  to  a  fine 
short  down  with  an  ordinary  knife-blade,  boiled 
with  hydrochloric  acid  to  remove  traces  of  iron 
or  other  soluble  matter,  washed  by  decantation,  and  set  aside 
for  use. 

*  Proceedings  of  Am.  Acad.  Arts  and  Sciences,  1878,  p.  342. 


Fig.  79. 


Fig.  80. 


§53,  a.]    GOOCH'S  METHOD  OF  FILTRATION  AND  IGNITION.    121 

Secondly.  A  platinum  crucible  of  ordinary  size,  preferably  of 
the  broad  low  pattern  (Fig.  78),  is  chosen,  and  the  bottom  (Fig.  79) 

perforated  with  fine  holes  (the  more 

numerous  and  the  finer  the  better) 

by  means  of  a  steel  point ;  or  better 

still,  the  bottom  may  be  made  of  fine 

platinum  gauze.  Next,  a  Bun  sen  fun- 
nel of  the  proper  size  is  selected,  and  over  the 
top  a  short  piece  of  rubber  tubing  *  is  stretched 
and  drawn  down  until  the  portion  above  the 
funnel  arranges  itself  at  right  angles  to  the 
stem.  Within  the  opening  in  the  rubber,  the 
perforated  crucible  is  fitted  as  shown  in  Fig.  80, 
and  the  funnel  is  connected  with  the  receiver 
of  a  Bunsen  pump  or  other  exhausting  appa- 
ratus in  the  ordinary  way. 

To  make  the  asbestos  filter,  the  pressure  of  the  pump  is  applied, 
and  a  little  of  the  asbestos  prepared  as  described,  and  suspended  in 
water,,  is  poured  into  the  crucible.  The  rubber  and  the  crucible 
are  held  together  by  the  exhaustion  of  the  vacuum  pump  with  suf- 
ficient force  to  make  an  air-tight  joint ;  the  water  is  drawn  through 
and  the  asbestos  is  deposited  almost  instantly  in  a  close  compact 
layer  on  the  perforated  bottom  ;  more  asbestos  (if  necessary)  in  sus- 
pension as  before  being  poured  upon  the  first  until  the  layer 
becomes  sufficiently  thick  for  the  purpose  for  which  it  is  intended. 
Finally  a  little  distilled  water  is  drawn  through  the  apparatus  to 
wash  away  any  filaments  which  might  cling  to  the  under  side,  and 
the  filter  is  ready  for  use  ;  the  whole  process  occupying  less  time 
than  is  required  to  fold  and  fit  an  ordinary  paper  filter  to  a  funnel. 
To  prepare  the  filter  for  the  weighing  of  a  precipitate,  the 
crucible  with  the  felt  of  asbestos  undisturbed  is  removed  from 
the  funnel  and  ignited.  In  case  the  precipitate  to  be  subse- 
quently collected  must  be  heated  to  high  temperature  for  a  long 
time,  it  is  better  to  enclose  the  perforated  crucible  with  its  felt 


*  If  suitable  rubber  tubing  is  not  at  hand  for  fitting  the  crucible  to  the 
funnel,  a  piece  of  strong  glass  tube,  preferably  tapering  slightly,  may  be  used 
in  place  of  a  funnel.  The  diameter  of  the  tube  should  exceed  that  of  the  cru- 
cible. One  end  is  drawn  down  to  size  of  a  common  funnel  stem;  the  crucible 
is  then  fitted  to  the  large  end  by  means  of  a  short  section  of  large  rubber  tub- 
ing, or  a  bored  rubber  stopper. 


122  OPERATIONS.  [§  54. 

within  another  crucible ;  because  in  such  cases  asbestos  felt  is  apt 
to  curl  at  the  edges,  and  without  such  precaution  some  of  the 
precipitate  might  drop  through  the  perforations  and  be  lost.  For 
drying  at  low  temperatures,  however,  and  even  for  ordinary  igni- 
tion, a  second  crucible  is  unnecessary ;  but,  during  the  ignition  of 
an  easily  reducible  substance  care  must  be  taken  to  avoid  contact 
of  unburnt  gas  with  the  perforated  bottom. 

To  perform  the  filtration,  the  crucible  is  replaced  in  the  funnel, 
the  pressure  applied,  and  the  process  conducted  precisely  as  in 
ordinary  filtration  by  the  Bunsen  pump.  It  is  necessary  to  observe 
that  the  vacuum  pump  be  started  before  pouring  the  liquid  upon 
the  filter.  The  final  drying  or  ignition,  as  the  case  may  be,  of  the 
precipitate  and  filter  is  made  without  difficulty,  or  need  of  extra 
precaution. 

For  turbid  liquids,  or  gelatinous  precipitates,  instead  of  the 
perforated  crucible  a  platinum  cone  may  be  used,  the  upper  part 
being  made  of  foil,  the  lower  part  of  gauze.  This  process  is  recom- 
mended not  only  for  such  precipitates  as  have  heretofore  usually 
been  collected  upon  weighed  paper  filters,  but  also  for  many  other 
precipitates  which  are  usually  ignited,  but  whose  proper  ignition 
is  more  or  less  interfered  with  by  the  presence  of  carbon. 

§54-. 
5.   ANALYSIS  BY  MEASURE  (VOLUMETRIC  ANALYSIS). 

The  principle  of  volumetric  analysis  has  been*  explained  already 
in  the  "  Introduction,"  where  we  have  seen  how  the  quantity  of 
iron  present  in  a  fluid  as  a  ferrous  salt  may  be  deter  mined  by  means 
of  a  solution  of  potassium  permanganate,  the  value  of  which  has 
been  previously  ascertained  by  observing  the  quantity  required  to 
convert  a  known  amount  of  iron  from  a  ferrous  to  a  ferric  salt. 

In  order  to  make  the  matter  as  clear  as  possible  a  few  more 
examples  are  here  given. 

Let  us  assume  that  we  have  prepared  a  sodium-chloride  solution, 
100  c.  c.  of  which  will  precipitate  exactly  1  grm.  of  silver,  in  the 
form  of  chloride,  from  a  silver-nitrate  solution ;  we  are  now  in  a 
position  to  determine  the  silver  content  of  unknown  silver  com- 
pounds. For  instance,  on  carefully  dissolving  1  grm.  of  a  silver- 
copper  alloy  in  nitric  acid,  and  adding  the  sodium-chloride  solution 


§  54.  j  VOLUMETRIC   ANALYSIS.  123 

as  above  described  just  until  all  the  silver  is  precipitated i.e., 

until  another  drop  causes  no  further  precipitate,  the  quantity  of 
silver  present  in  the  alloy  is  calculated  very  simply  from  the  volume 
of  the  sodium-chloride  solution  used  up.  Were,  for  instance, 
80  c.  c.  of  the  solution  used,  it  would  indicate  that  the  alloy  had  a 
silver  content  of  80  per  cent,  since,  as  100  c.  c.  of  solution  pre- 
cepitate  1  grm.  (or  100  per  cent.)  of  pure  silver,  each  c.  c.  of 
solution  will  correspond  to  1  per  cent,  of  silver. 

Again,  as  is  well  known,  iodine  and  hydrogen  sulphide  im- 
mediately react  when  brought  together,  sulphur  and  hydriodic  acid 
resulting  (I  -f-  H2S  =  HI  +  2S).  Hydriodic  acid  has  no  action 
on  starch-paste,  whereas  the  slightest  trace  of  iodine  colors  it  blue. 
Now,  if  we  prepare  a  solution  of  iodine  in  potassium -iodide  solu- 
tion, so  that  every  100  c.  c.  will  contain  0*3721  grm.  of  iodine,  we 
can,  with  such  a  solution,  decompose  exactly  0*1  grm.  of  hydrogen 
sulphide,  since 

34-086  :  126-85  : :  0-1  :  0-3721. 

Let  us  suppose  now  that  we  have  a  fluid  of  unknown  hydrogen- 
sulphide  content;  we  add  to  it  a  small  quantity  of  starch-paste, 
and  then,  drop  by  drop,  the  iodine  solution  made  as  above.  The 
blue  color  formed  will  not  be  permanent,  but  will  disappear  so  long 
as  any  iodine  and  hydrogen  sulphide  are  present  to  react ;  but  when 
all  the  latter  has  been  decomposed  the  liquid  suddenly  becomes  blue 
from  the  formation  of  starch  iodide.  The  quantity  of  hydrogen 
sulphide  may  hence  be  readily  calculated  from  the  volume  of  iodine 
solution  used  up,  since  100  c.  c.  of  the  latter  correspond  to  0-1  grm. 
hydrogen  sulphide.  Thus,  had  we  used  up  50  c.  c.  of  solution,  it 
would  indicate  0'5  grm.  of  hydrogen  sulphide. 

Solutions  of  accurately  known  composition  or  strength,  used  for 
the  purposes  of  volumetric  analysis,  are  called  standard  (or  titrat- 
ing*} solutions.  They  may  be  prepared  in  two  ways,  viz.,  (a)  by 
dissolving  a  weighed  quantity  of  a  substance  in  a  definite  volume  of 
fluid ;  or  (fj)  by  first  preparing  a  suitably  concentrated  solution  of 
the  reagent  required,  and  then  determining  its  exact  strength  by  a 
series  of  experiments  made  with  it  upon  weighed  quantities  of  the 
body  for  the  determination  of  which  it  is  intended  to  be  used. 

*From  the  French  tttre.  content  (of  gold  or  silver  in  coin). 


124  OPEKATIONS.  [§  54. 

In  the  preparation  of  standard  solutions  by  method  a,  the  weight 
of  the  reagent  taken  for  1000  c.  c.  may,  if  desired,  be  a  weight 
exactly  equivalent  to  1-008  gramme  of  hydrogen  (see  §  192,  c,  tf). 
In  the  case  of  standard  solutions  prepared  by  method  5,  this  may 
also  be  easily  done,  by  diluting  to  the  required  degree  the  still  some- 
what too  concentrated  solution,  after  having  accurately  determined 
its  strength ;  however,  as  a  rule,  this  latter  process  is  only  resorted 
to  in  technical  analyses,  where  it  is  desirable  to  avoid  all  calculation. 
Fluids  which  contain  the  eq.  number  of  grammes  of  a  substance  in 
one  litre  are  called  normal  solutions;  those  which  contain  -^  of 
this  quantity,  decinormal  solutions. 

The  determination  or  titration  of  a  standard  solution  intended 
to  be  used  for  volumetric  analysis  is  obviously  a  most  important 
operation,  since  any  error  in  this  will,  of  course,  necessarily  falsify 
every  analysis  made  with  it.  In  scientific  and  accurate  researches- 
it  is,  therefore,  always  advisable,  whenever  practicable,  to  examine 
the  standard  solution — no  matter  whether  prepared  by  method  a 
or  by  method  J,  with  subsequent  dilution  to  the  required  degree — 
by  experimenting  with  it  upon  accurately  weighed  quantities  of  the 
body  for  the  determination  of  which  it  is  to  be  used. 

In  the  previous  remarks  I  have  made  no  difference  between 
fluids  of  known  composition  and  those  of  known  power;  and  this 
has  hitherto  been  usual.  But  by  accepting  the  two  expressions 
as  synonymous,  we  take  for  granted  that  a  fluid  exercises  a  chemi- 
cal action  exactly  corresponding  to  the  amount  of  dissolved  sub- 
stance it  contains — that,  for  instance,  a  solution  of  sodium  chloride 
containing  1  mol.  Na  01  will  precipitate  exactly  1  at.  silver.  This. 
presumption,  however,  is  very  often  not  absolutely  correct,  as  will 
be  shown  with  reference  to  this  very  example,  §  115,  5,  5.  In  such 
cases,  of  course,  it  is  not  merely  advisable,  but  even  absolutely 
necessary,  to  determine  the  strength  of  the  fluid  by  experiment, 
although  the  amount  of  the  reagent  it  contains  may  be  exactly 
known,  for  the  power  of  the  fluid  can  be  inferred  from  its  com- 
position only  approximately  and  not  with  perfect  exactness.  If  a 
standard  solution  keeps  unaltered,  this  is  a  great  advantage,  as  it 
dispenses  with  the  necessity  of  determining  its  strength  before 
every  fresh  analysis. 

That  particular  change  in  the  fluid  operated  upon  by  means  of 
a  standard  solution  which  marks  the  completion  of  the  intended 


§  54.]  VOLUMETRIC   ANALYSIS.  125 

decomposition,  is  termed  the  FINAL  REACTION.  This  consists  either 
in  a  change  of  color,  as  is  the  case  when  a  solution  of  potassium  per- 
manganate acts  upon  an  acidified  solution  of  ferrous  salt,  or  a  solu- 
tion of  iodine  upon  a  solution  of  hydrogen  sulphide  mixed  with 
starch  paste ;  or  in  the  cessation  of  the  formation  of  a  precipitate 
iipon  further  addition  of  the  standard  solution,  as  is  the  case  when 
a  standard  solution  of  sodium  chloride  is  used  to  precipitate  silver 
from  its  solution  in  nitric  acid ;  or  in  incipient  precipitation,  as  is 
the  case  when  a  standard  solution  of  silver  is  added  to  a  solution  of 
hydrocyanic  acid  mixed  with  an  alkali ;  or  in  a  change  in  the  action 
<>f  the  examined  fluid  upon  a  particular  reagent,  as  is  the  case 
when  a  solution  of  sodium  arsenite  is  added,  drop  by  drop,  to  a 
solution  of  chlorinated  lime,  until  the  mixture  no  longer  imparts  a 
blue  tint  to  paper  moistened  with  potassium  iodide  and  starch- 
paste,  &c. 

The  more  sensitive  a  final  reaction  is,  and  the  more  readily,  posi- 
tively, and  rapidly  it  manifests  itself,  the  better  is  it  calculated  to 
serve  as  the  basis  of  a  volumetric  method.  In  cases  where  it  is  an 
object  of  great  importance  to  ascertain  with  the  greatest  practica- 
ble precision  the  exact  moment  when  the  reaction  is  completed,  the 
analyst  may  sometimes  prepare,  besides  the  actual  standard  solu- 
tion, another,  ten  times  more  dilute,  and  use  the  latter  to  finish  the 
process,  carried  nearly  to  completion  with  the  former. 

But  a  good  final  reaction  is  not  of  itself  sufficient  to  afford  a  safe 
basis  for  a  good  volumetric  method  ;  this  requires,  as  the  first  and 
most  indispensable  condition,  that  the  particular  decomposition 
which  constitutes  the  leading  point  of  the  analytical  process  should 
—at  least  under  certain  known  circumstances — remain  unalterably 
the  same.  Wherever  this  is  not  the  case — where  the  action  varies 
with  the  greater  or  less  degree  of  concentration  of  the  fluid,  or 
according  as  there  may  be  a  little  more  or  less  free  acid  present ;  or 
according  to  the  greater  or  less  rapidity  of  action  of  the  standard 
solution  ;  or  where  a  precipitate  formed  in  the  course  of  the  process 
has  not  the  same  composition  throughout  the  operation — the  basis 
of  the  volumetric  method  is  fallacious,  and  the  method  itself, 
therefore,  of  no  value. 

When  volumetric  analysis  first  began  to  find  favor,  many 
chemists  based  new  volumetric  methods  upon  final  end-reactions, 
without  carefully  studying  the  decompositions  involved ;  the  re- 


126  OPERATIONS.  [§  54. 

suit  was  a  great  number  of  volumetric  methods,  many  of  which 
were  useless.  These  have,  however,  been  subjected  to  a  sifting 
process,  particularly  by  FK.  MOHB  ;  *  and  in  the  special  part  of 
the  present  work  I  have  separated  the  really  good  methods  from 
the  unserviceable. 


*  Lehrbuch  d&r  Titrirmeihode,  3d  edit. 


SECTION    II. 
REAGENTS. 


FOR  general  information  respecting  reagents,  I  refer  the  stu- 
dent to  my  volume  on  ".Qualitative  Analysis." 

The  instructions  given  here  will  be  confined  to  the  preparation, 
testing,  and  most  important  uses  of  those  chemical  substances  which 
subserve  principally  and  more  exclusively  the  purposes  of  quanti- 
tative analysis.  Those  reagents  which  are  employed  in  qualitative 
investigations,  having  been  treated  of  already  in  the  volume  on  the 
qualitative  branch  of  the  analytical  science,  will  therefore  be  simply 
mentioned  here  by  name. 

The  reagents  used  in  quantitative  analysis  are  properly  arranged 
under  the  following  heads  :  — 

A.  Reagents  for  gravimetric  analysis  in  the  wet  way. 

B.  Reagents  for  gravimetric  analysis  in  the  dry  way. 

C.  Reagents  for  volumetric  analysis. 
2).  Reagents  used  in  organic  analysis. 

The  mode  of  preparing  the  fluids  used  in  volumetric  analysis, 
will  be  found  where  we  shall  have  occasion  to  speak  of  their  appli- 
cation. 


A.  REAGENTS  FOR  GRAVIMETRIC  ANALYSIS  IN  THE  WET  WAT. 
I.    SIMPLE    SOLVENTS. 

§56. 
1.  DISTILLED  WATER  (see  "  Qual.  Anal."). 

Water  intended  for  quantitative  investigations  must  be  perfectly 
pure.  Water  distilled  from  glass  vessels  leaves  a  residue  upon 
evaporation  in  a  platinum  vessel  (see  experiment  'No.  5),  and  is 
therefore  inapplicable  for  many  purposes  ;  as,  for  instance,  for  the 
determination  of  the  exact  degree  of  solubility  of  sparingly  soluble 

127 


128  BEAGENTS.  [§  57 

substances.     For  certain  uses  it  is  necessary  to  free  the  water  by 
ebullition  from  atmospheric  air  and  carbonic  acid. 

2.  ALCOHOL  (see  "  Qual.  Anal."). 

a.  Absolute  alcohol.     £.  Common  alcohol  of  various  degrees  of 

o 

strength. 

3.  ETHER. 

The  application  of  ether  as  a  solvent  is  very  limited.  It  is 
more  frequently  used  mixed  with  alcohol,  in  order  to  diminish  the 
solvent  power  of  the  latter  for  certain  substances,  e.g.,  ammonium 
platinic  chloride.  The  ordinary  ether  of  the  shops  will  answer  the 
purpose. 

4.  CARBON  BISULPHIDE  (see  "  Qual.  Anal."). 

This  should  be  purified,  if  necessary,  by  shaking  with  metallic 
mercury  (whereby  the  disagreeable  odor  of  the  commercial  article 
is  removed),  and  then  rectifying  over  the  water-bath.  In  con- 
ducting this  latter  operation  the  use  of  all  rubber  tubing  must  be 
avoided.  Carbon  disulphide  serves  for  removing  free  iodine  from 
aqueous  solutions,  and  for  freeing  sulphides  of  metals  from  ad- 
mixed sulphur. 

II.     ACIDS   AND  HALOGENS. 
a.  Oxygen  Acids. 

§57. 

1.  SULPHURIC  ACID. 

a.  Concentrated  sulphuric  acid  (commercial). 
1).   Concentrated  pure  sulphuric  acid. 
c.  Dilute  sulphuric  acid. 

See  "Qual.  Anal." 

2.  NITRIC  ACID. 

a.  Pure  nitric  acid  of  1*2  sp.  gr.  (see  "  Qual.  Anal."). 

5.  Red  fuming  nitric  acid  (concentrated  nitric  acid  containing 
some  hyponitric  acid). 

Preparation. — Mix  1000  grm.  of  pure  potassium  nitrate  with 
15  grm.  starch  in  lumps,  place  the  mixture  in  a  capacious  tubu- 
lated retort,  and  add  500  grm.  sulphuric  acid  and  500  grm. 
fuming  sulphuric  acid.  The  retort  is  then  placed  on  a  wire 


§  58.]  TCEAGENTS.  129 

gauze  over  a  gas-oven,  or  in  a  sand-bath.  The  distillation  will 
begin  without  the  application  of  heat.  If  the  potassium  nitrate 
is  not  perfectly  free  from  metallic  chlorides,  the  first  portion  of 
the  distillate  should  be  collected  separately  and  set  aside.  When 
the  distillation  slackens  gentle  heat  is  applied,  taking  care  not  to 
push  the  distillation  too  rapidly.  The  process  is  complete,  when, 
by  the  application  of  moderate  heat,  no  more  acid  distills  over. 
As  it  is  impossible  to  prevent  a  portion  of  the  hyponitric  acid  from 
escaping,  the  process  should  be  conducted  in  the  open  air,  or 
under  a  good  vapor  hood. 

Tests. — Red  fuming  nitric  acid  must  be  in  a  state  of  the  greatest 
possible  concentration,  and  perfectly  free  from  sulphuric  acid.  In 
•order  to  detect  minute  traces  of  the  latter,  evaporate  a  few  c.  c.  of 
the  specimen  in  a  porcelain  dish  nearly  to  dryness,  dilute  the  resi- 
due with  water,  add  some  barium  chloride,  and  observe  whether  a 
precipitate  forms  on  standing. 

Uses. — A  powerful  oxidizing  agent  and  solvent ;  it  serves  more 
especially  to  convert  sulphur  and  metallic  sulphides  into  sulphuric 
.acid  and  sulphates  respectively. 

3.  ACETIC  ACID  (see  "  Qual.  Anal."). 

4.  TARTARIC  ACID  (see  "  Qual.  Anal."). 

b.  Hydrogen  Acids  and  Hologens. 

§58. 
1.  HYDROCHLORIC  ACID. 

a.  Pure  hydrochloric  acid  of  1-12  sp.  gr.  (see  "Qual.  Anal.").* 

b.  Pure  fuming  hydrochloric  acid  of  about  1*18  sp.  gr. 

Preparation. — As  in  "  Qual.  Anal."  §  26,  with  this  modifica- 
tion, however,  that  only  3  or  4  parts  of  water,  instead  of  6,  are  put 
into  the  receiver,  to  4  parts  of  sodium  chloride  in  the  retort.  The 
greatest  care  must  be  taken  to  keep  the  receiver  cool,  and  to  change 
it  as  soon  as  the  tube  through  which  the  gas  is  conducted  into  it 
begins  to  get  hot,  since  it  is  now  no  longer  hydrochloric  acid  gas 
which  passes  over,  but  an  aqueous  solution  of  the  gas,  in  form  of 

*  For  BETTENDOKFF'S  process  for  the  preparation  of  arsenic-free  hydro- 
chloric acid,  and  based  upon  the  precipitation  of  arsenic  by  stannous  chloride, 
•see  Zeitschr.f.  analyt.  Chem.,  ix,  107. 


130  REAGENTS.  [§  58. 

vapor,  which  would  simply  weaken  the  fuming  acid,  if  it  were 
allowed  to  mix  with  it. 

Tests. — The  fuming  acid  must,  for  many  purposes,  be  perfectly 
free  from  chlorine  and  sulphurous  acid.  For  the  mode  of  testing 
for  these  impurities,  see  u  Qual  Anal."  loc.  cit.  Test  for  sulphuric 
acid  as  under  Nitric  Acid,  above. 

Uses. — Fuming  hydrochloric  acid  has  a  much  more  energetic 
action  than  the  dilute  acid;  it  is,  therefore,  used  instead  of  the 
latter  in  cases  where  a  more  rapid  and  energetic  action  is  desirable. 

2.  HYDROFLUORIC  ACID. 

This  is  employed  for  the  decomposition  of  silicates  and  borates, 
sometimes  in  the  gaseous  form,  sometimes  in  the  condition  of 
aqueous  solution.  In  the  first  case,  the  substance  to  be  decomposed 
is  introduced  into  the  leaden  box,  in  which  the  hydrofluoric  gas  is 
being  generated  ;  in  the  latter  case,  we  must  first  prepare  the  aque- 
ous acid.  The  raw  material  employed  is  fluor  spar  or  kryolite 
(LuBOLDT*).  Both  are  first  finely  powdered,  and  then  treated  with 
concentrated  sulphuric  acid.  To  1  part  kryolite,  2£  parts  sulphuric 
acid  are  used ;  to  1  part  fluor  spar,  2  parts  sulphuric  acid  are 
used.  If  the  latter  is  employed,  allow  the  mixture  to  stand  in 
a  dry  place  for  several  days,  stirring  every  now  and  then,  so  that 
the  silicic  acid  (which  is  generally  contained  in  fluor  spar)  may 
first  escape  in  the  form  of  fluosilicic  gas.  Convenient  distil- 
latory apparatus  have  been  described  by  LUBOLDT  (loc.  cit.)  and  by 
H.  BEiEGLEB.f  The  latter  commends  itself  especially  on  account 
of  its  relatively  small  cost.  It  consists  of  a  leaden  retort,  with  a 
movable  leaden  top,  which  can  be  luted  on.  The  receiver  belong- 
ing to  it  is  a  box  of  lead,  with  a  tubulure  at  the  side,  into  which 
the  neck  of  the  retort  just  enters.  The  cover  of  the  receiver  is 
raised  conical,  and  is  provided  at  the  top  with  an  exit  tube  of  le?.d. 
In  the  receiver  a  platinum  dish  containing  water  is  placed,  all 
joints  are  luted,  and  the  retort  is  carefully  heated  in  a  sand-bath. 
The  aqueous  hydrofluoric  acid  found"  at  the  end  of  the  operation  in 
the  platinum  dish  is  perfectly  pure.  The  small  quantity  of  impure 
hydrofluoric  acid  which  collects  on  the  bottom  of  the  receiver  is 
thrown  away.  The  hydrofluoric  acid  must  entirely  volatilize  when 
heated  in  a  platinum  dish  on  a  water-bath.  The  pure  acid  gives  no 

*  Jour,  fur  prakt.  Chem.,  LXXVI,  330. 
f  Annal.  d.  Chem.  u.  Pharm  ,  cxi,  380. 


§  59.]  REAGENTS.  131 

precipitate  when  neutralized  with  potash,  while  potassium  silico- 
fluoride  separates  if  the  acid  contains  hydrofluosilicic  acid.  The 
acid  is  best  preserved  in  gutta-percha  bottles,  as  recommended  by 
STADELER.  The  acid  is  now  obtainable  in  the  market  in  gutta- 
percha  bottles.  It  should  at  once  be  tested  ;  this  must  never  be 
neglected,  as  I  have  often  found  the  acid  to  be  impure.  The 
greatest  caution  must  be  observed  in  preparing  this  acid,  since, 
whether  in  the  fluid  or  gaseous  condition,  it  is  one  of  the  most 
injurious  substances. 

3.  CHLORINE  AND  CHLORINE-WATER  (see  u  Qual.  Anal."). 

4.  NITRO- HYDROCHLORIC  ACID  (see  "  Qual.  Anal."). 

5.  HTDROFLUOSILICIC  ACID  (see  "  Qual.  Anal."). 

This  should  be  kept  in  gutta-percha  bottles,  as  when  long 
kept  in  glass  it  attacks  the  latter  and  takes  up  some  of  its  constitu- 
ents. 

c.  Sulphur  Acids. 
1.  HYDROSULPHURIC  ACID  (see  "  Qual.  Anal."). 

III.   BASES  AND  METALS. 
a.  Oxygen  Bases  and  Metals. 

§  59. 
a.  Alkali  Eases. 

1.  POTASSIUM  HYDROXIDE  OR  POTASSA,  AND  SODIUM  HYDROXIDE  OR 
SODA  (see  "  Qual.  Anal."). 

All  the  four  sorts  of  the  caustic  alkalies  mentioned  in  the  quali- 
tative  part  are  required  in  quantitative  analysis,  viz.,  common  solu- 
tion of  soda,  potassa  purified  with  alcohol,  solution  of  potassa  pre- 
pared with  baryta,  and  absolutely  pure  soda.  Pure  solution  of 
potassa  may  be  obtained  also  by  heating  to  redness  for  half  an  hour 
in  a  copper  crucible,  a  mixture  of  1  part  of  potassium  nitrate,  and 
2  or  3  parts  of  thin  sheet  copper  cut  into  small  pieces,  treating  the 
mass  with  water,  allowing  the  oxide  of  copper  to  subside  in  a  tall 
vessel,  and  removing  the  supernatant  clear  fluid  by  means  of  a 
syphon  (WOHLER). 

2.  AMMONIA  (see  "  Qual.  Anal."). 


132  KEAGENTS.  [§  60. 

fi.   Alkali-earth  Bases. 

1.  BARIUM  HYDROXIDE,   OR  BARYTA  (see  "  Qual.  Anal."). 

2.  CALCIUM  HYDROXIDE,   OR  LIME. 

Finely  divided  calcium  hydroxide  mixed  with  water  (milk  of 
lime),  is  used  more  particularly  to  effect  the  separation  of  magne- 
sium, etc.,  from  the  alkali  metals.  Milk  of  lime  intended  to  be 
used  for  that  purpose  must,  of  course,  be  perfectly  free  from  alka- 
lies. To  insure  this  the  purest  lime  (calcined  white  marble) 
should  be  used,  and  slaked  lime  should  be  thoroughly  washed  by 
repeated  boiling  with  fresh  quantities  of  distilled  water.  This 
operation  is  best  conducted  in  a  silver  dish.  When  cold,  the 
inilk  of  lime  so  prepared  is  kept  in  a  well-stoppered  bottle. 

y.   Heavy  Metals,  and  their  Oxides. 

§60. 
1.  ZINC. 

Zinc  has  of  late  been  much  used  as  a  reagent  in  quantitative 
analysis.  It  serves  more  especially  to  effect  the  reduction  of  ferric 
to  ferrous  salts,  and  also  the  precipitation  of  copper  from  solutions 
of  its  salts.  Zinc  intended  to  be  used  for  the  former  purpose  must 
be  free  from  iron,  for  the  latter  free  from  lead,  copper,  and  other 
metals  which  remain  undissolved  upon  treating  the  zinc  with  dilute 
acids.  As  it  is  not  easy  to  prepare  in  quantity  zinc  that  will  answer 
both  purposes,  it  is  advisable  to  keep  on  hand,  besides  the  ordinary 
zinc  used  for  preparing  hydrogen,  the  two  following  kinds  also : 

a.  Zinc,  free  from  Iron. — The  distillation  of  zinc  in  the  labo- 
ratory is  a  tedious  and  costly  operation,  hence  as  a  rule  the  raw 
product  obtained  by  distillation  from  the  ore  is  used  in  the  prepara- 
tion of  the  iron-free  zinc.  This  product  contains,  at  least  in  many 
cases,  only  such  slight  traces  of  iron,  that  it  may  be  safely  used  for 
the  reduction  of  ferrous  salts  in  solution.  Ordinary  commercial 
zinc  contains  much  more  iron,  from  having  been  fused  in  iron 
vessels. 

h.  Zinc,  free  from  Lead,  Copper,  etc. — To  procure  zinc  which 
leaves  no  residue  upon  solution  in  dilute  sulphuric  acid,  there  is 
commonly  no  other  resource  but  to  re-distil  the  commercial  article. 

This  is  effected  in  a  retort  made  of  the  material  of  Hessian  or 


§  60.]  1JEAGENTS.  133 

black-lead  crucibles.  The  operation  is  conducted  in  a  wind-furnace 
with  good  draught.  The  neck  of  the  retort  must  hang  down  as 
perpendicularly  as  possible.  Over  this  is  placed  a  small  clay  drain- 
pipe, the  lower  end  of  which  dips  into  water  contained  in  a  tub  or 
large  stone- ware  dish.  The  joints  are  all  stopped  with  clay.  Under 
the  neck  is  placed  a  basin  or  small  tub,  filled  with  water.  The 
distillation  begins  as  soon  as  the  retort  is  at  a  bright  red  heat.  As 
the  neck  of  the  retort  is  very  liable  to  become  choked  up  with  zinc 
or  oxide  of  zinc,  it  is  necessary  to  keep  it  constantly  free  by  means 
of  a  pipe-stern.  The  zinc  obtained  by  this  re-distillation  is  nearly 
or  quite  free  from  lead,  but  still  contains  notable  traces  of  iron 
(from  the  wire).  If  the  presence  of  iron  is  to  be  totally  avoided,  a 
clay  pipe-stem  or  stick  of  wood  must  be  used  instead  of  the  iron 
wire. 

Tests. — The  following  is  the  simplest  way  of  testing  the  purity 
of  zinc :  Dissolve  the  metal  in  dilute  sulphuric  acid  in  a  small  flask 
provided  with  a  gas-evolution  tube,  place  the  outer  limb  of  the  tube 
under  water,  and  when  the  solution  is  completed,  let  the  water 
entirely  or  partly  recede  into  the  flask;  after  cooling,  add  to  the 
fluid,  drop  by  drop,  a  sufficiently  dilute  solution  of  potassium  per- 
manganate. If  a  drop  of  that  solution  imparts  the  same  red  tint 
to  the  zinc  solution  as  to  an  equal  volume  of  water,  the  zinc  may  be 
considered  free  from  iron.  I  prefer  this  way  of  testing  the  purity 
of  zinc  to  other  methods,  as  it  affords,  at  the  same  time,  an  approx- 
imate, or,  if  the  zinc  has  been  weighed  and  the  permanganate  solu- 
tion (which,  in  that  case,  must  be  considerably  diluted)  measured, 
an  accurate  and  precise  knowledge  of  the  quantity  of  iron  present. 
If  lead  or  copper  is  present,  the  metal  remains  undissolved  upon 
solution  of  the  zinc. 
2.  COPPEE. 

Preparation. — Commercial  copper,  with  the  exception  of  the 
Japanese,  which  is  not  always  obtainable,  is  seldom  sufficiently  pure 
for  analytical  purposes.  Hence  the  pure  metal  should  be  prepared 
by  the  chemist,  either  by  the  galvanoplastic  process,  or  by  the 
method  of  FCCHS,  in  which  copper-sulphate  solution  is  precipitated 
by  well-cleaned  iron,  the  precipitate  of  copper  boiled  with  hydro- 
chloric acid  to  remove  iron,  then  washed,  dried,  and  fused,  and  the 
regulus  so  obtained  rolled  out  into  thin  sheets. 

Tests. — Pure  copper  must  be  completely  soluble  in  nitric  acid, 


134  KEAGENTS.  [§  60. 

and  the  solution  must  afford  no  precipitate  with  excess  of  ammonia, 
even  on  long  standing  (iron,  lead,  etc.) ;  nor  should  it  be  rendered 
turbid  by  hydrochloric  acid  (silver).  After  precipitation  with 
hydrogen  sulphide,  the  filtrate  should  leave  no  residue  on  evapo- 
ration. 

Uses. — Copper  serves  occasionally  in  indirect  analysis ;  for  in- 
stance, in  estimating  the  copper  content  of  a  liquid,  for  estimating 
iron  according  to  FUCHS,  etc.  Since  the  development  and  use  of 
volumetric  analysis,  however,  it  is  but  seldom  used  in  quantitative 
analysis. 

2.  LEAD  OXIDE. 

Precipitate  pure  lead  nitrate  or  acetate  with  ammonium  car- 
bonate, wash  the  precipitate,  dry,  and  ignite  gently  to  complete 
decomposition . 

Lead  oxide  is  often  used  to  fix  an  acid,  so  that  it  is  not  expelled 
even  by  a  read  heat. 

3.  MEKCUKIC  OXIDE. 

Preparation. — Add  a  solution  of  mercuric  chloride  to  a  hot, 
moderately  dilute  caustic-soda  solution,  taking  care  that  the  soda 
be  always  in  excess.  The  yellow  precipitate  is  thoroughly  washed 
by  decantation,  then  mixed  with  water,  and  preserved  in  this  con- 
dition in  a  bottle. 

Test.- — Mercuric  oxide  must  leave  no  residue  on  ignition  in  a 
platinum  crucible. 

Uses. — This  reagent  serves  in  quantitative  analysis  for  decom- 
posing magnesium  chloride  in  the  process  of  separating  magnesia 
from  alkalies. 

J.  Sulphur  Bases. 

1.  AMMONIUM  SULPHIDE  (see  "  Qual.  Anal."). 

"We  require  both  the  colorless  monosulphide,  and  the  yellow 
polysulphide. 

2.  SODIUM  SULPHIDE  (see  "Qual.  Anal."). 


§  61.]  KEAGENTS.  135 

IV.  SALTS. 

a.  Salts  of  the  Alkalies. 
§61. 

1.  POTASSIUM  SULPHATE  (see  "Qual.  Anal."). 

2.  AMMONIUM  PHOSPHATE. 

Preparation. — Dilute  phosphoric  acid  (sp.  gr.  1*13),  prepared 
from  phosphorus,  is  mixed  with  an  equal  quantity  of  water,  and 
pure  ammonia  water  added  until  the  liquid  has  a  strongly  alkaline 
reaction,  when  it  is  set  aside  for  some  time,  then  filtered  if  neces- 
sary, and  kept  for  use. 

Tests.  —  Ammonium  phosphate  must  he  free  from  arsenic, 
nitric,  and  sulphuric  acids,  but  more  particularly  from  potassa  or 
soda.  To  test  it  for  these  two  last,  add  lead-acetate  solution  so 
long  as  a  precipitate  still  forms,  then  filter,  precipitate  the  lead 
excess  with  hydrogen  sulphide,  filter  again,  evaporate  the  filtrate 
to  dryness,  and  ignite  the  residue.  If  an  alkaline  residue  is  left, 
potassa  or  soda  was  present. 

In  most  cases  sodium  phosphate  (see  "  Qual.  Anal.")  may  be 
used  instead  of  ammonium  phosphate, 

3.  AMMONIUM  OXALATE  (see  u  Qual  Anal."). 

4.  SODIUM  ACETATE  (see  ".Qual.  Anal."). 

5.  AMMONIUM  SUCCINATE. 

Preparation. — Saturate  succinic  acid,  which  has  been  purified 
by  dissolving  in  nitric  acid  and  recrystallizing,  with  dilute  ammo- 
nia. The  reaction  of  the  new  compound  should  be  rather  slightly 
alkaline  than  acid. 

Uses. — This  reagent  serves  occasionally  to  separate  ferric  iron 
from  other  metals. 

6.  SODIUM  CARBONATE  (see  "Qual.  Anal."). 

This  reagent  is  required  both  in  solution,  and  in  pure  crystals ; 
in  the  latter  form  to  neutralize  an  excess  of  acid  in  a  fluid  which 
it  is  desirable  not  to  dilute  too  much. 

7.  AMMONIUM  CARBONATE  (see  "  Qual.  Anal."). 

8.  SODIUM  HYDROGEN  SULPHITE  (see  "  Qual.  Anal."). 

9.  SODIUM  THIOSULPHATK  (HYPOSULPHITE)  NaS2O3. 

This  salt  occurs  in  commerce.      It  should  be   dry,  clear,  well 


136  REAGENTS.  [§  61. 

crystallized,  and  completely  and  easily  soluble  in  water.  The  solu- 
tion must  give  with  silver  nitrate  at  first  a  white  precipitate,  must 
not  effervesce  with  acetic  acid,  and  when  acidified  must  give  no 
precipitate  with  barium  chloride,  or,  at  most,  only  a  slight  turbidity. 
The  acidified  solution  must,  after  a  short  time,  become  milky  from 
separation  of  sulphur. 

Uses. — Sodium  thiosulphate  is  used  for  the  precipitation  of 
several  metals,  as  sulphides,  particularly  in  separations,  for  instance, 
of  copper  from  zinc ;  it  also  serves  as  solvent  for  several  salts  (sil- 
ver chloride,  calcium  sulphate,  etc.) ;  lastly,  it  is  employed  in  volu- 
metric analysis,  its  use  here  depending  on  the  reaction  2(Na,S,Ot) 
+  21  =  2NaI  +  NasS4Oa. 

10.  POTASSIUM  NITRITE  (see  "  Qnal.  Anal."). 

11.  POTASSIUM  BICHROMATE  (see  "Qual.  Anal."). 

12.  AMMONIUM  MOLYBDATE  (see  "Qual.  Anal."). 

When  using  the  solution  of  ammonium  molybdate  in  nitric  acid 
for  the  estimation  of  phosphoric  acid,  the  filtrates  from  the  ammo- 
nium phospho-molybdate  and  magnesium-ammonium  phosphate 
will  contain  all  the  molybdic  acid.  If  the  filtrates  are  preserved, 
therefore,  there  will  be  no  loss,  and  the  acid  may  be  recovered  as 
follows :  Evaporate  the  residue  to  dryness  in  the  open  air  or  under 
a  good  draught,  and  heat  finally  until  most  of  the  ammonium 
nitrate  has  been  decomposed.  Digest  the  residue  with  ammonia, 
which  dissolves  the  molybdic  acid,  and  filter.  To  the  filtrate  add  a 
]ittle  magnesia  mixture  (§  62,  6)  in  order  to  precipitate  any  phos- 
phoric acid  present.  If  a  precipitate  occurs,  add  sufficient  mag- 
nesia mixture  to  assure  complete  precipitation  of  all  phosphoric 
acid.  After  allowing  to  stand  for  some  time,  filter,  acidulate  the 
filtrate  with  nitric  acid,  and  then  filter  off  the  precipitated  molybdic 
acid,  using  suction,  and  wash  it  with  the  smallest  quantity  of  water. 
The  acid  is  then  available  for  use  again  in  solution.  The  filtrate 
and  washings  from  the  acid  will  contain  but  little  acid ;  they  may 
be  worked  up  .with  the  next  residues  treated. 

13.  AMMONIUM  CHLORIDE  (see  "Qual.  Anal."). 

14.  POTASSIUM  CYANIDE  (see  "Qual.  Anal."). 
Preparation. — Besides  the  potassium  cyanide  prepared  accord- 
ing to  LIEBIG,  and  which  contains  potassium  cyanate  and  carbonate, 
there  is  required  also  a  pure  cyanide  for  use  in  certain  separations, 
e.g.,    as   in    WOHLER'S    method  of    separating  nickel  from    zinc. 


§  62.]  REAGENTS.  137 

The  pure  cyanide  is  prepared  as  follows:  2  parts  of  crystallized 
potassium  ferrocyanide  are  powdered  and  then  transferred  to  a 
retort,  wherein  it  is  heated  together  with  1J-  parts  concentrated 
sulphuric  acid  and  4  parts  water,  until  the  residue  begins  to  bump. 
The  vapors  of  hydrocyanic  acid  are  conducted  into  a  cooled 
receiver  containing  a  freshly  prepared  and  filtered  solution  of  1 
part  caustic  potassa  (not  fused,  but  evaporated  until  it  just  solidi- 
fies on  cooling)  in  3  to  4  parts  of  not  less  than  92-per-cent  alcohol. 
The  caustic  potassa  should  be  present  in  slight  excess  at  the  close 
of  the  operation.  The  crystalline  mass  is  filtered  by  the  aid  of 
suction,  then  washed  with  a  little  alcohol,  then  dried  in  a  porcelain 
dish  by  the  aid  of  heat,  and  finally  preserved  for  use  in  a  well- 
closed  bottle. 

b.  Salts  of  the  Alkali-earth  Metals. 


1.  BARIUM  CHLORIDE  (see  "  Qual.  Anal."). 

The  following  process  gives  a  very  pure  barium  chloride,  free 
from  calcium  and  strontium  :  —  Transmit  through  a  concentrated 
solution  of  impure  barium  chloride  hydrochloric  gas,  as  long  as  a 
precipitate  continues  to  form.  Nearly  the  whole  of  the  barium 
chloride  present  is  by  this  means  separated  from  the  solution,  in 
form  of  a  crystalline  powder.  Collect  this  on  a  filter,  let  the 
adhering  liquid  drain  off,  wash  the  powder  repeatedly  with  small 
quantities  of  pure  hydrochloric  acid,  until  a  sample  of  the  wash- 
ings, diluted  with  water,  and  precipitated  with  sulphuric  acid, 
gives  a  filtrate  which,  upon  evaporation  in  a  platinum  dish,  leaves 
no  residue.  The  hydrochloric  mother-liquor  serves  to  dissolve 
fresh  portions  of  witherite.  I  make  use  of  the  barium  chloride  so 
obtained,  principally  for  the  preparation  of  perfectly  pure  barium 
carbonate,  which  is  often  required  in  quantitative  analyses. 

2.  BARIUM  ACETATE. 

Preparation.  —  Dissolve  pure  barium  carbonate  in  moderately 
dilute  acetic  acid,  filter,  and  evaporate  to  crystallization. 

Tests.  —  Dilute  solution  of  barium  acetate  must  not  be  rendered 
turbid  by  solution  of  silver  nitrate.  See  also  "  Qual.  Anal.,"  Barium 
chloride,  the  same  tests  being  also  used  to  ascertain  the  purity  of 
the  acetate. 

Uses.  —  Barium  acetate  is  used  instead  of  barium  chloride,  to 
effect  the  precipitation  of  sulphuric  acid,  in  cases  where  it  is  desir- 


138  KEAGENTS.  [§  62. 

able  to  avoid  the  introduction  of  a  chloride  into  the  solution,  or 
to  convert  the  base  into  an  acetate.  As  the  reagent  is  seldom 
required,  it  is  best  kept  in  crystals. 

3.  BARIUM  CARBONATE  (see  "Qual.  Anal."). 

4.  STRONTIUM  CHLORIDE. 

Preparation. — Strontium  chloride  is  prepared  from  strontian- 
ite  or  celestine,  by  the  same  processes  as  barium  chloride.  The 
pure  crystals  obtained  are  dissolved  in  alcohol  of  96  per  cent.,  the 
solution  is  filtered,  and  kept  for  use. 

Uses. — The  alcoholic  solution  of  strontium  chloride  is  used  to 
effect  the  conversion  of  alkali  sulphates  into  chlorides,  in  cases 
where  it  is  desirable  to  avoid  the  introduction  into  the  fluid  of  a 
salt  insoluble  in  alcohol. 

5.  CALCIUM  CHLORIDE  (see  "  Qual.  Anal."). 

6.  MAGNESIUM  CHLORIDE,  MAGNESIUM  SULPHATE,  OR  • 

MAGNESIA  MIXTURE. 

Dissolve  1 1  parts  crystallized  magnesium  chloride  (MgCl2  -f-  6 
H2O)  and  28  parts  ammonium  chloride  in  130  parts  water,  add 
70  parts  dilute  ammonia  solution  (sp.  gr.  0*96).  Allow  the  mix- 
ture to  stand  one  or  two  days  and  filter.  Tins  solution,  commonly 
called  "  magnesia  mixture,"  is  used  to  precipitate  phosphoric 
acid,  and  also  arsenic  acid,  from  aqueous  solutions.  An  excess  is 
required  to  effect  complete  precipitation.  Prepared  as  here  de- 
scribed, about  10  c.  c.  should  be  used  in  ordinary  cases  for  every 
O'l  gramme  P3O6. 

A  solution  containing  the  same  per  cent,  (approximately)  of 
magnesium  chloride  and  other  constituents  may  also  be  prepared 
from  common  calcined  magnesia  (MgO),  provided  it  is  free  from 
the  other  alkali-earth  metals,  as  follows: — Add  to  11  parts  magnesia 
sufficient  hydrochloric  acid  to  effect  solution,  next  add  a  slight  ex- 
cess of  magnesia  and  boil  to  separate  traces  of  iron  ;  filter,  and  add 
140  parts  ammonium  chloride  and  350  parts  dilute  ammonia. 
Dilute  with  water  until  volume  equals  1000  c.  c.  for  every  11 
grammes  of  MgO  used.  Allow  the  mixture  to  stand  two  or  three 
days,  and  filter  if  necessary. 

Magnesia  mixture  may  be  also  made  as  follows:  Dissolve 
1  part  of  crystallized  magnesium  sulphate  and  2  parts  pure 
ammonium  chloride  in  8  parts  water  and  add  4  parts  ammonia 
water.  Let  stand  several  days,  then  filter. 


§  63.]  REAGENTS.  139 

c.  Salts  of  the  Heavy  Metals. 
§  63. 

1.  FERROUS  SULPHATE  (see  "Qual.  Anal."). 

2.  FERRIC  CHLORIDE  (see  "  Qual.  Anal."). 

3.  URANIO  ACETATE. 

Heat  finely  powdered  pitchblende  with  dilute  nitric  acid,  filter 
the  fluid  from  the  undissolved  portion,  and  treat  the  filtrate  with 
hydrosulphuric  acid  to  remove  the  lead,  copper,  and  arsenic ;  filter 
again,  evaporate  to  dryness,  extract  the  residue  with  water,  and  fil- 
ter the  solution  from  the  oxides  of  iron,  cobalt,  and  manganese. 
Uranic  nitrate  crystallizes  from  the  filtrate ;  purify  this  by  recrys- 
tallization,  and  then  heat  the  crystals  until  a  small  portion  of  uranic 
oxide  is  reduced.  Warm  the  yellowish-red  mass  thus  obtained 
with  acetic  acid,  filter  arid  let  the  filtrate  crystallize.  The  crystals 
are  uranic  acetate,  and  the  mother-liquor  contains  the  undecom- 
posed  nitrate  (WERTIIEIM). 

The  salt  may  be  more  conveniently  made  from  the  commer- 
cially obtainable  sodium  uranate  (manufactured  by  the  K.  K. 
Bergoberamt,  Joachim sthal).  Digest  1  part  of  this  salt  in  2  parts 
acetic  acid  (sp.  gr.  1*038),  then  add  25  parts  water,  heat,  filter, 
evaporate,  and  allow  to  crystallize.  The  uranic  oxide  in  the  last 
mother-liquors  (containing  also  sodium  acetate)  is  precipitated  by 
ammonia. 

Uranium  being  a  costly  metal,  all  the  residues  should  be 
saved,  and  worked  up  as  follows :  The  liquid  is  poured  off  fr^gn. 
any  sediment  of  uranium  phosphate.  All  the  uranium  in  it  is 
then  precipitated  by  adding  sodium  phosphate.  The  precipitate 
is  washed  by  decantation,  mixed  with  the  uranium  phosphate 
reserved,  the  whole  dissolved  in  hydrochloric  acid,  and  ferric 
chloride  added  until  a  sample  gives  a  brownish  precipitate  with 
ammonium  carbonate.  The  mixture  is  then  diluted,  and  to  the 
solution,  which  must  contain  a  sufficient  excess  of  hydrochloric 
acid,  add  a  solution  of  crystallized  sodium  carbonate  in  excess. 
All  the  phosphoric  acid  is  thus  precipitated  as  basic  ferric  phos- 
phate; the  uranium  oxide,  however,  remains  dissolved  in  the 
solution  of  sodium  bicarbonate  formed.  Filter  the  mixture,  wash, 
acidulate  the  filtrate  with  hydrochloric  acid,  warm  until  the  car- 
bon dioxide  is  completely  expelled,  warm  and  precipitate  the 


140  REAGENTS.  [§  64. 

uranium  oxide  with  ammonia.  After  washing,  dissolve  in  acetic 
acid  (E.  REICHARD).* 

Tests. — Solution  of  uranic  acetate  after  acidification  with 
hydrochloric  acid  must  not  be  altered  by  hydrosulphuric  acid ; 
ammonium  carbonate  must  produce  in  it  a  precipitate,  soluble  in 
an  excess  of  the  precipitant.  A  sample  of  the  dilute  solution 
should  acquire  a  red  tint  on  adding  a  little  sulphuric  acid  and  a 
drop  of  potassium  permanganate  solution  (absence  of  uranous 
salt). 

Uses. — Uranic  acetate  may  serve,  in  many  cases,  to  effect  the 
separation  and  determination  of  phosphoric  acid. 

4.  SILVER  NITRATE  (see  "  Qual.  Anal."). 

5.  LEAD  ACETATE  (see  "  Qual.  Anal."). 

6.  MERCURIC  CHLORIDE  (see  "  Qual.  Anal."). 

7.  STANNOUS  CHLORIDE  (see  "  Qua!.  Anal."). 

8.  PLATINIC  CHLORIDE  (see  "  Qual.  Anal."). 

It  is  convenient  to  know  approximately  the  strength  of  this 
solution.  I  usually  use  a  solution  10  or  20  c.  c.  of  which  contain 
1  grm.  platinum. 

9.  SODIUM  PALLADIO-CHLORIDE  (see  "  Qual.  Anal."). 

R     RE  A  GENTS  FOR  GRA  VIMETRIC  ANAL  T8I8  IN  THE  DR  T  WAY. 

§  64. 

1.  SODIUM  CARBONATE,  pure  anhydrous  (see  "  Qual.  Anal."). 

2.  MIXED  SODIUM  AND  POTASSIUM  CARBONATES  (see  "  Qual. 
Anal."). 

3.  BARIUM  HYDROXIDE  OR  BARYTA  (see  "  Qual.  Anal."  and 
§59). 

4.  POTASSIUM  NITRATE  (see  "  Qual.  Anal."). 

5.  SODIUM  NITRATE  (see  "  Qual.  Anal."). 

6.  BORAX  (fused). 

Preparation. — Heat  crystallized  borax  (see  "  Qual.  Anal.)  in  a 
platinum  or  porcelain  dish,  until  there  is  no  further  intumescence ; 
reduce  the  porous  mass  to  powder,  and  heat  this  in  a  platinum  cru- 
cible until  it  is  fused  to  a  transparent  mass.  Pour  the  semi-fluid, 

*  Zeitschr.f.  analyt.  Chem.,  vin,  116. 


§  64.]  REAGENTS.  141 

viscid  mass  upon  a  fragment  of  porcelain.  A  better  way  is  to  fuse 
the  borax  in  a  net-  of  platinum  gauze,  by  making  the  gas  blowpipe- 
flame  act  upon  it.  The  drops  are  collected  in  a  platinum  dish. 
The  vitrified  borax  obtained  is  kept  in  a  well -stoppered  bottle. 
But  as  it  is  always  necessary  to  heat  the  vitrified  borax  previous  to 
use,  to  make  quite  sure  that  it  is  perfectly  anhydrous,  the  best  way 
is  to  prepare  it  only  when  required. 

Uses. — Vitrified  borax  is  used  to  effect  the  expulsion  of  car- 
bonic acid  and  other  volatile  acids,  at  a  red  heat. 

7.  POTASSIUM  DISULPHATE. 

Preparation. — Mix  87  parts  of  normal  potassium  sulphate  (see 
"  Qual.  Anal."),  in  a  platinum  crucible,  with  49  parts  of  concen- 
trated pure  sulphuric  acid,  and  heat  to  gentle  redness  until  the  mass 
is  in  a  state  of  uniform  and  limpid  fusion.  Pour  the  fused  salt  on 
a  fragment  of  porcelain,  or  into  a  platinum  dish  standing  in  cold 
water.  After  cooling,  break  the  mass  into  pieces,  and  keep  for 
use. 

Uses. — This  reagent  serves  as  a  flux  for  certain  native  com- 
pounds of  alumina  and  chromic  oxide.  Potassium  disulphate  is 
used  also,  as  we  have  already  had  occasion  to  state,  for  the  cleansing 
of  platinum  crucibles ;  for  this  latter  purpose,  however,  the  salt 
which  is  obtained  in  the  preparation  of  nitric  acid  will  be  found 
sufficiently  pure. 

8.  SODIUM  DISULPHATE. 

Preparation. — This  is  prepared  like  the  potassium  salt,  usiu^ 
71  parts  of  pure,  normal  sodium  sulphate  and  49  parts  of  concen- 
trated pure  sulphuric  acid. 

Uses. — Sodium  disi^phate  is  used  like  the  potassium  disul- 
phate, but  is  to  be  substituted  for  the  latter  when,  as  in  fusing 
corundum,  the  analysis  is  hampered  by  alum  crystallizing  out 
{L.  SMITH,  Zeitsohr.  f.  analyt.  Chem.,  iv,  412). 

9.  HYDROGEN-POTASSIUM  FLUORIDE. 

Neutralize  a  definite  quantity  of  hydrofluoric  acid  in  a  plati- 
num dish  with  pure  potassium  carbonate  or  potassium  hydroxide, 
applying  heat  toward  the  last;  then  add  a  quantity  of  hydrofluoric 
acid  equal  to  that  first  taken,  and  evaporate  the  whole  to  dryness. 
The  preparation  is  usually  made  just  before  required  ;  if  it  is  to  be 
preserved,  gutta-percha  vessels  must  be  used. 


142  KEAGENTS.  [§  64. 

Tests. — Solution  of  hydrogen -potassium  fluoride  must  remain 
unaffected  on  adding  hydrogen  sulphide,  ammonia,  ammonia  and 
ammonium  sulphide,  or  ammonium  carbonate  and  sodium  phosphate 
with  addition  of  ammonia. 

'Uses. — This  preparation  is  an  excellent  flux  for  many  minerals 
which  are  usually  very  refractory,  e.g.,  tinstone,  chrome  iron 
(GIBBS,  Zeitschr.f.  analyt.  Chem.,  m,  399). 

10.  HYDROGEN- AMMONIUM  FLUORIDE. 

Preparation. — Add  ammonia  in  considerable  excess  to  hydro- 
fluoric acid  or  hydrosilicofluoric  acid  in  a  platinum  dish,  gently 
heat  for  some  time,  filter  if  necessary,  and  evaporate  the  filtrate  to 
dryness  in  a  platinum  dish.  Half  the  ammonia  escapes,  while  hy- 
drogen-ammonium fluoride  remains  behind.  If  this  is  to  be  pre- 
served, a  gutta-percha  vessel  must  be  used. 

Tests. — Like  those  of  hydrogen-potassium  fluoride.  In  addi- 
tion, a  sample  heated  in  a  platinum  dish  (in  the  open  or  under  a 
good  vapor  hood)  must  leave  no  fixed  residue. 

Uses. — The  preparation  may  be  advantageously  used  instead  of 
hydrofluoric  acid  in  the  analysis  of  silicates. 

11.  AMMONIUM  CARBONATE  (solid). 

,  Preparation. — See  "  Qual.  Anal."  —This  reagent  serves  to 
convert  the  acid  alkali  sulphates  into  normal  salts.  It  must  com- 
pletely -volatilize  when  heated  in  a  platinum  dish. 

12.  AMMONIUM  TITRATE. 

Preparation. — Neutralize  pure  ammonium  carbonate  with  pure 
nitric  acid,  warm,  and  add  ammonia  to  slightly  alkaline  reaction; 
filter,  if  necessary,  and  let  the  filtrate  crystallize.  Fuse  the  crys- 
tals in  a  platinum  dish,  and  pour  the  fused  mass  upon  a  piece  of 
porcelain ;  break  into  pieces  whilst  still  warm,  and  keep  in  a  well- 
stoppered  bottle. 

Tests. — Ammonium  nitrate  must  leave  no  residue  when  heated 
in  a  platinum  dish. 

Uses.—  Ammonium  nitrate  serves  as  an  oxidizing  agent;  for 
instance,  to  convert  lead  into  lead  oxide,  or  to  effect  the  com- 
bustion of  carbon,  in  cases  where  it  is  desired  to  avoid  the  use  of 
fixed  salts. 


§  84.  J  REAGENTS.  143 

13.  AMMONIUM  CHLORIDE. 
Preparation  and  Tests. — See  "  Qual.  Anal." 

Uses. — Ammonium  chloride  is  often  used  to  convert  metallic 
oxides  and  adds,  e.g.,  lead  oxide,  zinc  oxide,  stannic  oxide,  arsenic 
acid,  antimonic  acid,  &c.,  into  chlorides  (ammonia  and  water  escape 
in  the  process).  Many  metallic  chlorides  being  volatile,  and  others 
volatilizing  in  presence  of  ammonium  chloride  fumes,  they  may  be 
completely  removed  by  igniting  them  with  ammonium  chloride  in- 
excess,  and  thus  many  compounds,  e.g.,  alkali  antimonates,  may 
be  easily  arid  expeditiously  analyzed.  Ammonium  chloride  is 
also  used  to  convert  various  salts  of  other  acids  into  chlorides,  e.g., 
small  quantities  of  alkali  sulphates. 

14.  HYDROGEN   GAS. 

Preparation. — Hydrogen  gas  is  evolved  when  dilute  sulphuric 
acid  is  added  to  granulated  zinc.  It  may  be  purified  from  traces 
of  foreign  gases  either  by  passing  first  through  mercuric-chloride 
solution,  then  through  potassa  solution,  or  as  recommended  by 
STENHOUSE,  by  passing  through  a  tube  filled  with  pieces  of  char- 
coal. If  the  gas  is  desired  dry,  pass  through  sulphuric  acid  or  a 
calcium-chloride  tube.  If  the  zinc  used  is  new,  the  evolution  of  gas 
may  be  facilitated  by  adding  a  drop  of  platinic-chloride  solution. 

Tests. — Pure  hydrogen  gas  is  inodorous.  It  ought  to  burn  with 
a  colorless  flame,  which,  when  cooled  by  depressing  a  porcelain 
dish  upon  it,  must  deposit  nothing  on  the  surface  of  the  dish  except 
pure  water  (free  from  acid  reaction). 

Uses. — Hydrogen  gas  is  frequently  used,  in  quantitative  analy- 
sis, to  reduce  oxides,  chlorides,  sulphides,  &c.,  to  the  metallic  state ; 
also  to  protect  certain  substances,  like  metallic  sulphides,  from  the 
action  of  atmospheric  oxygen  during  ignition. 

12.   CHLORINE. 

Preparation. — See  "Qua!.  Anal." — Chlorine  gas  is  purified 
and  dried  by  transmitting  it  through  a  U-tube  containing  frag- 
ments of  manganese  dioxide,  then  concentrated  sulphuric  acid,  or  a 
calcium-chloride  tube. 

Uses. — Chlorine  gas  serves  principally  to  produce  chlorides, 
and  to  separate  the  volatile  from  the  non-volatile  chlorides;  it  is 
also  used  to  displace  and  indirectly  determine  bromine  and  iodine, 
as  well  as  to  convert  lower  chlorine  compounds  into  higher. 


144  REAGENTS.  [§  65. 

C.   REAGENTS    USED  IN  VOLUMETRIC  ANALYSIS. 
§65. 

Under  this  head  are  arranged  the  most  important  of  those 
substances  which  serve  for  the  preparation  and  testing  of  the 
fluids  required  in  volumetric  analysis  and  have  not  been  given 
under  A  and  B. 

1 .    PURE  CRYSTALLIZED  OXALIC  ACID,   HaCa04  -f-  2H20. 

The  introduction  of  crystallized  oxalic  acid  as  a  basis  for  alkal- 
imetry and  acidimetry  is  due  to  FR.  MOHR.  It  is  also  employed 
to  determine  the  strength  of,  or  to  standardize,  a  solution  of  potas- 
sium permanganate,  1  molecule  of  potassium  permanganate  being 
required,  in  the  presence  of  free  sulphuric  acid,  to  convert  5  mole- 
cules of  oxalic  acid  into  carbon  dioxide  and  water  (K2Mn2O8  -f- 
5H.CA  +  3H,S04  =  K2S04  +  2MnSO4  +  8H.O  +  lOCO',). 
"We  use  in  most  cases  the  pure  crystallized  acid  which  has  the 
formula  HaCaO4  +  2HaO,  and  of  which  the  molecular  weight  is 
accordingly  126*048. 

Preparation. — The  pure  acid  is  prepared  by  shaking  powdered 
commercial  oxalic  acid  with  a  small  quantity  of  warm  water  (so  that 
considerable  of  the  acid  remains  undissolved),  then  filtering,  and 
crystallizing  by  rapidly  cooling  (MOHR).  Spread  the  crystals, 
after  draining,  on  blotting  paper,  and  set  aside  to  dry  in  a  dust- 
free  place  at  the  ordinary  temperature  (not  too  high),  or  press 
them  gently  between  renewed  layers  of  blotting  paper  until  the 
latter  take  up  no  more  moisture. 

Tests. — The  crystals  of  oxalic  acid  must  not  show  the  least  sign 
of  efflorescence  (to  which  they  are  liable  even  at  20°  in  a  dry 
atmosphere) ;  they  must  dissolve  in  water  to  a  perfectly  clear  fluid ; 
when  heated  in  a  platinum  dish,  they  must  leave  no  fixed  and 
incombustible  residue  (calcium  carbonate,  potassium  carbonate, 
&c.).  If  the  acid  obtained  by  a  first  crystallization  fails  to  satisfy 
these  requirements,  it  must  be  recrystallized.  In  this  case  the 
strength  of  the  solution  must  be  such  that  only  10  or  20  per  cent 
of  the  dissolved  oxalic  acid  crystallizes  out,  and  this,  containing  the 
impurities,  is  then  removed,  after  which  the  mother-liquor  is  con- 
centrated by  evaporation.  The  crop  of  crystals  next  obtained  is 
purer. 


§  65.]  REAGENTS.  145 

2.  TINCTURE  OF  LITMUS. 

Preparation. — Digest  1  part  of  litmus  of  commerce  witli  6 
parts  of  water  on  the  water-bath  for  some  time,  filter,  divide  the 
blue  fluid  into  2  portions,  and  saturate  in  one  half  the  free  alkali, 
by  stirring  repeatedly  with  a  glass  rod  dipped  in  very  dilute  nitric 
acid,  until  the  color  just  appears  red ;  add  the  remaining  blue  half, 
together  with  1  part  of  strong  spirit  of  wine,  and  keep  the  tincture 
which  is  now  ready  for  use,  in  a  small  open  bottle,  not  quite  full, 
in  a  place  protected  from  dust.  In  a  stoppered  bottle  the  tincture 
would  speedily  lose  color. 

Tests. — Litmus  tincture  is  tested  by  coloring  with  it  about  100 
cubic  centimetres  of  water  distinctly  blue,  dividing  the  fluid  into 
two  portions,  and  adding  to  the  one  the  least  quantity  of  a  dilute 
acid,  to  the  other  a  trace  of  solution  of  soda.  If  the  one  portion 
acquires  a  distinct  red,  the  other  a  distinct  blue  tint,  the  litmus 
tincture  is  fit  for  use,  as  neither  acid  nor  alkali  predominates. 

3.  POTASSIUM  PERMANGANATE. 

Preparation. — Mix  8  parts  of  very  finely  powdered  pure  pyro- 
lusite,  or  manganese  dioxide,  with  7  parts  of  potassium  chlorate, 
put  the  mixture  into  a  shallow  cast-iron  pot,  and  add  37  parts  of  a 
solution  of  potassa  of  1/27  specific  gravity  (the  same  solution  as 
is  used  in  organic  analysis  *) ;  evaporate  to  dry  ness,  stirring  the 
mixture  during  the  operation ;  put  the  residue  before  it  has  ab- 
sorbed moisture,  into  an  iron  or  Hessian  crucible,  and  expose  to  a 
dull-red  heat,  with  frequent  stirring  with  an  iron  rod  or  iron  spa- 
tula, until  no  more  aqueous  vapors  escape  and  the  mass  is  in  a  faint 
glow.  Remove  the  crucible  now  from  the  fire,  and  transfer  the 
friable  mass  to  an  iron  pot.  Reduce  to  coarse  powder,  and  transfer 
this,  in  small  portions  at  a  time,  to  an  iron  vessel  containing  100 
parts  of  boiling  water;  keep  boiling,  replacing  the  evaporating 
wrater,  and  passing  a  stream  of  carbon  dioxide  through  the-  fluid 
(  MULDER f).  The  originally  dark-green  solution  of  potassium 
manganate  soon  changes,  with  separation  of  hydrated  manganese 
dioxide,  to  the  deep  violet-red  of  the  permanganate.  When  it  is 
considered  that  the  conversion  is  complete,  allow  to  settle,  take 
out  a  small  quantity  of  the  clear  liquid,  boil  and  pass  carbon 
dioxide  through  it.  If  a  precipitate  forms,  the  conversion  is  not 
yet  complete. 

*  Or,  instead  of  the  solution,  use  10  parts  of  the  hydroxide  KOH.  In  this 
case  fuse  the  potash  and  the  chlorate  together  first,  and  then  project  the  man- 
ganese into  the  crucible. 

\  Jahresbericht  von  KOPP  uud  WILL.  1858.  581. 


146  REAGENTS.  [§  65. 

The  following  method,  recommended  by  STAEDELER,*  is  more 
rapid  :  Powder  the  fused  mass  and  macerate  it  with  an  equal  weight 
of  cold  water  in  a  flask  ;  then  add  an  equal  quantity  of  water,  and 
conduct  chlorine  into  the  mixture  until  the  latter  has  lost  its  green 
color  and  become  a  pure  red.  Then  dilute  with  four  times  its  vol- 
ume of  water  and  allow  to  settle.  By  this  method  the  yield  of 
potassium  permanganate  is  increased  one-half,  because  no  manga- 
nese dioxide  is  precipitated.  On  the  other  hand,  if  the  fused 
mass  contains  potassium  hydroxide  in  excess,  potassium  chlorate 
may  form,  and  which  will  render  somewhat  difficult  the  purifica- 
tion of  the  permanganate  crystals. 

The  clear,  red  solution,  obtained  by  any  one  of  the  above 
methods,  is  decanted  from  the  precipitate,  the  latter  washed  by 
decantation,  and  the  united  liquids  evaporated  over  an  open  fire 
to  the  crystallizing  point,  and  then  allowed  to  cool.  The  mother- 
liquor,  on  evaporation,  yields  a  new  crop  of  crystals.  The  last 
mother-liquor  contains  much  potassium  chloride,  hence  it  can  only 
be  employed  for  the  preparation  of  manganese  dioxide.  If  the 
crystals  obtained  are  not  sufficiently  pure,  they  may  be  readily 
purified  by  recrystallization.  They  are  freed  from  adhering 
in  other-liquor  by  exposure  on  a  block  of  plaster  of  Paris.  The 
solution  of  potassium  permanganate  may  be  filtered,  if  necessary, 
through  gun-cotton,  asbestos  or  previously  ignited  sand. 

4.   AMMONIUM  FERROUS  SULPHATE. 


FR.  MOHR  has  proposed  to  employ  this  double  salt,  which  is 
not  liable  to  efflorescence  and  oxidation,  as  an  agent  to  determine 
the  strength  of  the  permanganate  solution. 

Preparation.  —  Take  two  equal  portions  of  dilute  sulphuric 
acid,  ,and  warm  the  one  with  a  moderate  excess  of  small  iron 
nails  free  from  rust,  until  the  evolution  of  hydrogen  gas  has  alto- 
gether or  very  nearly  ceased  ;  neutralize  the  other  portion  exactly 
with  ammonium  carbonate,  and  then  add  to  it  a  few  drops  of  dihue 
sulphuric  acid.  Filter  the  solution  of  the  ferrous  sulphate  into  that 
of  the  ammonium  sulphate,  evaporate  the  mixture  a  little,  if  neces- 
sary, and  then  allow  the  salt  to  crystallize.  Let  the  crystals,  which 
are  hard  and  of  a  pale-green  color,  drain  in  a  funnel,  then  wash 

*  Journ.  f.  prakt.  Chem.,  cur,  107. 


§  65.]  REAGENTS.  147 

them  in  a  little  water,  dry  thoroughly  on  blotting-paper  in  the  air, 
and  keep  for  use. 

The  molecular  weight  of  the  salt  (892  '376)  is  almost  exactly 

7  times  the  atomic  weight  of  iron  (55  -9).      The  solution  of  the 

salt   in  water  which   has   been  just   acidified  with   sulphuric  acid 

must  not  become  red  on  the  addition  of  potassium  sulphocyanate. 

5.    AMMONIA-  IRON-ALUM. 


Preparation.  —  Bring  into  a  large  porcelain  dish  58  grins,  of 
pure  crystallized  ferrous  sulphate  (see  Fresenius'  "Qual.  Anal." 
Am.  ed.,  p.  73),  together  with  a  quantity  of  sulphuric  acid  equiva- 
lent to  8'3  grms.  of  sulphuric  anhydride  (SO,),  (see  Table,  §  191). 
Heat  upon  a  sand-bath,  adding  nitric  acid  from  time  to  time,  in 
small  portions,  until  the  iron  has  all  passed  into  ferric  sulphate,  or 
until  a  drop  of  the  solution  gives  no  blue  coloration  with  potassium 
ferricyamde.  Heat  further,  and  evaporate  until  the  excess  of 
nitric  acid  is  expelled,  then  add  14  grms.  of  ammonium  sulphate,* 
and,  if  need  be,  hot  water  sufficient  to  bring  the  salt  into  solution; 
iilter  into  a  porcelain  capsule  and  set  aside,  under  cover,  to  crys- 
tallize. 

The  iron-alum  separates  in  cubo-octahedrons,  wh\ch  may  be  yel- 
lowish, lilac,  or  colorless.  If  dark  in  color,  dissolve  in  wrarm  water, 
add  a  few  drops  of  oil  of  vitriol,  and  crystallize  again.  Rinse  the 
pale  or  colorless  crystals,  after  separation  from  the  mother-liquor, 
with  cold  water,  wrap  up  closely  in  iilter  paper,  and  allow  them  to 
dry  at  the  ordinary  temperature.f 

*  If  not  on  hand,  this  salt  may  be  prepared  by  saturating  sulphuric  acid 
with  ammonium  carbonate  and  evaporating  to  dryness.  30  grammes  of  sul- 
phuric acid  give  somewhat  more  than  is  required  above. 

t  Examinations  of  iron-alum  thus  prepared  show  that  the  variations  in  the 
color  of  the  salt,  from  colorless  to  rose,  are  not  connected  with  appreciable 
differences  of  composition. 

J.  H.  Grove,  of  the  Sheffield  Laboratory,  obtained  the  following  results  in  the 
examination  of  ammonia-iron-alum  crystals,  the  ferric  oxide  being  estimated  by 
ignition  :  — 


Fe2O3 

'    ( 

16-59 

1st 

j 
•< 

16'55 

\ 

16-59 

2d 

16-53 

3d 

16-57 

4th 

16-57 

5th 

16-58 

6th 

j 

16-50 
16-56 

7th 

1655 

Calculated 

16-60 

148  KEAGENTS.  [§  65. 

The  yield  should  be  about  80  grms.  The  dry  salt  should 
be  pulverized,  pressed  between  folds  of  paper  until  freed  from 
mechanically  adhering  water,  and  preserved  in  a  wrell-stoppered 
bottle. 

Uses. — Ammonia-iron-alum  furnishes  the  best  means  of  obtain- 
ing a  definite  quantity  of  iron  in  a  ferric  salt  for  making  standard 
solutions,  being  easily  obtained  pure  and  inalterable  if  kept 
away  from  acid  vapors.  Its  purity  may  be  readily  controlled  by 
ascertaining  the  loss  on  careful  ignition,  which  should  leave  a  resi- 
due of  16 '6  per  cent,  of  ferric  oxide,  corresponding  to  11 '62  per 
cent,  of  metallic  iron. 

6.  PURE  IODINE. 

Preparation. — Triturate  iodine  of  commerce  with  -J-  part  of  its 
weight  of  potassium  iodide,  dry  the  mass  in  a  large  watch-glass  with 
ground  rim,  warm  this  gently  on  a  sand-bath,  or  on  an  iron  plate, 
and  as  soon  as  violet  fumes  begin  to  escape,  cover  it  with  another 
watch-glass  of  the  same  size.  Continue  the  application  of  heat 
until  all  the  iodine  is  sublimed,  and  keep  in  a  well-closed  glass 
bottle.  The  chlorine  or  bromine,  which  is  often  found  in  iodine 
of  commerce,  combines,  in  this  process,  with  the  potassium,  and 
remains  in  the  lower  watch-glass,  together  with  the  excess  of 
potassium  iodide. 

Tests. — Iodine  purified  by  the  process  just  now  described,  must 
leave  no  fixed  residue  when  heated  on  a  watch-glass.  But,  even 
supposing  it  should  leave  a  trace  on  the  glass,  it  would  be  of  no 
great  consequence,  as  the  small  portion  intended  for  use  has  to  be 
resublimed  immediately  before  weighing. 

Uses. — Pure  iodine  is  used  to  determine  the  amount  of  iodine 
contained  in  the  solution  of  iodine  in  potassium  iodide,  employed 
in  many  volumetric  processes. 

7.  POTASSIUM  IODIDE. 

Small  quantities  of  this  article  may  be  procured  cheaper  in 
commerce  than  prepared  in  the  laboratory.  For  the  preparation  of 
potassium  iodide  intended  for  analytical  purposes  I  recommend 
BAUP'S  method,  improved  by  FKEDEKKING,  because  the  product 
obtained  by  tjiis  process  is  free  from  iodic  acid. 

Tests. — Put  a  sample  of  the  salt  in  dilute  sulphuric  acid.  If 
the  iodide  is  pure,  it  will  dissolve  without  coloring  the  fluid ;  but 
if  it  contain  potassium  iodate,  the  fluid  will  acquire  a  brown  tint, 


§  65.]  REAGENTS.  149 

from  the  presence  of  free  iodine,  the  sulphuric  acid  setting  free 
iodic  and  hydriodic  acids  which  react  on  each  other  (HIO,  -f-  5HI» 
=  3H,O  +  61)  with  liberation  of  iodine  which  remains  in  solution. 
Mix  the  solution  of  another  sample  with  silver  nitrate,  as  long  as 
a  precipitate  continues  to  form ;  add  solution  of  ammonia  in  excess, 
shake  the  mixture,  filter,  and  supersaturate  the  filtrate  with  nitric 
acid.  The  formation  of  a  white,  curdy  precipitate  indicates  the 
presence  of  chloride  in  the  potassium  iodide.  Presence  of  potassium 
sulphate  is  detected  by  means  of  solution  of  barium  chloride, 
with  addition  of  some  hydrochloric  acid. 

Uses. — Potassium  iodide  is  used  as  a  solvent  for  iodine  in  the 
preparation  of  standard  solutions  of  iodine ;  it  is  employed  also  to 
absorb  free  chlorine.  In  the  latter  case  every  atom  of  chlorine  lib- 
erates an  atom  of  iodine,  which  is  retained  in  solution  by  the  agency 
of  the  excess  of  potassium  iodide.  The  potassium  iodide  intended 
for  these  uses  must  be  free  from  potassium  iodate  and  carbonate; 
the  presence  of  trifling  traces  of  potassium  chloride  or  potassium 
sulphate  is  of  no  consequence. 

If  a  potassium- iodide  solution  of  accurately  known  strength 
is  to  be  made,  the  salt  must  be  dried  before  weighing.  This  may 
be  accomplished  by  exposing  it  in  powdered  form  to  a  tempera- 
ture of  180°  until  its  weight  is  constant.  Exposure  to  a  tempera- 
ture much  above  200°  is  to  be  avoided,  as  then  potassium  iodate 
is  likely  to  be  formed,  and  this  would  render  the  iodide  impure. 
(PETTERSSON,  Zeitschr.  f.  analyt.  Chem.,  ix,  362). 

8.  SULPHUROUS  ACID. 

Preparation. ^-The  sulphurous  acid  gas  evolved  by  the  action 
of  sulphuric  acid  on  copper  turnings  (see  "Qual.  Anal.")  is 
washed  and  then  passed  into  water  until  the  latter  is  saturated. 
The  solution  is  best  preserved  in  small,  well- stoppered  bottles. 

This  concentrated  solution  serves  for  preparing  the  diluted 
solution  of  sulphurous  acid  used  in  BUNSEN'S  method  of  estimat- 
ing iodine. 

9.  ARSENOUS  OXIDE  (As2O3).    • 

The  arsenous  oxide  sold  in  the  shops  in  large  pieces,  externally 
opaque,  but  often  still  vitreous  within,  is  generally  quite  pure. 
The  purity  of  the  article  is  tested  by  moderately  heating  it  in  a 
glass  tube,  open  at  both  ends,  through  which  a  feeble  current  of 
air  is  transmitted.  Pure  arsenous  oxide  must  completely  volatilize 
in  this  process;  no  residue  must  be  left  in  the  tube  upon  the 
expulsion  of  the  sublimate  from  it.  If  a  non-volatile  residue  is  left 


150  REAGENTS.  [§  65. 

which,  when  heated  in  a  current  of  hydrogen  gas,  turns  black,  the 
arsenous  oxide  contains  antimony  trioxide,  and  is  unfit  for  use  in 
analytical  processes.  Dissolve  about  10  grms.  of  the  arsenous 
oxide  to  be  tested  in  soda,  and  add  1  to  2  drops  lead-acetate  solu- 
tion. If  a  brownish  color  develops,  the  arsenous  oxide  contains 
arsenous  sulphide  and  cannot  be  used.  Arsenous  oxide  dissolves 
in  a  solution  of  sodium  carbonate,  forming  sodium  arsenite,  which 
is  used  to  determine  hypochlorous  acid,  free  chlorine,  iodine,  &c. 
10.  SODIUM  CHLORIDE. 

Perfectly  pure  rock-salt  is  best  suited  for  analytical  purposes. 
It  must  dissolve  in  water  to  a  clear  fluid ;  ammonium  oxalate, 
sodium  phosphate,  and  barium  chloride  must  not  render  the  solu- 
tion cloudy.  Pure  sodium  chloride  may  be  prepared  also  by 
MARGUERITTE'S  process,  viz.,  conduct  into  a  concentrated  solution 
of  common  salt  hydrochloric  gas  to  saturation,  collect  the  small 
crystals  of  sodium  chloride  which  separate  on  a  funnel,  let  them' 
thoroughly  drain,  wash  with  hydrochloric  acid,  and  dry  the  sodium 
chloride  finally  in  a  porcelain  dish,  until  the  hydrochloric  acid 
adhering  to  it  has  completely  evaporated.  The  mother-liquor 
which  contains  the  small  quantities  of  calcium  sulphate,  magne- 
sium chloride,  &c.,  originally  present  in  the  salt,  is  used  instead 
of  a  corresponding  quantity  of  water,  when  next  preparing  hydro- 
chloric acid. 

Uses. — Sodium  chloride  serves  as  a  volumetric  precipitating 
agent  in  the  determination  of  silver,  and  also  to  determine  the 
strength  of  solutions  of  silver  intended  for  the  estimation  of  chlo- 
rine. We  usually  fuse  it  before  weighing.  The  operation  must 
be  conducted  with  caution,  and  must  not  be  continued  longer  than 
necessary ;  for  if  the  gas-flame  acts  on  the  salt,  hydrochloric  acid 
escapes,  while  sodium  carbonate  is  formed. 
11.  METALLIC  SILVER. 

The  silver  obtained  by  the  proper  reduction  of  the  pure  chlo- 
ride of  the  metal  alone  can  be  called  chemically  pure.  The  silver 
precipitated  by  copper  is  never  absolutely  pure,  but  contains  gener- 
ally about  TfiVff  of  copper. 

Chemically  pure  silver  is  only  used  in  small  quantity  for  stand- 
ardizing the  NaCl  solution  employed  for  the  determination  of 
silver.  The  solution  of  silver  required  for  the  estimation  of 
chlorine  need  not  be  made  with  absolutely  pure  silver,  as  the 
strength  of  this  solution  had  always  best  be  determined  after  the 
preparation,  by  means  of  pure  sodium  chloride. 


§  66.]  REAGENTS.  151 

D.    REAGENTS  USED  IN  ORGANIC  ANALYSIS. 


1.  CUPRIC  OXIDE. 

Preparation. — Stir  pure*  copper  scales  (which  should  first  be 
ignited  in  a  muffle)  with  pure  nitric  acid  in  a  porcelain  dish  to  a 
thick  paste  ;  after  the  effervescence  has  ceased,  heat  gently  on  the 
sand-bath  until  the  mass  is  perfectly  dry.  Transfer  the  green  basic 
salt  produced  to  a  Hessian  crucible,  and  heat  to  a  moderate  redness, 
until  no  more  fames  of  hyponitric  acid  escape  ;  this  may  be  known 
by  the  smell,  or  by  introducing  a  small  portion  of  the  mass  into  a 
test  tube,  closing  the  latter  with  the  finger,  heating  to  redness,  and 
then  looking  through  the  tube  lengthways.  The  uniform  decom- 
position of  the  salt  in  the  crucible  may  be  promoted  by  stirring 
the  mass  from  time  to  time  with  a  hot  glass  rod.  When  the  cruci- 
ble has  cooled  a  little,  reduce  the  mass,  which  now  consists  of  pure 
cupric  oxide,  to  a  tolerably  fine  powder,  by  triturating  it  in  a  brass 
or  porcelain  mortar ;  pass  through  a  metal  sieve,  and  keep  in  a 
well-stoppered  bottle  for  use.  It  is  always  advisable  to  leave  a 
small  portion  of  the  oxide  in  the  crucible,  and  to  expose  this  again 
to  an  intense  red  heat.  This  agglutinated  portion  is  not  pounded, 
but  simply  broken  into  small  fragments. 

Another  method  is  to  dissolve  pure  copper  in  pure  nitric  acid, 
evaporate  to  dryness  in  a  porcelain  dish,  ignite  the  copper  nitrate 
thus  obtained  in  a  Hessian  crucible  until  no  fumes  arise  on  stirring 
the  top  of  the  mass  with  a  rod.  A  portion  in  the  bottom  of  the 
crucible  will  be  sintered  if  a  proper  heat  has  been  applied,  while 
the  upper  part  will  be  pulverulent.  Treat  the  sintered  portion  as 
above,  and  reserve  each  separately.  This  method  gives  a  reliable 
product. 

Tests. — Pure  cupric  oxide  is  a  compact,  heavy,  deep-black  pow- 
der, gritty  to  the  touch  ;  upon  exposure  to  a  red  heat  it  must  evolve 
no  hyponitric  acid  fumes,  nor  carbon  dioxide ;  the  latter  would 
indicate  presence  of  fragments  of  charcoal,  or  particles  of  dust.  It 
must  contain  nothing  soluble  in  water.  That  portion  of  the  oxide 
which  has  been  exposed  to  an  intense  red  heat  should  be  hard, 
and  of  a  grayish-black  color. 

*  If  the  scales  contain  lime,  digest  them  with  water,  containing  a  little  nitric 
acid,  for  a  long  time,  wash,  and  then  proceed  as  above. 


l.)2  REAGENTS.  [§  66. 

Uses. — Cupric  oxide  serves  to  oxidize  the  carbon  and  hydrogen 
of  organic  substances,  yielding  up  its  oxygen  wholly  or  in  part, 
according  to  circumstances.  That  portion  of  the  oxide  which  has 
been  heated  to  the  most  intense  redness  is  particularly  useful  in  the 
analysis  of  volatile  fluids. 

N.B.  The  cupric  oxide,  after  use,  may  be  regenerated  by  oxi- 
dation with  nitric  acid,  and  subsequent  ignition.  Should  it  have 
become  mixed  with  alkali  salts  in  the  course  of  the  analytical  pro- 
cess, it  is  first  digested  with  very  dilute  cold  nitric  acid,  and  washed 
afterwards  with  water.  To  purify  cupric  oxide  containing  chlo- 
ride, E.  ERLENMEYER  recommends  to  ignite  it  in  a  tube,  first  in  a 
stream  of  moist  air,  and  finally,  when  the  escaping  gas  ceases  to 
redden  litmus  paper,  in  dry  air.  By  these  operations  any  oxides 
of  nitrogen  that  may  have  remained  are  also  removed. 

2.  LEAD  CHROMATE. 

Preparation. — Precipitate  a  clear  filtered  solution  of  lead  ace- 
tate, slightly  acidulated  with  acetic  acid,  with  a  small  excess  oi 
potassium  dichromate  ;  wash  the  precipitate  by  decantation,  and  at 
last  on  a  linen  strainer ;  dry,  put  in  a  Hessian  crucible,  and  heat  to 
bright  redness  until  the  mass  is  fairly  in  fusion.  Pour  out  upon  a 
stone  slab  or  iron  plate,  break,  pulverize,  pass  through  a  fine 
metallic  sieve,  and  keep  the  tolerably  fine  powder  for  use. 

Tests. — Lead  chromate  is  a  heavy  powder,  of  a  dirty  yellowish- 
brown  color.  It  must  evolve  no  carbon  dioxide  upon  the  applica- 
tion of  a  red  heat ;  the  evolution  of  carbon  dioxide  would  indicate 
contamination  with  organic  matter,  dust,  &c.  It  must  contain 
nothing  soluble  in  water. 

Uses. — Lead  chromate  serves,  the  same  as  cupric  oxide,  for 
the  combustion  of  organic  substances.  It  is  converted,  in  the  pro- 
cess of  combustion,  into  chromic  oxide  and  basic  lead  chromate. 
It  suffers  the  same  decomposition,  with  evolution  of  oxygen,  when 
heated  by  itself  above  its  point  of  fusion.  The  property  of  lead 
chromate  to  fuse  at  a  red  heat  renders  it  preferable  to  cupric  oxide 
as  an  oxidizing  agent,  in  cases  where  we  have  to  act  upon  difficultly 
combustible  substances. 

N.B.  Lead  chromate  may  be  used  a  second  time.  For  this 
purpose  it  is  fused  again  (being  first  roasted,  if  necessary),  and 
then  powdered.  After  having  been  twice  used  it  is  powdered, 
moistened  with  nitric  acidj  dried,  and  fused.  In  this  way  the 


§  66.]  REAGENTS.  153 

lead   chromate   may   be   used   over   and   over   again   indefinitely 

(VOHL*). 

3.  OXYGEN    GAS. 

Preparation. — Triturate  100  grammes  of  potassium  chlorate 
with  5  grammes  of  finely  pulverized  manganese  binoxide,  and 
introduce  the  mixture  into  a  plain  retort,  which  must  not  be  more 
than  half  full ;  expose  the  retort  over  a  charcoal  fire  or  a  gas-lamp, 
at  first  to  a  gentle,  and  then  to  a  gradually  increased  heat.  As 
soon  as  the  salt  begins  to  fuse,  shake  the  retort  a  little,  that  the 
contents  may  be  uniformly  heated.  The  evolution  of  oxygen 
speedily  commences,  and  proceeds  rapidly  at  a  relatively  low  tem- 
perature, provided  the  above  proportions  be  adhered  to.  As  soon 
as  the  air  is  expelled  from  the  retort,  connect  the  glass  tube  fixed 
in  the  neck  of  the  retort  by  means  of  a  tight-fitting  cork,  with  an 
india-rubber  tube  inserted  in  the  lower  orifice  of  the  gasometer ; 
the  glass  tube  must  be  sufficiently  wide,  and  there  must  be  sufficient 
space  left  around  the  india-rubber  to  permit  the  free  efflux  of  dis- 
placed water.  Continue  the  application  of  heat  to  the  retort  till 
the  evolution  of  gas  has  ceased.  100  grammes  of  potassium 
chlorate  give  about  27  litres  of  oxygen. 

The  oxygen  produced  by  this  process  is  moist,  and  may  con- 
tain traces  of  carbon  dioxide,  and  also  of  chlorine.  These  impuri- 
ties must  be  removed  and  the  oxygen  thoroughly  dried,  before  it 
can  be  used  in  organic  analysis.  The  gas  is  therefore  passed  from 
the  gasometer  first  through  a  solution  of  potassa  of  1*27  sp.  gr., 
then  through  U  tubes  containing  granulated  soda  lime,  and  finally  > 
according  to  circumstances,  through  II  tubes  containing  calcium 
chloride  or  pumice-stone  moistened  with  sulphuric  acid. 

Tests. — A  chip  of  wood  which  has  been  kindled  and  blown  out 
so  as  to  leave  a  spark  at  the  extremity  must  immediately  burst  into 
flame  in  oxygen  gas.  The  gas  must  not  render  lime-water  or  a 
solution  of  silver  nitrate  turbid  when  transmitted  through  these 
fluids. 

4.  SODA-LIME. 

Preparation. — T^ke  solution  of  soda  (]S"aOH),  ascertain  its 
specific  gravity,  weigh  out  a  certain  quantity,  calculate  the  weight  of 
sodium  hydroxide  present,  add  twice  this  latter  weight  of  the  best 
quick-lime,  allow  the  lime  to  slake,  and  then  evaporate  to  drym-» 

*  Annalen  d.  CJiem.  u.  Pharm.,  cvi,  127. 


154  EEAGENTS.  [§  66. 

in  an  iron  vessel.  Heat  the  residue  in  an  iron  or  Hessian  crucible ; 
keep  for  some  time  at  a  low  red  heat.  Break  up  while  still  warm 
in  an  iron  mortar,  and  pass  the  whole  through  a  sieve  with  meshes 
about  3  mm.  wide.  Reject  the  finest  portion  (removing  it  with 
a  fine  sieve  having  2  mm.  meshes)  and  keep  the  granulated  prod- 
uct in  a  well-closed  bottle. 

Tests. — Soda-lime  must  not  effervesce  much  when  treated 
with  dilute  hydrochloric  acid ;  nor  should  it,  more  particularly, 
•evolve  ammonia  when  mixed  with  pure  sugar  and  ignited. 

Use. — Granulated  soda-lime  prepared  as  above  described  forms 
an  excellent  absorbent  for  carbon  dioxide.  It  was  formerly  also 
used  for  nitrogen  determination  instead  of  the  following  : 

5.  SODA-LIME  FOR  NITROGEN  DETERMINATIONS.* 
Preparation. — Equal  weights  of   sal-soda   in   clean    (washed) 

large  crystals  and  of  good  white  promptly  slaking  quick-lime  are 
separately  so  far  pulverized  as  to  pass  through  holes  of  -^w  inch, 
then  well  mixed  together,  placed  in  an  iron  pot  which  should  not 
be  more  than  half  filled,  and  gently  heated,  at  first  without  stir- 
ring. The  lime  soon  begins  to  combine  with  the  crystal  water  of 
the  sodium  carbonate,  the  whole  mass  heats  strongly,  swells  up,  and 
in  a  short  time  yields  a  fine  powder,  which  may  then  be  stirred  to 
effect  intimate  mixture  and  to  drive  off  the  excess  of  water  so  that 
the  mass  is  not  perceptibly  moist  and  yet  short  of  the  point  at 
which  it  rises  in  dust  on  handling.  When  cold  it  is  secured  in 
well-closed  bottles  or  fruit  jars,  and  is  ready  for  use. 

6.  METALLIC  COPPER. 

Metallic  copper  serves,  in  the  analysis  of  nitrogenous  substances, 
to  effect  the  reduction  of  the  nitric  oxide  gas  that  may  form  in 
the  course  of  the  analytical  process. 

It  is  used  either  in  the  form  of  turnings,  or  copper  scales 
reduced  by  hydrogen ;  or  of  small  rolls  made  of  fine  copper  wire 
gauze.  A  length  of  from  7  to  10  centimetres  is  given  to  the 
spirals  or  rolls,  and  just  sufficient  thickness  to  admit  of  their  being 
inserted  into  the  combustion  tube.  To  have  it  perfectly  free  from 
dust,  oxide,  &c.,  it  is  first  heated  to  redness  in  the  open  air,  in  a 
crucible,  until  the  surface  is  oxidized  ;  it  is  then  put  into  a  glass  or 
porcelain  tube,  through  which  an  uninterrupted  current  of  dry 
hydrogen  gas  is  transmitted ;  and  when  all  atmospheric  air  has 
been  expelled  from  the  evolution  apparatus  and  the  tube,  the 
*  S.  W.  Johnson.  Report  of  the  Conn.  Agr.  Expr.  Station,  1878,  p.  111. 


§  66.]  REAGENTS.  155 

latter  is  in  its  whole  length  heated  to  redness.  The  operator  should 
make  sure  that  the  atmospheric  air  has  been  thoroughly  expelled, 
before  he  proceeds  to  apply  heat  to  the  tube ;  neglect  of  this  pre- 
caution may  lead  to  an  explosion. 

7.  POTASSIUM  HYDROXIDE  OR  POTASSA. 
a.  Solution  of  Potassa. 

Solution  of  potassa  is  prepared  from  the  carbonate,  with  the 
aid  of  milk  of  lime,  in  the  way  described  in  the  "  Qualitative 
Analysis,"  for  the  preparation  of  solution  of  soda.  The  propor- 
tions are — 1  part  of  potassium  carbonate  to  12  parts  of  water,  and 
•f  part  of  lime,  slaked  to  paste  with  three  times  the  quantity  of 
warm  water. 

The  decanted  clear  solutio"h  is  evaporated,  in  an  iron  vessel, 
over  a  strong  fire,  until  it  has  a  specific  gravity  of  1-27  ;  it  is  then, 
whilst  still  warm,  poured  into  a  bottle,  which  is  well  closed,  and 
allowed  to  stand  at  rest  until  all  solid  particles  have  subsided.  The 
clear  solution  is  finally  drawn  off  from  the  deposit,  and  kept  for 
use. 

J.  Fused  Potassa  (common). 

The  commercial  potassa  in  sticks  (impure  KOH  usually  com- 
bined with  more  or  less  H2O)  will  answer  the  purpose.  If  you 
wish  to  prepare  it,  evaporate  solution  of  potassa  (a)  in  a  silver  ves- 
sel, over  a  strong  fire,  until  the  residuary  hydroxide  flows  like 
oil,  and  white  fumes  begin  to  rise  from  the  surface.  Pour  the 
fused  mass  out  on  a  clean  iron  plate,  and  break  it  up  into  small 
pieces.  Keep  in  a  well-stoppered  bottle  for  use. 

c.  Potassa  (purified  with  alcohol),  see  "  Qual.  Anal.,"  p.  43. 

Uses. — Solution  of  potassa  serves  for  the  absorption,  and  at 
the  same  time  for  the  estimation  of  carbon  dioxide.  In  many 
cases,  a  tube  filled  with  fragments  of  fused  potassa  is  used,  in 
addition  to  the  apparatus  filled  with  solution  of  potassa.  Potassa 
purified  with  alcohol^  which  is  perfectly  free  from  potassium  sul- 
phate, is  employed  for  the  determination  of  sulphur  in  organic 
substances. 

8.  CALCIUM  CHLORIDE. 

a.  Pure  Calcium  Chloride. 

Preparation. — Dissolve  marble  in  commercial  hydrochloric 
acid  diluted  with  four  or  five  times  its  volume  of  water.  (The 
waste  solution  resulting  from  the  preparation  of  carbon  dioxide 


156  REAGENTS.  [§  66- 

may  be  used.)  Add  to  this  solution  with  stirring  lime,  flaked 
with  sufficient  water  to  give  it  the  consistency  of  thin  paste  until 
it  gives  an  alkaline  reaction  and  a  pellicle  of  calcium  carbonate 
forms  on  the  surface  on  standing  exposed  to  the  air.  Iron,  man- 
ganese, and  especially  magnesium  are  usually  present  in  such  a 
solution,  and  are  precipitated  by  the  calcium  hydroxide — the  iron, 
however,  not  completely.  After  a  few  hours,  filter  and  pass  hydro- 
gen sulphide  through  the  alkaline  solution  until  a  filtered  portion 
is  no  longer  blackened  by  this  reagent.  Let  the  solution  stand  for 
twelve  hours,  then  filter  from  the  iron  sulphide.  Add  next  hydro- 
chloric acid  to  strongly  acid  reaction  to  convert  the  calcium  sul- 
phide and  calcium  oxychloride  which  may  be  present  into  chloride. 
Concentrate  in  a  porcelain  dish.  If  sulphur  separates,  after  a  short 
time  filter  again,  and  continue  the  evaporation  to  dry  ness  with 
addition  of  a  little  more  hydrochloric  acid  toward  the  end  of  the 
process.  Finally  expose  the  residue  to  a  tolerably  strong  heat 
about  (200°)  on  the  sand-bath,  until  it  is  changed  throughout  to  a 
white  porous  perfectly  opaque  mass,  which  point  can  be  ascertained 
by  breaking  up  a  piece  detached  from  the  top.  The  product  is 
CaCl2-f-  2HaO.  Reduce  while  still  hot  to  granules  of  the  proper 
size  (•§•  to  ^  of  an  inch)  by  means  of  suitable  sieves  and  a  mortar 
previously  warmed,  and  keep  in  well-closed  bottles. 

b.   Crude  fused  Calcium  Chloride. 

Preparation.— Neutralize  the  alkaline  solution  obtained  in  a 
(without  separating  the  little  iron  present  with  H2S)  exactly  with 
hydrochloric  acid,  and  evaporate  to  dryness  in  an  iron  pan  ;  fuse 
the  residue  in  an  iron  or  Hessian  crucible,  pour  out  the  fused  mass> 
and  break  into  pieces.  Preserve  it  in  well-stoppered  bottles. 

Uses. — The  crude  fused  calcium  chloride  serves  to  dry  moist 
gases ;  the  pure  chloride  is  used  in  elementary  organic  analysis  for 
the  absorption  and  estimation  of  water  formed  by  the  hydrogen 
contained  in  the  analyzed  substance.  A  solution  of  the  pure  cal- 
cium chloride  should  not  show  an  alkaline -reaction.  A  calcium 
chloride  tube  filled  with  it  should  not  gain  weight  when  a  very 
slow  current  of  perfectly  dry  carbon  dioxide  is  passed  through  it 
an  hour. 

9.  POTASSIUM  BICHROMATE. 

Potassium  dichromate  of  commerce  is  purified  by  repeated 
recrystallization,  until  barium  chloride  produces,  in  the  solution  of 


§  66.]  REAGENTS.  157 

a  sample  of  it  in  water,  a  precipitate  which  completely  dissolves  in 
hydrochloric  acid.  Potassium  dichromate  thus  perfectly  free  from 
sulphuric  acid  is  required  more  particularly  for  the  oxidation  of 
organic  substances  with  a  view  to  the  estimation  of  the  sulphur 
contained  in  them.  Where  the  salt  is  intended  for  other  purposes, 
e.g.,  to  determine  the  carbon  of  organic  bodies,  by  heating  them 
with  potassium  dichromate  and  sulphuric  acid,  one  recrystallizatioa 
is  sufficient. 


SECTION    III. 

FOKMS  AND  COMBINATIONS  IN  WHICH  SUB- 
STANCES ARE  SEPARATED  FROM  EACH  OTHER, 
OR  IN  WHICH  THEIR  WEIGHT  IS  DETERMINED. 

§67. 

THE  quantitative  analysis  of  a  compound  substance  requires, 
as  the  first  and  most  indispensable  condition,  a  correct  and  accurate 
knowledge  of  the  composition  and  properties  of  the  new  combina- 
tions into  which  it  is  intended  to  convert  its  several  individual 
constituents,  for  the  purpose  of  separating  them  from  one  another, 
and  determining  their  several  weights.  Regarding  the  properties 
of  the  new  compounds,  we  have  to  inquire  more  particularly,  in 
the  first  place,  how  they  behave  with  solvents ;  secondly,  what  is 
their  deportment  in  the  air ;  and,  thirdly,  what  is  their  behavior  on 
ignition  ?  It  may  be  laid  down  as  a  general  rule,  that  compounds 
are  the  better  adapted  for  quantitative  determination  the  more 
insoluble  they  are,  and  the  less  alteration  they  undergo  upon 
exposure  to  air  or  to  a  high  temperature. 

The  composition  of  a  substance  is  expressed  either  in  per  cents 
or  in  stoichiometrical  formulas ;  the  latter  enable  the  composition 
of  the  more  frequently  recurring  compounds  to  be  readily  re- 
membered. In  this  section  the  chemical  formula  is  stated  in  the 
first  column ;  the  second  gives  the  equivalents  (O  =  1 6),  while  the 
third  gives  the  percentage  composition. 

A  compound  is  the  better  adapted  for  quantitative  determina- 
tion, with  respect  to  its  composition,  the  less  it  contains  relatively 
of  the  substance  to  be  determined,  since  every  operative  error, 
loss,  or  inaccuracy  is  distributed  over  a  larger  mass  on  weighing, 
and  the  error  will,  hence,  be  so  much  the  less  for  the  substance  to 
be  determined.  Thus  platinum-ammonium  chloride  is  better 
adapted  for  the  estimation  of  nitrogen  than  is  ammonium  chloride 
because  the  former  contains  only  6*295  per  cent,  of  nitrogen, 
whereas  the  latter  contains  26*24  per  cent. 

1158 


§  67.]  FORMULAE.  159 

Suppose  we  analyze  a  nitrogenous  compound,  and,  with  abso- 
lutely accurate  manipulation,  obtain  1  grai.  of  platinum-ammo- 
nium chloride  from  0*3  grm.  of  the  compound.  100  parts  of  this 
platinum  salt  contain  6-295  parts  of  nitrogen,  hence  1  grm.  will 
contain  0 '06295  parts.  Since  this  is  afforded  by  0*3  grm.  of  the 
substance,  it  follows  that  100  parts  of  the  latter  will  contain 
20-983  parts  of  nitrogen. 

Let  us  now  make  a  second  analysis,  and  convert  the  nitrogen 
into  ammonium  chloride.  "Working  with  equal  accuracy  we  ob- 
tain from  0-3  grm.  of  the  substance  0*2399  grm.  ammonium, 
chloride,  corresponding  to  0*06295  grm.  nitrogen,  or  20*983  per 
cent.  Assuming,  now,  that  in  both  analyses  a  loss  of  10  milli- 
grammes had  occurred,  we  would  have  obtained  in  the  first  opera- 
tion only  0'99  parts  'of  platinum-ammonium  chloride  instead  of 
100,  corresponding  to  0-06232  nitrogen,  or  20*77  per  cent.  The 
loss  would,  hence,  have  been  0*213  per  cent.  In  the  case  of 
ammonium  chloride,  however,  we  would  have  obtained  0*2299 
parts  instead  of  0*2399,  corresponding  to  0-0603  nitrogen,  or 
20'1  per  cent — a  loss  of  0-873  per  cent.  We  thus  see  that  a 
similar  error  would  occasion  in  one  case  a  loss  in  nitrogen  of  0*213 
per  cent,  while  in  the  other  the  loss  would  be  0-873  per  cent. 

Having  thus  touched  generally  upon  the  requirements  a  com- 
pound must  possess  in  order  to  be  adapted  for  quantitative  ex- 
amination, we  will  proceed  to  enumerate  those  compounds  best 
adapted  and  which  are  as  a  rule  employed.  Of  course  the  de- 
scription of  the  external  form  and  appearance  relates  more  par- 
ticularly to  the  condition  in  which  they  are  obtained  in  our  analy- 
sis. In  enumerating  tho  properties  of  substances  reference  will 
be  had  exclusively  to  those  which  bear  directly  upon  the  object 
immediately  in  view. 

[The  percentage  compositions  of  these  compounds  are  stated  in 
connection  with  their  description.  For  this  purpose  the  symbols 
of  the  constituent  elements  of  the  compounds  in  many  cases 
(viz.  :  when  they  are  oxygen  salts)  are  grouped  in  a  manner 
different  from  that  used  to  express  their  chemical  constitution. 
This  grouping  constitutes  a  kind  of  formulae  differing  from  either 
the  empirical  or  rational  in  ordinary  use  in  modern  text-books  of 
chemistry,  but  identical  with  that  formerly  in  general  use  (the  old 
system).  These  formulae  are  based  upon  the  fact  that  in  all 
oxygen  salts,  whether  normal,  acid,  basic,  ortho-,  meta-,  or  pyro~~ 


160  FORMS.  [§  67. 

salts,  there  is  just  enough  oxygen  to  form  with  the  radicals  present, 
both  basic  and  acid,  their  corresponding  oxides  or  anhydrides,  and 
with  hydrogen,  if  present,  water.  They  represent  oxides  (and 
water)  jointly  equivalent  in  weight  to  the  radicals,  hydrogen,  and 
remaining  oxygen,  which  rational  formulae  represent  as  existing  in 
oxygen  salts 

EXAMPLES. 

Potassium  sulphate,     SO2  <  QK  =  KASO»- 
Hydrogen  potassium  sulphate, 


Potassium  disulphate, 


0  <  18  i  OK  =  K°°'2SO" 


Ammonium  magnesium  phosphate, 

/       , 

\       \ 

Magnesium  pyrophosphate, 

/  PO  <  g  >  Mg 
O  <  =2MffO,P,O,. 

\  PO  <  g  >  Mg 

Most  analytical  chemists  prefer  to  present  the  results  of  analyses 
of  oxygen  salts  in  percentages  of  oxides  (or  anhydrides)  and  water 
on  account  of  the  simplicity  of  computations  required.  Accord- 
ingly, in  the  following  section,  the  percentage  composition  of 
oxygen  salts  is  given  in  this  manner,  accompanied  by  correspond- 
ing formulae  and  molecular  weights.  These  formulae  are  in  every 
case  preceded  by  rational  formulae  constructed  in  accordance  with 
the  theory  of  the  constitution  of  oxygen  salts  which  is  now 
generally  accepted.] 


§  68.]  BASES    OF   GROUP   I.  161 

A.     FORMS  IN  WHICH  THE  BASIC  RADICALS  ARE  WEIGHED  OR 

PRECIPITA  TED. 

BASIC  RADICALS   OF   THE  FIRST   GROUP. 

.      §  68. 

1.  POTASSIUM. 

The  combinations  best  suited  for  the  weighing  of  potassium 
•are  POTASSIUM  SULPHATE,  POTASSIUM  CHLORIDE,  and  POTASSIUM 

PLATINIC    CHLORIDE. 

a.  Potassium  sulphate  crystallizes  usually  in  small,  hard, 
straight,  four-sided  prisms,  or  in  double  six-sided  pyramids ;  in 
the  analytical  process  it  is  obtained  as  a  white  crystalline  mass. 
It  dissolves  with  some  difficulty  in  water  (1  part  requiring  10  parts 
of  water  of  12°),  it  is  almost  absolutely  insoluble  in  pure  alcohol, 
but  slightly  more  soluble  in  alcohol  containing  sulphuric  acid 
(Expt.  No.  6).  It  does  not  affect  vegetable  colors ;  it  is  unalter- 
able in  the  air.  The  crystals  decrepitate  strongly  when  heated, 
yielding  at  the  same  time  a  little  water,  which  they  hold  mechani- 
cally confined.  The  decrepitation  of  crystals  that  have  been  kept 
long  drying  is  less  marked.  At  a  good  red  heat  the  salt  fuses 
without  volatilizing  or  decomposing.  At  a  white  heat  a  little  of 
the  salt  volatilizes  and  also  some  sulphuric  acid,  so  that  the  residue 
possesses  an  alkaline  reaction  (AL.  MITSCHEKLICH,*  BoussiNGAuurf-). 
When  exposed  to  a  red  heat,  in  conjunction  with  ammonium 
chloride,  potassium  sulphate  is  partly,  and,  upon  repeated  applica- 
tion of  the  process,  wholly  converted,  with  effervescence,  into 
potassium  chloride  (H.  ROSE). 

COMPOSITION. 

qo        OK    _  KaO     .     .     .       94-22  54-06 

^3<OK=-SO8      .     .     .      80-07  45-94 

174-29  100-00 

The  acid  potassium  sulphate  (KHS04),  which  is  produced  when 
the  normal  salt  is  evaporated  to  dryness  with  free  sulphuric  acid, 
is  readily  soluble  in  water,  and  fusible  even  at  a  moderate  heat, 

*  Journ.f.  prakt.  Chem.,  IAXXIII,  486.     f  Zeitschr.f.  anal.  Chem.,  vn,  244. 


162  FORMS.  [§  68. 

At  a  red  heat  it  loses  sulphuric  acid,  and  is  converted  into  normal 
potassium  sulphate,  but  not  readily  —  the  complete  conversion  of 
the  acid  into  the  normal  salt  requiring  the  long-continued  applica- 
tion of  an  intense  red  heat.  However,  when  heated  in  an  atmos- 
phere of  ammonium  carbonate  —  which  may  be  readily  procured  by 
repeatedly  throwing  into  the  faint  red-hot  crucible  containing  the 
acid  sulphate  small  lumps  of  pure  ammonium  carbonate,  and 
putting  on  the  lid  —  the  acid  salt  changes  readily  and  quickly  to 
the  normal  sulphate.  The  transformation  may  be  considered 
complete  as  soon  as  the  salt,  which  was  so  readily  fusible  before,  is 
perfectly  solid  at  a  faint  red  heat. 

b.  Potassium  nitrate  crystallizes  ordinarily  in  the  form  of 
long,  striated  prisms.  In  analysis  it  is  obtained  as  a  white  saline 
mass.  It  is  readily  soluble  in  water,  almost  insoluble  in  absolute 
alcohol,  and  but  sparingly  soluble  in  alcohol.  It  does  not  affect 
vegetable  colors,  and  is  unchangeable  in  air.  On  being  heated  it 
melts  far  below  a  red  heat,  without  decomposition  or  loss  of 
weight.  Strongly  heated  it  evolves  oxygen,  and  becomes  con- 
verted into  potassium  nitrite  ;  intensely  heated  (to  redness)  oxy- 
gen arid  nitrogen  are  evolved,  the  residue  being  then  caustic 
potassa.  On  being  ignited  with  ammonium  chloride,  or  in  a 
current  of  dry  hydrochloric-acid  gas,  it  is  readily  and  completely 
converted  into  potassium  chloride.  Repeatedly  evaporated  with 
oxalic  acid  in  excess  (4  to  6  times),  it  is  completely  converted  into 
potassium  chloride. 

COMPOSITION. 


.  .  .  46-04     45-52 

101-15     100-00 

D.  Potassium  chloride  crystallizes  usually  in  cubes,  often 
lengthened  to  columns  ;  rarely  in  octahedra.  In  analysis  we 
obtain  it  either  in  the  former  shape,  or  as  a  crystalline  mass.  It  is 
readily  soluble  in  water,  bat  much  less  so  in  dilute  hydrochloric 
acid  ;  in  absolute  alcohol  it  is  nearly  insoluble,  and  but  slightly 
soluble  in  common  alcohol.  It  does  not  affect  vegetable  colors, 
and  is  unalterable  in  the  air.  "When  heated,  it  decrepitates,  unless 
it  has  been  kept  long  drying,  with  expulsion  of  a  little  water 


§  68.]  BASES   OF   GROUP  I.  163 

mechanically  confined  in  it.  At  a  moderate  red  heat,  it  fuses 
unaltered  and  without  diminution  of  weight ;  when  exposed  to  a 
higher  temperature,  it  volatilizes  in  white  fumes  ;  this  volatilization 
proceeds  the  more  slowly  the  more  effectually  the  access  of  air  is 
prevented  (Expt.  No.  T).  When  repeatedly  evaporated  with 
solution  of  oxalic  acid  in  excess,  it  is  converted  into  potassium 
oxalate.  "When  evaporated  with  excess  of  nitric  acid,  it  is  con- 
verted readily  and  completely  into  nitrate.  On  ignition  with 
ammonium  oxalate,  potassium  carbonate  and  potassium  cyanide 
are  formed  in  noticeable  quantities. 


COMPOSITION. 

K       ....       39-11  52-455 

Cl  35-45  47-545 


74-56  100-000 

d.  Potassium  platinic  chloride  presents  either  small  reddish- 
yellow  octahedra,  or  a  lemon-colored  -powder.  It  is  difficultly 
soluble  in  cold,  more  readily  in  hot  water;  nearly  insoluble  in 
absolute  alcohol,  and  but  sparingly  soluble  in  common  alcohol — 
one  part  requiring  for  its  solution,  respectively,  12083  parts  of 
absolute  alcohol,  3775  parts  of  alcohol  of  76  per  cent,  and 
1053  parts  of  alcohol  of  55  per  cent.  (Expt.  No.  8,  a.}  Presence 
of  free  hydrochloric  acid  sensibly  increases  the  solubility  (Expt. 
No.  8,  I).  In  caustic  potassa  it  dissolves  completely  to  a  yellow 
fluid.  It  is  unalterable  in  the  air,  and  at  100°.  On  exposure  to 
an  intense  red  heat,  four  atoms  of  chlorine  escape,  metallic  plati- 
num and  potassium  chloride  being  left ;  but  even  after  long-con- 
tinued fusion,  there  remains  always  a  little  potassium  platinic 
chloride  which  resists  decomposition.  Complete  decomposition  is 
easily  effected,  by  igniting  the  double  salt  in  a  current  of  hydrogen 
gas,  or  with  some  oxalic  acid. 

According  to  ANDREWS,  potassium  platinic  chloride,  even 
though  dried  at  a  temperature  considerably  exceeding  100°,  retains 
still  -0055  of  its  weight  of  water. 


164  FORMS.  [§  69. 

COMPOSITION. 

(KC1)3.     .     .  149-12       30-69         K,  .     .     .     78-22  16-10 

PtCL    .     .     .   336-70       69-31         Ft  ...   194-90  40-12 

01..     .     .  212-70  43  78 
485-82     100-00 

485-82  100-00 

e.  Potassium  silicofluoride  is  obtained  on  mixing  a  solution  of 
a  potassium  salt  with  hydrofluosilicic  acid  in  the  form  of  a  trans- 
lucent iridescent  precipitate,  which  increases  and  completely 
separates,  when  an  equal  volume  of  strong  alcohol  is  added  to  the 
fluid.  After  being  filtered  off,  washed  with  weak  alcohol  and  dried, 
it  is  a  soft  white  powder.  It  is  difficultly  soluble  in  cold  water,  far 
more  readily  in  boiling  water,  not  at  all  or  in  merest  traces  soluble 
in  a  mixture  of  water  and  strong  alcohol  in  equal  parts,  but  it  is 
decidedly  more  soluble  in  the  presence  of  any  considerable  quan-, 
tity  of  free  acid,  especially  hydrochloric  or  sulphuric  acid.  When 
potassa  is  added  to  the  boiling  aqueous  solution  of  the  salt  the 
following  change  takes  place :  (KF)2SiF4  +  4KOH  :  :  6KF  + 
Si(OH)4,  the  solution  turning  from  acid  to  neutral  (principle  of 
STOLBA'S  volumetric  method  of  estimating  potassium).  As  soon  as 
it  is  ignited  the  salt  fuses,  gives  off  silicon  fluoride  and  leaves 
potassium  fluoride. 

§69. 
2.  SODIUM. 

Sodium  is  usually  weighed  as  SODIUM  SULPHATE,  SODIUM  CHLO- 
RIDE, or  SODIUM  CARBONATE.  It  is  separated  from  potassium  in  the 
form  of  SODIUM  PLATINIC  CHLORIDE,  from  other  bodies  occasionally 
in  the  form  of  sodium  silicofluoride. 

a.  Anhydrous  normal  sodium  sulphate  is  a  white  powder  or  a 
white  very  friable  mass.  It  dissolves  readily  in  water ;  but  is 
sparingly  soluble  in  absolute  alcohol ;  presence  of  free  sulphuric 
acid  slightly  increases  its  solubility  in  that  menstruum ;  it  is  some- 
what more  readily  soluble  in  common  alcohol  (Expt.  No.  9).  It 
does  not  a-ffect  vegetable  colors  ;  upon  exposure  to  moist  air,  it 
slowly  absorbs  water  (Expt.  No.  10).  At  a  gentle  heat  it  is  un- 
altered, at  a  strong  red  heat  it  fuses  without  decomposition  or  loss 
of  weight.  At  a  white  heat  it  loses  weight  by  volatilization  of 
sodium  sulphate  and  also  of  sulphuric  acid  (AL.  MITSCIIERLICII, 


§  69.]  BASKS    OF   GROUP  I.  165 

BOUSSINGAULT).     When  ignited  with  ammonium  chloride,  it  be* 
haves  like  potassium  sulphate. 

COMPOSITION. 

j  ONa       Na,O  .....     62  10  43  '68 

u*  <  ONa  '—  SO3      ....     80-07  56-32 

142-17          100-00 

The  acid  sodium  sulphate  (sodium  hydrogen  sulphate,  NaHSO4) 
which  is  always  produced  upon  the  evaporation  of  a  solution  of  the 
normal  salt  with  sulphuric  acid  in  excess,  fuses  even  at  a  gentle 
heat  ;  it  may  be  readily  converted  into  the  normal  salt  in  the  same 
manner  as  the  acid  potassium  sulphate  (see  §  68,  a). 

1).  Sodium  nitrate  crystallizes  as  obtuse  rhombohedra.  In 
analysis  it  is  usually  obtained  as  an  amorphous  saline  mass.  It  is 
readily  soluble  in  water,  is  practically  insoluble  in  absolute  alcohol, 
and  is  but  very  slightly  soluble  in  alcohol.  It  is  indifferent 
towards  vegetable  colors.  Under  ordinary  circumstances  it  is 
unalterable  in  the  air,  but  attracts  moisture  from  very  moist  air. 
It  fuses  far  below  a  red  heat,  and  without  decomposition  (comp. 
Expt.  No.  11);  at  a  higher  temperature  it  is  decomposed  like 
potassium  nitrate  (§  68).  Ignited  with  ammonium  chloride,  or 
in  hydrochloric-acid  gas,  and  evaporated  with  oxalic  acid  or 
aqueous  hydrochloric  acid,  it  behaves  like  the  corresponding 
potassium  salt.  The  decomposition  with  aqueous  hydrochloric 
acid  is  more  readily  effected,  i.e.,  with  fewer  evaporations,  than 
is  the  case  with  potassium  nitrate  (BAUMHATTER  *). 

COMPOSITION. 

•NT        n       -vrXO_]SraO  .     .     .   39.05         45-89 
~  .     .      .  46-04         54-11 


85-09        100-00 
c.  Sodium  chloride  crystallizes  in  cubes,  octahedra,  and  hollow 

*  Jour.  f.  prakt.  Chem.,  LXXVIII,  213. 


166  FORMS.  [§69,. 

four-sided  pyramids.  In  analysis  it  is  frequently  obtained  as  an 
amorphous  mass.  It  dissolves  readily  in  water,  but  is  much  less 
soluble  in  hydrochloric  acid  ;  it  is  nearly  insoluble  in  absolute 
alcohol,  and  but  sparingly  soluble  in  common  alcohol ;  100  parts 
of  alcohol  of  75  per  cent,  dissolve,  at  a  temperature  of  15°,  0*7  part 
(WAGNEK).  It  is  neutral  to  vegetable  colors.  Exposed  to  a 
somewhat  moist  atmosphere,  it  slowly  absorbs  water  (Expt.  No.  12). 
Crystals  of  this  salt  that  have  not  been  kept  drying  a  considerable 
time  decrepitate  when  heated,  yielding  a  little  water,  which  they 
hold  mechanically  confined.  The  salt  fuses  at  a  red  heat  without 
decomposition ;  at  a  white  heat,  and  in  open  vessels  even  at  a 
bright  red  heat,  it  volatilizes  in  white  fumes  (Expt.  "No.  13).  If  a 
carburetted  hydrogen  flame  acts  on  fusing  sodium  chloride,  hydro- 
chloric acid  escapes,  and  some  sodium  carbonate  is  formed.  On 
evaporation  with  oxalic  or  nitric  acid  as  well  as  by  ignition  with 
ammonium  oxalate,  it  behaves  like  the  corresponding  potassium 
salt. 

COMPOSITION. 

!Na  .  .  .  .  23-05     39-40 
Cl  35-45      60  60 


58-50     100-00 

d.  Anhydrous  sodium  carbonate  is  a  white  powder  or  a  white 
very  friable  mass.     It  dissolves  readily  in  water,  but  much  less  so 
in  solution  of  ammonia  (MARGUERITTE)  ;  it  is  insoluble  in  alcohol. 
Its  reaction  is  strongly  alkaline.     Exposed  to  the  air,  it  absorbs 
water  slowly.     On  moderate  ignition  to  incipient  fusion  it  scarcely 
loses  weight ;  on  long  fusion,  however,  it  volatilizes  to  a  consider- 
able extent  (Comp.  Exp  fc.  14). 

COMPOSITION. 

.ONa_    ]STaaO  .     .     .     .     62-10  58-53 

<ONa  ~  CO,    ....     44-00  41-47 

106-10  100-00 

e.  Sodium  platinic  chloride  crystallizes  with  6  mol.  water, 
(NaCl)a.PtCl4  -f-  6H5O,    in  light  yellow,    transparent,   prismatic 
crystals  which   dissolve   readily  both   in  water  and  in  common 
alcohol. 


§  70.]  BASES    OF    GROUP    I.  167 

f.  Sodium  silicqfluoride  is  similar  in  properties  to  the  corre- 
sponding potassium  salt.  It  has  an  analogous  composition,  and  is 
decomposed  in  the  same  way  by  alkalies.  It  is,  however,  con- 
siderably more  soluble  in  water  and  in  diluted  alcohol. 


§  TO. 
3.  AMMONIUM. 

Ammonium  is  most  appropriately  weighed  as  AMMONIUM 
OHLORIDE,  or  as  AMMONIUM  pLATiNic  CHLORIDE,  or  it  may  be  esti- 
mated from  the  weight  of  the  PLATINUM  in  the  latter  compound. 

Under  certain  circumstances  ammonium  may  also  be  estimated 
from  the  volume  of  the  NITROGEN  GAS  eliminated  from  it ;  and  it 
is  frequently  estimated  by  alkalimetry. 

a.  Ammonium  chloride  crystallizes  in  cubes  and  octahedra,  but 
more  frequently  in  feathery  crystals.  In  analysis  we  obtain  it 
uniformly  as  a  white  mass.  It  dissolves  readily  in  water,  but  is 
difficultly  soluble  in  common  alcohol.  It  does  not  alter  vegetable 
colors,  and  remains  unaltered  in  the  air.  Solution  of  ammonium 
chloride,  when  evaporated  on  the  water-bath,  loses  a  small  quantity 
of  ammonia,  and  becomes  slightly  acid.  The  diminution  of  weight 
occasioned  by  this  loss  of  ammonia  is  very  trifling  (Expt.  No.  15). 
At  100°  ammonium  chloride  loses  nothing,  or  very  little  of  its 
weight  (comp.  same  Expt.).  At  a  higher  temperature  it  volatilizes 
readily,  and  without  undergoing  decomposition. 

COMPOSITION. 

NH4  .     .     18-072       33-77  NH,  .     .     17-064       31-88 

Cl.  35-450       06-23  HC1  36-458       68-12 


53-522     100-00  53'522     100-00 

100  parts  of  ammonium  chloride  correspond  to  48*72  parts  of 
ammonium  oxide. 

b.  Ammonium  platinic  chloride  occurs  either  as  a  heavy, 
lemon-colored  powder,  or  in  small,  hard  octahedral  crystals  of  a 
bright  yellow  color.  It  is  difficultly  soluble  in  cold,  but  more 
readily  in  hot  water.  It  is  very  sparingly  soluble  in  absolute 
alcohol,  but  more  readily  in  common  alcohol — 1  part  requiring  of 
absolute  alcohol,  26535  parts ;  of.  alcohol  of  76  per  cent.,  1406 


168  FORMS.  [§  71. 

parts;  of  alcohol  of  55  per  cent.,  665  parts.  The  presence  of 
free  acid  sensibly  increases  its  solubility  (Expt.  No.  16).  It 
remains  unaltered  in  the  air,  and  at  100°.  It  loses  a  little  water 
between  100°  and  125°.  Upon  ignition  chlorine  and  ammonium 
chloride  escape,  leaving  the  metallic  platinum  as  a  porous  masa 
(spongy  platinum).  However,  if  due  care  be  not  taken  in  this 
process  to  apply  the  heat  gradually,  the  escaping  fumes  will  carry 
off  particles  of  platinum,  which  will  coat  the  lid  of  the  crucible. 
For  properties  of  metallic  platinum,  see  §  89,  a. 

COMPOSITION. 

(NI-I4C1)S.  .107-044  24-12  (NH4)3  .  .  36-144   8'15 
FtCl4  .   .  .  336-700  75-88  Ft  ...  194-900  43-92 

Cle  .  .  .  212-700  47-93 

443-744  100-00 

443-744  100  00 

N,  .  .  .  .  28-080  6-33  (NH3)a  .  .  34-128  7-691 

He  ...  8-064  1-82 

Pt  ...  194-900  43-92  (HCl)f  .  .  72-916  16-432 

01.  ...  212-700  47.93  FtCl4   .  .  336-700  76-010 


443-744  100-00  443-744  100-000 

100  parts  of  ammonium  platinic  chloride  correspond  to  11  '76 
parts  of  ammonium  oxide. 

c.  Nitrogen  gas  is  colorless,  tasteless,  and  inodorous ;  it  mixes 
with  air,  without  producing  the  slightest  coloration ;  it  does  not 
affect  vegetable  colors.  Its  specific  gravity  is  0-996971.*  One 
litre  weighs  at  0°,  and  0-76  metre  bar.,  1*254035  grm.  It 
is  difficultly  soluble  in  water,  1  volume  of  water  absorbing,  at  0°, 
and  0-76  pressure,  0-02035  vol. ;  at  10°,  0-01607  vol. ;  at  15% 
0-01478  vol.  of  nitrogen  gas  (BUNSEN). 

BASIC  RADICALS  OF  THE  SECOND  GROUP. 

§71. 

1.  BARIUM. 
Barium  is  weighed  as  BARIUM   SULPHATE,  BARIUM  CARBONATE, 

and  BARIUM  SILICOFLUORIDE. 

a.f  Artificially  prepared  barium  sulphate  presents  the  appear- 
ance of  a  fine  white  powder.  When  recently  precipitated,  it  is 

*  According  to  REGNAULT,  0-97137. 


§  71.]  BASES    OF    GROUP   II. 

difficult  to  obtain  a  clear  filtrate,  especially  if  the  precipitation  wa& 
effected  in  the  cold,  and  the  solution  contains  neither  hydrochloric 
acid  nor  ammonium  chloride.  It  is  as  good  as  insoluble  in  cold 
and  in  hot  water.  (1  part  of  the  salt  requires  more  than  400,000 
| parts  of  water  for  solution.)  It  has  a  great  tendency,  upon  pre- 
cipitation, to  carry  down  with  it  other  substances  contained  in  the 
solution  from  which  it  separates,  more  particularly  barium  nitrate, 
nitrates  and  chlorates  of  the  alkali  metals,  ferric  oxide,  &c.  Several 
of  the  impurities,  such,  for  instance,  as  potassium  or  sodium  chlo- 
rates, may  be  removed  by  igniting  the  barium  sulphate,  moistening 
with  hydrochloric  acid,  evaporating  the  latter  off  and  exhausting 
the  residue  with  water ;  other  impurities  again,  such  as  potassium 
or  sodium  nitrates,  cannot  be  removed  by  this  treatment.  Even 
the  precipitate  obtained  from  a  solution  of  barium  chloride  by 
means  of  sulphuric  acid  in  excess  contains  traces  of  barium  chloride, 
which  it  is  impossible  to  remove,  even  by  washing  with  boiling 
water,  but  which  are  dissolved  by  nitric  acid  (SIEGLE).  Cold  dilute 
acids  dissolve  trifling,  yet  appreciable  traces  of  barium  sulphate ; 
for  instance,  1000  parts  of  nitric  acid  of  1  -032  sp.  gr.  dissolve  0-062 
parts  (CALVERT),  1000  parts  of  hydrochloric  acid  containing  3  per 
cent,  dissolve  0*06  parts.*  Cold  concentrated  acids  dissolve  con- 
siderably more ;  thus,  1000  parts  of  nitric  acid  of  1 '167  sp.  gr.  dis- 
solve 2  parts  (CALVERT).  Boiling  hydrochloric  acid  also  dissolves 
appreciable  traces;  thus 230  c.  c.  hydrochloric  acid  of  1*02  sp.  gr., 
were  found,  after  a  quarter  of  an  hour's  boiling  with  0'679  grm. 
barium  sulphate,  to  have  dissolved  of  it  O'O-IS  grm.  Acetic  acid 
dissolves  less  barium  sulphate  than  the  other  acids;  thus,  80  c.  c. 
acetic  acid  of  1*02  sp.  gr.  were  found,  after  a  quarter  of  an  hour's 
boiling  with  0'4  grm.,  to  have  dissolved  only  0'002  grm.  (SIEGLE). 
Free  chlorine  considerably  increases  its  solubility  (O.  L.  ERDMANN). 
Several  salts  more  particularly  interfere  with  the  precipitation  of 
barium  by  sulphuric  acid.  I  observed  this  some  time  ago  with 
magnesium  chloride,  but  ammonium  nitrate  (MITTENTZWEY),  alkali 
nitrates  generally,*  and  more  particularly  alkali  citrates  (SPILLER), 
possess  this  property  in  a  high  degree.  In  the  last  case  the  pre- 
cipitate appears  on  the  addition  of  hydrochloric  acid.  If  a  fluid 
contains  metaphosphoric  acid,  barium  cannot  be  completely  pre- 
cipitated out  of  it  by  means  of  sulphuric  acid  ;  the  resulting  pre- 
cipitate too  contains  phosphoric  acid  (SCHEERER,  RUBE).  Barium 
*  Zeitschr.f.  anal.  Chem.,  ix,  62. 


170  FOKMS.  [§  71. 

sulphate  dissolves  in  considerable  quantity  in  concentrated  sulphuric 
acid,  but  separates  again  on  dilution.  It  is  as  good  as  insoluble 
in  a  boiling  solution  of  ammonium  sulphate  (1  in  4).  Barium 
sulphate  remains  quite  unaltered  in  the  air,  at  100°,  and  even  at 
a  red  heat.  At  a  strong  white  heat  it  loses  sulphuric  acid  (Bous- 
SINGAULT).*  On  ignition  with  charcoal,  or  under  the  influence  of 
reducing  gases,  it  is  converted  comparatively  easily,  but  as  a  rule 
only  partially,  into  barium  .sulphide.  On  ignition  with  ammonium 
chloride,  barium  sulphate  undergoes  partial  decomposition.  It  is 
not  affected,  or  affected  but  very  slightly,  by  cold  solutions  of  the 
hydrogen  carbonates  of  the  alkali  metals  or  of  ammonium  carbo- 
nate ;  solutions  of  normal  sodium  and  potassium  carbonates  when 
cold  have  only  a  slight  decomposing  action  upon  it ;  but  when 
boiling,  and  upon  repeated  application,  they  effect  at  last  the 
complete  decomposition  of  the  salt  (H.  KOSE).  By  fusion  with 
sodium  or  potassium  carbonate,  barium  sulphate  is  readily  decom- 
posed. 

COMPOSITION. 

BaO     .     .     .     .     153-40       65-70 
SOS      ....        80-07       34-30 

233-47     100  00 

I.  Artificially  prepared  barium  carbonate  presents  the  appear- 
ance of  a  white  powder.  It  dissolves  in  14137  parts  of  cold,  and  in 
15421  parts  of  boiling  water  (Expt.  No.  17).  It  dissolves  far  more 
readily  in  solutions  of  ammonium  chloride  or  ammonium  nitrate; 
from  these  solutions  it  is,  however,  precipitated  again,  though  not 
completely,  by  caustic  ammonia.  In  water  containing  free  carbonic 
acid,  barium  carbonate  dissolves  to  an  acid  carbonate.  In  water  con- 
taining ammonia  and  ammonium  carbonate,  it  is  nearly  insoluble, 
one  part  requiring  about  141000  parts  (Expt.  No.  18).  Its  solution 
in  water  has  a  very  faint  alkaline  reaction.  Alkali  citrates  and 
motaphosphates  impede  the  precipitation  of  barium  by  ammonium 
carbonate.  It  is  unalterable  in  the  air,  and  at  a  red  heat.  When 
exposed  to  the  strongest  heat  of  a  blast-furnace,  it  slowly  yields  up 
the  whole  of  its  carbonic  acid  ;  this  expulsion  of  the  carbonic  acid 
is  promoted  by  the  simultaneous  action  of  aqueous  vapor.  Upon 
heating  it  to  redness  with  charcoal,  caustic  baryta  is  formed,  with 
evolution  of  carbon  monoxide. 

*  Z&itschr.f.  analyt.  Chem.,  vn,  244. 


§  72.]  BASES    OF   GROUP   II.  171 

COMPOSITION. 

_BaO      ....    153-4         77-71 
-  COa      ....      44  22-29 

197-4       100-00 

c.  Bcvrium  silicofluoride  forms  small,  hard,  and  colorless  crys- 
tals, or  (more  generally)  a  crystalline  powder.  It  dissolves  in  3800 
parts  of  cold  water ;  in  hot  water  it  is  more  readily  soluble  (Expt. 
No.  19).  The  presence  of  free  hydrochloric  acid  increases  its  solu- 
bility considerably  (Expt.  No.  20).  Ammonium  chloride  acts  also 
in  the  same  way  (1  part  siliconuoride  of  barium  dissolves  in  428 
parts  of  saturated,  and  589  parts  of  dilute  solution  of  ammonium 
chloride.  J.  W.  MALLET).  In  common  alcohol  it  is  almost  insoluble. 
It  is  unalterable  in  the  air,  and  at  100°  ;  when  ignited,  it  is  decom- 
posed into  silicon  fluoride,  which  escapes,  and  barium  fluoride, 
which  remains. 

COMPOSITION. 

BaF3     .     .     .    175-5     62-66       Ba  .     .     .   137-4  49-05 

SiF4      ...     104-6     37-34       Si    ...     28-4  10-14 

F.   .     .      .   114.3  40-81 

280-1    100-00 

280-1  100-00 

§72. 

2.  STRONTIUM. 
Strontium   is  weighed  either  as   STRONTIUM   SULPHATE,  or  as 

STRONTIUM  CARBONATE. 

a.  Strontium  sulphate,  artificially  prepared,  is  a  white  powder, 
sometimes  dense  and  crystalline,  sometimes  loose  and  bulky.  It 
dissolves  in  6895  parts  of  cold,  and  9638  parts  of  boiling  water 
(Expt.  No.  21).  In  water  containing  sulphuric  acid,  it  is  still  more 
difficultly  soluble,  requiring  from  11000  to  12000  parts  (Expt.  No. 
22).  Of  cold  hydrochloric  acid  of  8'5  per  cent.,  it  requires  474  parts ; 
of  cold  nitric  acid  of  4'8  per  cent.,  432  parts ;  of  cold  acetic  acid  of 
15-6  per  cent,  of  HC2H3Oa,  as  much  as  7843  parts  (Expt.  No.  23). 
It  dissolves  in  solutions  of  potassium  chloride  and  magnesium  chlo- 
ride, in  quantity  which  increases  with  the  concentration,  also  in  solu- 
tions of  sodium  chloride  and  calcium  chloride  in  greatest  quantity 


172  FOKMS.  L§  7%- 

when  the  solutions  are  of  medium  concentration  (A.  VIRCK*)  ;  it 
it  is  precipitated  from  these  solutions  by  sulphuric  acid.  Meta- 
phosphoric  acid  (SCHEERER,  RTJBE),  and  also  alkali  citrates,  but  not 
free  citric  acid  (SPILLER),  impede  the  precipitation  of  strontium  by 
sulphuric  acid.  It  is  as  good  as  insoluble  in  absolute  alcohol,  in 
common  alcohol,  and  in  a  boiling  solution  of  ammonium  sulphate 
(1  in  4).  It  does  not  alter  vegetable  colors  ;  and  remains  unaltered 
in  the  air,  and  at  a  red  heat.  When  exposed  to  a  most  intense  red 
heat,  it  fuses  with  loss  of  a  small  quantity  of  sulphuric  acid  (M. 
DARMSTADT  f) ;  all  the  sulphuric  acid  will  escape  on  very  strong 
ignition  continued  for  a  length  of  time  (BOUSSINGAULT  $).  When 
ignited  with  charcoal,  or  under  the  influence  of1  reducing  gases,  it 
is  converted  into  strontium  sulphide.  Solutions  of  acid  and  nor- 
mal carbonates  of  potassium,  sodium,  and  ammonium  decompose 
strontium  sulphate  completely  at  the  common  temperature,  even 
when  considerable  quantities  of  alkali  sulphates  are  present  (EL 
ROSE).  Boiling  promotes  the  decomposition. 

COMPOSITION. 

_SrO     .     .     .     103-00  56-41 

SO      .     .     .       80-0.7  43-59 


183-67         100-00 

J.  Strontium  carbonate,  artificially  prepared,  is  a  white,  soft, 
loose  powder.  It  dissolves,  at  the  common  temperature,  in  18045 
parts  of  water  (Expt.  No.  24) :  presence  of  ammonia  diminishes 
its  solubility  (Expt.  No.  25).  It  dissolves  pretty  readily  in  solu- 
tions of  ammonium  chloride  and  ammonium  nitrate,  but  is  precipi- 
tated again  from  these  solutions  by  ammonia  and  ammonium  car- 
bonate, and  more  completely  than  barium  carbonate  under  similar 
circumstances.  Water  impregnated  with  carbonic  acid  dissolves  it 
as  an  acid  carbonate.  Its  reaction  is  very  feebly  alkaline.  Alkali 
citrates  and  metaphosphates  impede  the  precipitation  of  strontium 
by -alkali  carbonates.  Ignited  with  access  of  air  it  is  infusible, 
but  when  exposed  to  a  most  intense  heat,  it  fuses  and  gradually 
loses  its  carbonic  acid.  On  ignition  with  charcoal,  strontium  oxide 
is  formed,  with  evolution  of  carbon  monoxide  gas. 

*  Zeitschr.f.  analyt.  Chem.,  i,  473.  ~\  lb.,  vi,  370.  \lb.,  vn,  244. 


§  73.]  BASES    OF   GROUP   II.  173 

COMPOSITION. 

^O.  «  _  SrO  .  .  .   103-6     70*19 
-O'    "CO,  ...    44-0     29-81 


'147-6         100-00 

§73. 
3.  CALCIUM. 

Calcium  is  weighed  either  as  CALCIUM  SULPHATE,  CALCIUM  CAR- 
BONATE, or  CALCIUM  OXIDE  ;  to  convert  it  into  the  latter  forms,  it 
is  first  usually  precipitated  as  calcium  oxalate. 

a.  Artificially  prepared  anhydrous  calcium  sulphate  is  a  loose, 
white  powder.     It  dissolves,  at  the  common  temperature,  in  430 
parts,  at  100°,  in  460  parts  of  water  (POGGIALE).      Presence  of 
hydrochloric  acid,  nitric  acid,  ammonium  chloride,  sodium  sulphate, 
or  sodium  chloride,  increases  its  solubility.     It  dissolves  with  com- 
parative ease,  especially  on  gently  warming,  in  aqueous  solution  of 
sodium  thiosulphate  (DIEHL),  and  also  in  a  boiling  solution  of 
ammonium  sulphate  (1  in  4).     The  aqueous  solution  of  calcium 
sulphate  does  not  alter  vegetable  colors.     In  alcohol  of  90  per  cent 
or  stronger  it  is  almost  absolutely  insoluble.     Exposed  to  the  air, 
it  slowly  absorbs  water.     It  remains  unaltered  at  a  dull-red  heat. 
Heated  to  intense  bright  redness,  it  fuses,  losing  weight  consider- 
ably from  loss  of  sulphuric  acid  (AL.  MITSCHEELICH  *).     On  long 
ignition  at  a  white  heat  all  the  sulphuric  acid  escapes  (BoussiN- 
OAULTf).      On  ignition  with  charcoal,  or  under  the  influence  of 
reducing  gases,  it  is  converted  into  calcium  sulphide.     Solutions 
of  normal  and  acid  carbonates  of  the  alkali  metals  decompose  cal- 
cium sulphate  more  readily  still  than  strontium  sulphate. 

COMPOSITION. 

^O^  p         CaO     .     .     .     56-10         41-20 
U'<0>          =  SO,      ...     80-07         58-80 

136-17        100-00 

b.  Calcium  carbonate  artificially  produced  by  the  precipitation 
of  a  calcium  salt  with  ammonium  carbonate  is  at  first  loose  and 

*Jour.f.  prakt.  Chem.,  LXXXIII,  485.  f  Zeitschr.f.  analyt.  Chem.,  vn,  224. 


174  FORMS.  [§  73. 

amorphous,  but  after  some  time  becomes  a  white,  fine,  crystalline 
powder,  which  under  the  microscope  has  sometimes  the  form  of 
calcite,  sometimes  that  of  aragonite.  It  is  very  slightly  soluble 
in  water.  By  protracted  boiling  1  litre  of  water  dissolves  0*034: 
grm.  according  to  A.  "W.  HOFMANN,  or  0-036  grm.  according  to 
C.  WELTZIEN;  so  one  part  requires  28500  parts  of  water  for  solu- 
tion. The  solution  has  a  barely-perceptible  alkaline  reaction.  In 
water  containing  ammonia  and  ammonium  carbonate  the  crystal- 
lized salt  dissolves  much  more  sparingly  (Expt.  No.  26),  one 
part  requiring  about  65000  parts ;  this  solution  is  not  precipitated 
by  ammonium  oxalate.  Amorphous  calcium  carbonate  is  also 
much  more  insoluble  in  water  containing  ammonia  than  in  pure 
water  (DIVEKS*).  Presence  of  ammonium  chloride  and  of  ammo- 
nium nitrate  increases  the  solubility  of  calcium  carbonate  ;  but  the 
salt  is  precipitated  again  from  these  solutions  by  ammonia  and 
ammonium  carbonate,  and  more  completely  than  barium  carbonate 
under  similar  circumstances.  Normal  salts  of  potassium  and  sodium, 
and  also  normal  calcium  and  magnesium  salts  (HUNT),  likewise 
increase  its  solubility,  the  precipitation  of  calcium  by  the  alkali 
carbonates  is  completely  prevented  or  considerably  interfered  with 
by  the  presence  of  alkali  citrates  (SPILLER)  or  metaphosphates 
(RUBE).  Water  impregnated  with  carbonic  acid  dissolves  calcium 
carbonate  as  acid  carbonate.  Calcium  carbonate  remains  unaltered 
in  the  air  at  100°,  and  even  at  a  low  red  heat ;  but  upon  the  appli- 
cation of  a  stronger  heat,  more  particularly  with  free  access  of  air, 
it  gradually  loses  its  carbonic  acid.  By  means  of  a  gas  blowpipe- 
lamp,  calcium  carbonate  (about  0*5  grm.),  in  an  open  platinum 
crucible,  is  without  difficulty  reduced  to  calcium  oxide ;  attempts 
to  effect  complete  reduction  over  a  spirit  lamp  with  double  draught 
have,  however,  failed  (Expt.  No.  27).  It  is  decomposed  far  more 
readily  when  ignited  with  charcoal,  giving  off  its  carbonic  acid  in, 
the  form  of  carbon  monoxide. 

COMPOSITION. 

0.    pa_CaO     .     .     .     .     56-1-       56-04 
O  '          ~  CO,     .     .     .     .     44-0         43-96 

100-1       100*00 
*  Jour.  Chem.  Soc.  1870,  362. 


§  73.]  BASES    OF   GROUP   II.  175 

c.  Calcium  oxalate,  precipitated  from  hot  or  concentrated  solu* 
tions,  is  a  fine  white  powder  consisting  of  infinitely  minute  indis- 
tinct crystals,  and  almost  absolutely  insoluble  in  water.  The  salt 
has  the  formula,  CaC2O4  -|-  H9O.  When  precipitated  from  cold, 
extremely -dilute -solutions,  the  salt  presents  a  more  distinctly  crys- 
talline appearance,  and  consists  of  a  mixture  of  CaC2O4  -|-  H2O  and 
CaC2O4  +  3II3O  (SOUCHAY  and  LENSSEN).  Presence  of  free  oxalic 
acid  and  acetic  acid  slightly  increases  the  solubility  of  calcium 
oxalate.  The  stronger  acids  (hydrochloric  acid,  nitric  acid)  dissolve 
it  readily  ;  from  these  solutions  it  is  precipitated  again  unaltered, 
by  alkalies,  and  also  (provided  the  excess  of  acid  be  not  too  great) 
by  alkali  oxalates  or  acetates  added  in  excess.  Calcium  oxalate 
does  not  dissolve  in  solutions  of  potassium  chloride,  sodium  chlo- 
ride, ammonium  chloride,  barium  chloride,  calcium  chloride,  and 
strontium  chloride,  even  though  these  solutions  be  hot  and  concen- 
trated ;  but,  on  the  other  hand,  it  dissolves  readily  and  in  appreci- 
able quantities,  in  hot  solutions  of  the  salts  belonging  to  the  mag- 
nesium group.  From  these  solutions  it  is  reprecipitated  by  an 
excess  of  alkali  oxalate  (SOUCHAY  and  LENSSEN).  Alkali  citrates 
(SPILLEK)  and  metaphosphates  (RUBE)  impede  the  precipitation  of 
lime  by  alkali  oxalates.  When  treated  with  solutions  of  many  of 
the  heavy  metals,  e.g.,  with  solution  of  cupric  chloride,  silver 
nitrate,  &c.,  calcium  oxalate  suffers  decomposition,  a  soluble  cal- 
cium salt  being  formed,  and  an  oxalate  of  the  heavy  metal,  which 
separates  immediately,  or  after  some  time  (REYNOSO).  Calcium 
oxalate  is  unalterable  in  the  air,  and  at  100°.  Dried  at  the  latter 
temperature,  it  has  invariably  the  following  composition  (Expt.  No. 
28,  also  SOUCHAY  and  LENSSEN*). 

CO  — O\  CaO     .     .     .     56-100       38-39 

|  ;>  Ca  4-  HfO  =  C203    .     .     .     72-000    .   49-28 

CO-0/  H20     .     .     ,     18-016       12-33 


146-116     100-00 

At  205°  calcium  oxalate  loses  its  water,  without  undergoing 
decomposition ;  at  a  somewhat  higher  temperature,  still  scarcely 
reaching  dull  redness,  the  anhydrous  salt  is  decomposed,  without 
actual  separation  of  carbon,  into  carbon  monoxide  and  calcium 
carbonate.  The  powder,  which  was  previously  of  snowy  whiteness, 

*  Annal.  d.  Chem.  und  Pharm.,  c,  322. 


176  FORMS.  L§  74. 

transiently  assumes  a  gray  color  in  the  course  of  this  process,  even 
though  the  oxalate  be  perfectly  pure.  Upon  continued  applica- 
tion of  heat  this  gray  color  disappears  again.  If  the  calcium 
oxalate  is  heated  in  small,  coherent  fragments,  such  as  are  obtained 
upon  drying  the  precipitated  salt  on  a  filter,  the  commencement 
and  progress  of  the  decomposition  can  be  readily  traced  by  this 
transient  appearance  of  gray.  If  the  process  of  heating  be  con- 
ducted properly,  the  residue  will  not  contain  a  trace  of  calcium 
oxide.  Hydrated  calcium  oxalate  exposed  suddenly  to  a  dull-red 
heat,  is  decomposed  with  considerable  separation  of  carbon.  By 
ignition  over  the  gas  blowpipe  calcium  oxalate  is  converted  into 
calcium  oxide. 

d.  Calcium  oxide  obtained  by  continued  strong  ignition  of  the 
oxalate  or  carbonate  appears  as  a  white,  infusible  powder,  unalter- 
able by  ignition.  By  standing  in  the  air  it  attracts  water  and  car- 
bonic acid,  but  not  rapidly  enough  to  interfere  with  accurate 
weighing  (Expt.  No.  29).  By  treatment  with  a  little  water 
calcium  hydroxide  is  formed  with  evolution  of  much  heat  ;  on 
igniting  again  the  water  of  hydration  is  readily  and  completely 
removed.  Pure  calcium  oxide  dissolves  in  dilute  hydrochloric 
acid  with  evolution  of  heat,  but  without  effervescence. 


4.  MAGNESIUM. 

Magnesium  is  weighed  as  MAGNESIUM  SULPHATE,  MAGNESIUM 
PYROPHOSPHATE,  or  MAGNESIUM  OXIDE.  To  convert  it  into  the  pyro- 
phosphate,  it  is  precipitated  as  NOKMAL  AMMONIUM  MAGNESIUM  PHOS- 
PHATE. 

a.  Anhydrous  magnesium  sulphate  presents  the  appearance  of 
a  white,  opaque  mass.  It  dissolves  readily  in  water.  It  is  nearly 
altogether  insoluble  in  absolute  alcohol,  but  it  is  somewhat  soluble 
in  common  alcohol. 

It  does  not  alter  vegetable  colors.  Exposed  to  the  air  it  absorbs 
water  rapidly.  At  a  moderate  red  heat,  it  remains  unaltered  ;  but 
when  heated  to  intense  redness,  it  undergoes  partial  decomposition, 
losing  part  of  its  acid,  after  which  it  is  no  longer  perfectly  soluble 
in  water.  By  means  of  a  gas  blowpipe  it  as  tolerably  easy  to  expel 


§  74.]  BASES   OF   GROUP   II.  177 

the  whole  of  the  sulphuric  acid  from  small  quantities  of  magne- 
sium sulphate  (Expt.  No.  30).  Ignited  with  ammonium  chloride 
magnesium  sulphate  is  not  decomposed. 


COMPOSITION. 


so   /O     Mo._MgO     ....     40-30         33-48 
°*<0>MS-S03       ....     80-07         66-52 


120-37       100-00 

b.  Ammonium  magnesium  phosphate  is  a  white  crystalline 
powder.  It  dissolves,  at  the  common  temperature,  in  15293  parts 
of  cold  water  (Expt.  No.  31).  In  water  containing  ammonia,  it  is 
much  more  insoluble.  1000  grin,  of  a  mixture  of  3  parts  water 
and  1  part  ammonia  solution,  dissolved  only  a  quantity  correspond- 
ing to  0-004  grm.  pyrophosphate  (KISSEL  *) ;  the  salt  was  consid- 
erably more  soluble  when  ammonium  chloride  was  also  present ; 
thus,  in  one  of  KISSEL'S  experiments  a  quantity  corresponding  to 
0-011  grm.  pyrophosphate  was  dissolved  by  1000  grm.  fluid  con- 
taining 18  grm.  ammonium  chloride.  Presence  of  excess  of  mag- 
nesium sulphate  diminishes  the  solubility  in  dilute  ammonia,  even 
in  the  presence  of  ammonium  chloride,  to  such  an  extent  that  the 
quantity  dissolved  by  1000  grm.  fluid  cannot  be  estimated  (KISSEL); 
the  precipitate,  under  these  circumstances,  is  liable,  especially  in 
the  absence  of  much  ammonium  chloride,  and  when  a  large  excess 
of  magnesium  sulphate  is  present,  to  contain  some  magnesium 
hydroxide  or  basic  magnesium  sulphate  (KuBEL,f  KISSEL).  Sodium 
phosphate  also  diminishes  (to  about  the  same  extent  as  magnesium 
sulphate)  the  solubility  of  the  salt  in  water  containing  ammonium 
chloride  and  ammonia  (W.  HEINTZ  J).  It  dissolves  readily  in  acids, 
even  in  acetic  acid.  Its  composition  is  expressed  by  the  formula 
NH4MgPO4  +  6H2O.  5  mol.  of  water  escape  at  100°,  the  remain- 
ing water  together  with  ammonia  are  expelled,  at  a  red  heat,  leav- 
ing MgQP2O7.  On  the  application  of  a  stronger  heat  the  mass 
passes  through  a  state  of  incandescence,  if  the  salt  were  pure ;  the 
weight  of  the  residue  is  not  affected.  The  incandescence  may  not 
take  place  at  all  in  the  presence  of  small  quantities  of  calcium  salts, 

*  Zeitschr.  j.  analyt.  Chem.,  viir,  173.          f  lb.,  vm,  125.          $lb.t  ix,  16. 


178  FOKMS.  [§  74. 

of  other  magnesium  salts,  or  of  silicic  acid.  It  is  occasioned  not 
by  the  passage  of  the  orthophosphate  into  the  pyrophosphate,  but 
by  the  passage  from  the  crystalline  to  the  amorphous  condi- 
tion (O.  POPP  *).  If  ammonium  magnesium  phosphate  is  dissolved 
in  dilute  hydrochloric  or  nitric  acid  and  ammonia  be  then  added 
to  the  solution,  the  salt  is  reprecipitated  completely,  or  more  cor- 
rectly, only  so  much  remains  in  solution  as  corresponds  to  its 
ordinary  solubility  in  water  containing  ammonia  and  ammonium 
salt. 

c.  Magnesium  pyrophosphate  presents  the  appearance  of  a 
white  mass,  often  slightly  inclining  to  gray.  It  is  barely  soluble  in 
water,  but  readily  so  in  hydrochloric  acid,  and  in  nitric  acid.  It 
remains  unaltered  in  the  air,  and  at  a  red  heat ;  at  a  very  intense 
heat  it  fuses  unaltered.  Exposed  at  a  white  heat  to  the  action  of 
hydrogen,  Mg3(PO4)2  is  formed,  while  PH3,  P  and  P2O3  escape. 
3Mg2P2Q7  =  2Mg,(PO4)1  +  PaOB  (STRUVEf).  It  leaves  the  color 
of  moist  turmeric-,  and  of  reddened  litmus-paper  unchanged.  If 
we  dissolve  it  in  hydrochloric  or  nitric  acid,  add  water  to  the  solu- 
tion, boil  for  some  time,  and  then  precipitate  with  ammonia  in 
excess,  we  obtain  a  precipitate  of  ammonium  magnesium  phosphate 
which,  after  ignition,  affords  less  Mg2P2O7,  than  was  originally 
employed.  WEBER  £  gives  the  loss  as  from  1-3  to  2-3  per  cent. 
My  experiments  (Expt.  No.  32)  confirm  this,  and  show  under 
what  conditions  the  loss  is  smallest.  By  long-continued 
fusion  with  mixed  potassium  and  sodium  carbonates,  magnesium 
pyrophosphate  is  completely  decomposed,  the  pyrophosphoric 
acid  being  re- converted  into  orthophosphoric.  If,  therefore, 
we  treat  the  fused  mass  with  hydrochloric  acid,  and  then, 
add  water  and  ammonia,  we  re-obtain  on  igniting  the  precipitate 
the  whole  quantity  of  the  salt  used.  If  the  solution  of  magnesium 
pyrophosphate  in  nitric  acid  is  evaporated  to  dry  ness  a  white  resi- 
due is  left ;  if  this  is  heated  more  strongly  hyponitric  acid  is  liber- 
ated, and  the  residue  turns  the  color  of  cinnamon  ;  on  cooling  it  is 
yellowish-white.  By  heating  still  more  strongly  to  incipient  red- 
ness, rapid  decomposition  sets  in,  more  hyponitric  acid  is  evolved, 
and  pure-white  magnesium  pyrophosphate  is  left.  Unless  the  heat 
is  applied  with  care  the  evolution  of  gas  may  be  so  rapid  as  to- 
carry  away  particles  of  the  substance  (E.  LUCK). 

*  Zeitschr.  f.  analyt.  Chem.,  xiu,  305.    '    f  Jour.  f.  prakt.  Chem.,  LXXIX,  349s, 
\Pogg.  Ann.,  LXXIIT,  146. 


75.]  BASES    OF   GROUP    III.  179 

COMPOSITION. 

XPO<()>Mg_2MgO     .     .     .      80-6         36-21 
^ 


222-6       100-00 

d.  Magnesium  oxide  is  a  white,  light,  loose  powder.  It  dis- 
solves in  55,368  parts  of  cold,  and  in  the  same  proportion  of  boil- 
ing water  (Expt.  No.  33).  Its  aqueous  solution  has  a  very  slightly 
alkaline  reaction.  It  dissolves  in  hydrochloric  and  in  other  acids, 
without  evolution  of  gas.  Magnesium  oxide  dissolves  readily  and 
in  quantity,  in  solutions  of  normal  ammonium  salts,  and  also  in 
solutions  of  potassium  chloride  and  sodium  chloride  (Expt.  No. 
34)  and  potassium  sulphate  and  sodium  sulphate  (R.  WARINGTON, 
Jr.)  it  is  more  soluble  than  in  water.  Exposed  to  the  air,  it  slowly 
absorbs  carbonic  acid  and  water.  Magnesium  oxide  is  highly  infusi- 
.ble,  remaining  unaltered  at  a  strong  red  heat,  and  fusing  super- 
ficially only  at  the  very  highest  temperature. 

COMPOSITION. 

Mg     ........    24-3         60-30 

O  16-0         39-70 


40-3       10000 

BASIC  RADICALS  OF  THE  THIRD  GROUP. 

§  T5. 
1.  ALUMINIUM. 

Aluminium  is  usually  precipitated  as  HYDROXIDE,  occasionally  as 
BASIC  ACETATE  or  BASIC  FORMATE,  and  always  weighed  as  ALUMINIUM 
OXIDE. 

a.  Aluminium  hydroxide,  recently  precipitated  from  a  solu- 
tion of  an  aluminium  salt  by  an  alkali  is  translucent,  and  when 
dried  at  100°  has  the  formula,  A13(OH)6.  The  precipitate  inva- 
riably retains  a  minute  proportion  of  the  acid  with  which  the 
aluminium  was  previously  combined,  as  well  as  of  the  alkali  which 
has  served  as  the  precipitant ;  it  is  freed  with  difficulty  from  these 
admixtures  by  repeated  washing.  It  is  insoluble  in  pure  water ; 


180  FORMS.  [§  75. 

but  it  readily  dissolves  in  soda,  potassa,  and  ethylamine  (SONNEN- 
SCHEIN)  ;  it  is  sparingly  soluble  in  ammonia,  and  insoluble  in  am- 
monium carbonate;  presence  of  ammonium  salts  greatly  diminishes 
its  solubility  in  ammonia  (Expt.  No.  35).  The  correctness  of  this 
statement  of  mine  in  the  first  edition  of  the  present  work,  has 
been  amply  confirmed  since  by  MALAGUTI  and  DUEOCHEK  ;*  and 
also  by  experiments  made  by  my  former  assistant,  Mr.  J.  FUCHS. 
The  former  chemists  state  also  that,  when  a  solution  of  aluminium 
is  precipitated  with  ammonium  sulphide,  the  fluid  may  be  filtered 
off  five  minutes  after,  without  a  trace  of  aluminium  in  it.  FUCHS 
did  not  find  this  to  be  the  case  (Expt.  No.  36).  Aluminium 
hydroxide,  recently  precipitated,  dissolves  readily  in  hydrochloric 
or  nitric  acid ;  but  after  filtration,  or  after  having  remained  for 
some  time  in  the  fluid  from  which  it  has  been  precipitated,  it 
does  not  dissolve  in  these  acids  without  considerable  difficulty, 
and  long  digestion.  Aluminium  hydroxide  shrinks  considerably 
on  drying,  and  then  presents  the  appearance  of  a  hard,  translucent, 
yellowish,  or  of  a  white,  earthy  mass.  "When  ignited,  it  loses 
water,  and  this  loss  is  frequently  attended  with  slight  decrepitation, 
and  invariably  with  considerable  diminution  of  bulk.  Aluminium 
hydroxide  precipitated  from  a  solution  of  aluminium  in  potassa  or 
soda  by  ammonium  chloride  is  milk-white,  denser,  easier  to  wash, 
and  much  less  soluble  in  ammonia  than  the  variety  above  de- 
scribed. When  dried  at  100°,  it  has  the  formula  A12O3  -f  (II.O), 
(J.  LowEf). 

b.  Aluminium  oxide  or  alumina,  prepared  by  heating  the 
hydroxide  to  a  moderate  degree  of  redness,  is  a  loose  and  soft  mass ; 
but  upon  the  application  of  a  very  intense  degree  of  heat,  it  con- 
cretes into  small,  hard  lumps.  At  the  most  intense  white  heat,  it 
fuses  to  a  clear  glass.  Ignited  alumina  is  dissolved  by  dilute  acids 
with  very  great  difficulty  ;  in  fuming  hydrochloric  acid,  it  dis- 
solves upon  long-continued  digestion  in  a  warm  place,  slowly,  but 
completely.  It  dissolves  tolerably  easily  and  quickly  by  first  heat- 
ing with  a  mixture  of  8  parts  of  concentrated  sulphuric  acid  and 
3  parts  of  water,  and  then  adding  water  (A.  MITSCHERLICH^:). 
Ignition  in  a  current  of  hydrogen  gas  leaves  it  unaltered.  By 
fusion  with  potassium  disulphate,  it  is  rendered  soluble  in  water. 
Upon  igniting  alumina  with  ammonium  chloride,  aluminium 

*4nn.  de  GMm.  et  de  'Phys.t  3  Ser.  17,  421. 

\Zeitochr.f.  analyt.  Chem.,  iv,  350.  £  Jour.  f.  prakt.  Chem.,  LXXXI,  110. 


§76.]  BASES    OF   GROUP   III.  181 

chloride  escapes;  but  the  process  fails  to  effect  complete  volatili- 
zation of  the  alumina  (II.  ROSE).  When  alumina  is  fused  at  a 
very  high  temperature,  with  ten  times  its  quantity  of  sodium  car- 
bonate, sodium  aluminate  is  formed,  which  is  soluble  in  water 
(R.  RICHTER).  Placed  upon  moist  red  litmus-paper,  pure  alumina 
does  not  change  the  color  to  blue. 

COMPOSITION. 

A12 54-2  53-03 

O3 48-0  46-97 

102-2  100-00 

c.  If  to  the  solution  of  a  salt  of  aluminium,  sodium  carbonate 
or  ammonium  carbonate  be  added,  till  the  resulting  precipitate  only 
just  redissolves  on  stirring,  and  then  sodium  acetate  or  ammonium 
acetate  poured  in  in  abundance  and  the  mixture  boiled  some  time, 
the  aluminium  is  precipitated  almost  completely  as  basic  acetate  in 
the  form  of  translucent  flocks,  so  that  if  the  filtrate  be  boiled  with 
ammonium  chloride  and  ammonia  only  unweighable  traces  of 
aluminium  hydroxide  separate.  If  the  quantity  of  sodium  acetate 
employed  be  too  small,  the  precipitate  appears  more  granular,  the 
filtrate  would  then  contain  a  larger  amount  of  aluminium.  The 
precipitate  cannot  be  very  conveniently  filtered  and  washed.  In 
washing  it  is  best  to  use  boiling  water,  containing  a  little  sodium 
acetate  or  ammonium  acetate.  The  precipitate  is  readily  soluble 
in  hydrochloric  acid. 

<L  If,  instead  of  the  acetates  mentioned  in  c,  the  corresponding 
formates  be  used,  a  flocculent  voluminous  precipitate  of  basic 
aluminium  formate  is  obtained,  which  may  be  very  readily  washed 
(FR.  SCHULZE*). 

§  76. 
2.  CHROMIUM. 

Chromium  is  usually  precipitated  as  CHROMIC  HYDROXIDE,  and 
always  weighed  as  chromic  oxide. 

a.  Chromic  hydroxide  recently  precipitated  from  a  green  solu- 
tion, is  greenish-gray,  gelatinous,  insoluble  in  water :  it  dissolves 
readily,  in  the  cold,  in  solutions  of  potassa  or  soda,  to  a  dark  green 
fluid ;  it  dissolves  also  in  the  cold,  but  rather  sparingly,  in  solution 

*  Chem,  Centralbl.  1861,  3. 


182  FORMS.  [§  77. 

of  ammonia,  to  a  light  violet  red  fluid.  In  acids  it  dissolves 
readily,  with  a  dark  green  color.  Presence  of  ammonium  chloride 
exercises  no  influence  upon  the  solubility  of  the  hydroxide  in 
ammonia.  Boiling  effects  the  complete  separation  of  the  hydroxide 
from  its  solutions  in  potassa,  or  ammonia  (Expt.  No.  37).  The 
dried  hydroxide  is  a  greenish-blue  powder  ;  it  is  converted  into 
oxide  with  loss  of  water  at  a  gentle  red  heat. 

5.  Chromic  oxide,  produced  by  heating  the  hydroxide  to  dull 
redness,  is  a  dark  green  powder ;  upon  the  application  of  a  higher 
degree  of  heat,  it  assumes  a  lighter  tint,  but  suffers  no  diminution 
of  weight ;  the  transition  from  the  darker  to  the  lighter  tint  is 
marked  by  a  vivid  incandescence  of  the  powder.  The  feebly 
ignited  oxide  is  difficultly  soluble  in  hydrochloric  acid,  and  the 
strongly  ignited  oxide  is  altogether  insoluble  in  that  acid.  It 
remains  unaltered  when  ignited  with  ammonium  chloride,  or  in 
a  current  of  hydrogen.  By  fusion  with  sodium  carbonate  and 
potassium  nitrate,  potassium  chromate  is  formed. 

COMPOSITION. 

Cra  .  .  .  .  104-2      68-46 
O3   ....   48-0      31-54 

152-2      100-00 

• 

BASIC  RADICALS  OP  THE  FOURTH  GROUP. 

|TT. 
1.  ZINC. 

Zinc  is  weighed  in  the  form  of  OXIDE  or  SULPHIDE  ;  it  is  precipi- 
tated as  BASIC  CARBONATE,  Or  aS  SULPHIDE. 

a.  Basic  zinc  carbonate,  recently  precipitated,  is  white,  floccu- 
lent,  nearly  insoluble  in  water — (one  part  requiring  44600  parts, 
Expt.  No.  38  —but  readily  soluble  in  potassa,  soda,  ammonia,  am- 
monium carbonate,  and  acids.  The  solutions  in  soda  or  potassa,  if 
concentrated,  are  not  altered  by  boiling ;  but  if  dilute,  nearly  all 
the  zinc  present  is  thrown  down  as  a  white  precipitate.  From  the 
solutions  in  ammonia  and  ammonium  carbonate,  especially  if  they 
are  dilute,  zinc  is  likewise  separated  upon  boiling.  When  a  neutral 
solution  of  zinc  is  precipitated  with  sodium  carbonate  or  potassium 
carbonate,  carbonic  acid  is  set  free,  since  the  precipitate  formed  is  not 


§  77.]  BASES    OF   GROUP   IV.  183 

ZnCO3,  but  consists  of  a  compound  of  zinc  hydroxide,  with  normal 
carbonate  in  proportions  varying  according  to  the  concentration  of 
the  solution,  and  to  the  mode  of  precipitation.  Owing  to  the 
presence  and  action  of  this  carbonic  acid,  part  of  the  zinc  remains 
in  solution ;  if  filtered  cold,  therefore,  the  nitrate  gives  a  precipi- 
tate with  ammonium  sulphide.  Bat  if  the  solution  is  precipitated 
boiling,  and  kept  at  that  temperature  for  some  time,  the  precipi- 
tation of  the  zinc  is  complete  to  the  extent  that  the  nitrate  is  not 
rendered  turbid  by  ammonium  sulphide ;  still,  if  the  nitrate,  mixed 
with  ammonium  sulphide,  be  allowed  to  stand  at  rest  for  many 
hours,  minute  and  almost  unweighable  flakes  of  zinc  sulphide  will 
separate  from  the  fluid.  The  precipitate  of  zinc  carbonate, 
obtained  in  the  manner  just  described,  may  be  completely  freed 
from  all  admixture  of  alkali  by  washing  with  hot  water.  If 
ammonium  salts  be  present,  the  precipitation  is  not  complete  till 
every  trace  of  ammonia  is  expelled.  If  the  solution  of  a  zinc  salt 
is  mixed  with  potassium  or  sodium  carbonate  in  excess,  the  mix- 
ture evaporated  to  dryness,  at  a  gentle  heat,  and  the  residue 
treated  with  cold  water,  a  perceptible  proportion  of  the  zinc  is 
obtained  in  solution  as  double  carbonate  of  zinc  and  potassium  or 
sodium  ;  but  if  the  mixture  is  evaporated  to  dryness,  at  a  boiling 
heat,  and  the  residue  treated  with  hot  water,  the  whole  of  the 
zinc,  with  the  exception  of  an  extremely  minute  proportion,  as  we 
have  already  had  occasion  to  observe,  is  obtained  as  zinc  carbonate. 
The  dried  basic  zinc  carbonate  is  a  brilliant,  white,  loose  powder ; 
exposure  to  a  red  heat  converts  it  into  oxide. 

J.  Zinc  oxide,  produced  from  the  carbonate  by  ignition,  is  a 
white  light  powder,  with  a  slightly  yellow  tint.  When  heated,  it 
acquires  a  yellow  color,  which  disappears  again  on  cooling.  Upon 
ignition  writh  charcoal,  carbon  monoxide  and  zinc  fumes  escape. 
By  igniting  in  a  rapid  current  of  hydrogen,  metallic  zinc  is  pro- 
duced ;  whilst  by  igniting  in  a  feeble  current  of  hydrogen, 
•crystallized  zinc  oxide  is  obtained  (Srr.  CLAIRE  DEVILLE).  In  the 
latter  case,  too,  a  portion  of  the  metal  is  reduced  and  volatilized. 
Zinc  oxide  is  insoluble  in  water.  Placed  on  moist  turmeric  paper, 
it  does  not  change  the  color  to  brown.  In  acids,  zinc  oxide  dis- 
solves readily  and  without  evolution  of  gas.  Ignited  with  ammo- 
nium chloride,  fused  zinc  chloride  is  produced  which  volatilizes 
with  very  great  difficulty  if  the  air  is  excluded :  but  readily  and 
completely,  with  free  access  of  air,  and  wTith  ammonium  chloride 


184  FORMS.  [§  77. 

fumes.  Mixed  with  a  sufficiency  of  powdered  sulphur  and  ignited 
in  a  stream  of  hydrogen,  the  corresponding  amount  of  sulphide  is 
obtained  (II.  ROSE). 

COMPOSITION. 

Zn 65-4  80-344 

O  16-0  19-656 


81-4  100-000 

c.  Zinc  sulphide,  recently  precipitated,  is  a  white,  loose  hydrate. 
The  following  facts  should  here  be  mentioned  with  regard  to  its 
precipitation.*  Colorless  ammonium  sulphide  precipitates  dilute 
solutions  of  zinc,  but  only  slowly ;  yellow  ammonium  sulphide 
does  not  precipitate  dilute  solutions  of  zinc  (1  :  5000)  at  all.  Am- 
monium chloride  favors  the  precipitation  considerably.  Free 
ammonia  acts  so  as  to  keep  the  precipitate  somewhat  longer  in 
suspension,  otherwise  it  exerts  no  injurious  influence.  If  the  con- 
ditions which  I  shall  lay  down  are  strictly  observed,  zinc  may  be 
precipitated  by  ammonium  sulphide  from  a  solution  containing 
only  -ginAnnr'  Hydrated  zinc  sulphide  on  account  of  its  slimy 
nature  easily  stops  up  the  pores  of  the  filter,  and  cannot  therefore 
be  washed  without  difficulty  on  a  filter.  The  washing  is  best 
performed  by  using  water  containing  ammonium  sulphide,  and 
continually  diminished  quantities  of  ammonium  chloride  (at  last 
none)  (see  Expt.  No.  39).  The  hydrate  is  insoluble  in  water,  in 
caustic  alkalies,  alkali  carbonates,  and  the  monosulphides  of  the 
alkali  metals.  It  dissolves  readily  and  completely  in  hydrochloric 
and  in  nitric,  but  only  very  sparingly  in  acetic  acid.  When  dried, 
the  precipitated  zinc  sulphide  is  a  white  powder  ;  when  air-dried 
its  composition  is  3ZnS  +  2II2O  ;  dried  at  100°,  2ZnS  +  H2O  ; 
at  150°,  4ZnS  +  H,O  (A.  SouciiAYf).  On  ignition  it  loses  the 
whole  of  its  water.  During  the  latter  process  some  hydrogen 
sulphide  escapes,  and  the  residue  contains  some  oxide.  By  roast- 
ing in  the  air,  and  intense  ignition,  small  quantities  of  zinc 
sulphide  may  be  readily  converted  into  the  oxide.  On  igniting 
the  dried  zinc  sulphide,  mixed  with  powdered  sulphur,  in  a  stream 
of  hydrogen,  the  pure  anhydrous  sulphide  is  obtained  (II.  ROSE). 
The  latter  suffers  no  loss  of  weight  worth  mentioning  by  ignition 
for  five  minutes  over  the  gas  blowpipe ;  but  if  such  ignition  is 

*  Jour.f.  prakt.  Chem.,  LXXXII,  263.         f  Zeitschr.f.  analyt.  Ohem.,  vii,  78. 


§  78.]  BASES   OF   GROUP   IV.  185 

very  protracted  the  loss   of   weight   becomes   considerable    (AL. 

CLASSEN*). 

COMPOSITION. 

Zn 65-40  67-10 

S     .....    32-07  32-90 

97-47  100-00 

§  78. 

2.  MANGANESE. 
Manganese  is  weighed  either  as  PROTOSESQUIOXIDE  (MANGANOSO- 

MANGANIC     OXIDE),    as     SULPHIDE,    as     MANGANOUS     SULPHATE,    Or     dS 

PYROPHOSPHATE.  With  the  view  of  converting  it  into  these  formsr 
it  is  precipitated  as  MANGANOUS  CARBONATE,  MANGANOUS  HYDROX- 
IDE, MANGANESE  DIOXIDE,  Or  AMMONIUM  MANGANESE  PHOSPHATE. 

a.  Manganese  carbonate,  recently  precipitated,  is  white,  floccu- 
lent,  nearly  insoluble  in  pure  water,  but  somewhat  more  soluble  in 
water  impregnated  with  carbonic  acid.  Presence  of  sodium  car- 
bonate or  potassium  carbonate  does  not  increase  its  solubility. 
Recently  precipitated  manganese  carbonate  dissolves  pretty  readily 
in  ammonium  chloride :  it  is  owing  to  this  property  that  a  solution 
of  manganese  cannot  be  completely  precipitated  by  potassium  or 
sodium  carbonate,  in  presence  of  ammonium  chloride  (or  any  other 
ammonium  salt),  until  the  latter  is  completely  decomposed.  If 
the  precipitate,  while  still  moist,  is  exposed  to  the  air,  or  washed 
with  water  impregnated  with  air,  especially  if  it  is  in  contact  with 
alkali  carbonate,  it  slowly  assumes  a  dirty  brownish-white  color,  part 
of  it  becoming  converted  into  hydrated  protosesquioxide.  Even 
long-continued  washing  will  not  remove  the  last  traces  of  alkali 
salt  from  the  precipitate.  The  wash-water  often  comes  through 
turbid.  If  the  filtrate  and  wash-water  are  evaporated  to  dryness 
and  the  residue  is  treated  with  boiling  water,  the  small  traces  of 
manganous  carbonate  which  were  partly  dissolved  and  partly  sus- 
pended will  remain  behind  in  the  form  of  hydrated  protosesqui- 
oxide. Dried  by  pressure  the  precipitate  is  white,  and  consists  of 
MnOO,  +  H3O  ;  dried  in  a  vacuum  it  consists  of  2(MnCO3)  +  H2O 
(E.  PRioRf)  ;  when  dried  with  free  access  of  air,  the  powder  is  of 
a  dirty-white  color.  When  strongly  heated  with  access  of  air, 

*  Zeitschr.f.  analyt.  ('hem.,  iv,  421.  f  lb.,  vni,  428. 


186  FORMS.  [§  78. 

this  powder  first  turns  black,  and  changes  subsequently  to  brown 
protosesquioxide  of  manganese.  However,  this  conversion  takes 
some  time,  and  must  never  be  held  to  be  completed  until  two 
weighings,  between  which  the  precipitate  has  been  ignited  again 
with  free  access  of  air,  give  perfectly  corresponding  results.  On 
igniting  the  manganous  carbonate,  mixed  with  powdered  sulphur, 
in  a  stream  of  hydrogen,  manganese  sulphide  is  obtained  (II.  ROSE). 
&.  Manganous  hydroxide  recently  thrown  down  forms  a 
white,  flocculent  precipitate,  barely  soluble  in  water  and  alkalies, 
but  soluble  in  ammonium  chloride  ;  it  immediately  absorbs  oxygen 
from  the  air,  and  turns  brown,  owing  to  the  formation  of  hydrated 
protosesquioxide.  On  drying  it  in  the  air,  a  brown  powder  is 
obtained  which,  when  heated  to  intense  redness,  with  free  access 
•of  air,  is  converted  into  protosesquioxide,  and  on  ignition  with 
sulphur,  in  a  stream  of  hydrogen,  is  converted  into  sulphide. 

c.  Protosesquioxide  of  manganese,   artificially  produced,  is  a 
brown  powder.     All  the  oxides  of  manganese  are  finally  converted 
into  this  by  strong  ignition  in  the  air.     Each  time  it  is  heated  it 
assumes  a  darker  color,  but  its  weight  remains  unaltered.     It  is 
insoluble  in  water,  and  does  not  alter  vegetable  colors.     If  ignited 
with   ammonium  chloride,   it  is  converted   into   the   manganous 
chloride.     When  heated  with  concentrated  hydrochloric  acid,  it 
dissolves  to  chloride  with  evolution  of  chlorine  (Mn3O4  -f-  8IIC1  = 
3MnCl2  -f-  2C1  +  4H2O).     On  ignition  with  sulphur  in  a  stream 
of  hydrogen  it  is  converted  into  sulphide  (H.  ROSE).     On  ignition 
in  oxygen  it  is  converted  into  manganic  oxide  (SCHNEIDER).     On 
ignition  in  hydrogen  it  is  converted  into  manganous  oxide. 

COMPOSITION. 

Mn3    ....     165  72-05 

O4 64:  27-95 

229  100-00 

d.  Manganese  dioxide  is  occasionally  produced  in  analysis  b} 
exposing   a   concentrated   solution    of    manganous    nitrate    to    ?, 
gradually  increased  temperature.     At  140°  brown  flakes  separate, 
at  155°  much  nitrous  acid  is  disengaged,  and  the  whole  of  the 
manganese  separates  as  anhydrous  dioxide.     It  is  brownish-black, 
and  is  deposited  on  the  sides  of  the  vessel,  with  metallic  lustre.    It 
is  insoluble  in  weak  nitric  acid,  but  dissolves  to  a  small  amount  in 


§  78.]  BASES    OF   GROUP   IV. 

hot  and  concentrated  nitric  acid  (DEVILLE).  In  hydrochloric  acid 
it  dissolves  with  evolution  of  chlorine,  in  concentrated  sulphuric 
acid  with  liberation  of  oxygen.  The  dioxide  is  also  sometimes 
obtained  in  the  hydrated  condition  in  analytical  separations,  tlms 
when  we  precipitate  a  solution  of  a  manganous  salt  with  sodium 
hypochlorite,  or,  after  addition  of  sodium  acetate,  with  bromine  or 
chlorine  in  the  heat.  The  brownish-black  flocculent  precipitate 
thus  obtained,  contains  alkali,  from  which  it  cannot  be  well  freed 
by  washing. 

e.  Manganese  sulphide,  prepared  in  the  wet  way,  generally 
forms  a  flesh-colored  precipitate.  I  must  make  a  few  remarks 
with  reference  to  its  precipitation.*  This  is  effected  but  incom- 
pletely if  we  add  to  a  pure  manganous  solution  only  ammonium 
sulphide,  no  matter  whether  it  be  colorless  or  yellow,  while  it  is 
perfectly  effected  if  ammonium  chloride  be  used  in  addition.  A 
large  quantity  even  of  ammonium  chloride  does  not  impede  the 
precipitation.  Ammonia  in  small  quantity  is  not  injurious,  but  in 
large  quantity  it  interferes  with  complete  precipitation,  especially 
in  the  presence  of  ammonium  polysulphide  (A.  CLASSEN-)-).  In  all 
cases  we  must  allow  to  stand  at  least  24  hours,  and  with  very 
dilute  solutions  48  hours,  before  filtering.  Colorless  or  slightly 
yellow  ammonium  sulphide  is  the  most  appropriate  precipitant. 
In  the  presence  of  ammonium  chloride  even  a  large  excess  of 
ammonium  sulphide  is  uninjurious.  If  the  precipitation  is  con- 
ducted as  directed,  the  manganese  can  be  precipitated  from  solu- 
tions which  contain  an  amount  equivalent  to  only  ^nroVw  °^  ^10 
manganous  oxide.  If  the  flesh-colored  hydrated  sulphide  remains 
some  time  under  the  fluid,  from  which  it.  was  precipitated,  it 
sometimes  becomes  converted  into  the  green  anhydrous  sulphide.:): 
This  conversion  is  more  likely  to  take  place  when  a  large  excess  of 
ammonium  sulphide  has  been  used  ;  heating  favors  it,  ammonium 
chloride  hinders  it.  The  conversion  is  occasionally  rapid.  The 
green  sulphide  thus  obtained  consists  of  eight-sided  tables  dis- 
tinctly visible  under  the  microscope  (F.  MUCK§).  In  acids  (hydro- 
chloric, sulphuric,  acetic,  (fee.)  the  hydrated  sulphide  dissolves  with 
evolution  of  hydrogen  sulphide.  If  the  precipitate,  while  still 
moist,  is  exposed  to  the  air,  or  washed  with  water  impregnated  with 
air,  it  changes  to  brown,  hydrated  protosesquioxide  of  manganese 

*  Journ.f.  prakt.  Chem.,  LXXXII,  265.  \  Zeitsehr.f.  analyt.  Chem.,  vin,  370. 
%Journ.f.prakt.  Chem.,  LXXXII,  268.  §  Zeitsehr.f.  Chem.,  N.  F.  vr,  6. 


Of   THE 

UNIVERSITY 


188  FORMS.  [§  78. 

being  formed,  together  with  a  small  portion  of  manganous  sulphate. 
Hence  in  washing  the  hydrate  we  always  add  some  ammonium  sul- 
phide to  the  wash-water,  and  keep  the  filter  as  full  as  possible  with 
the  same.  We  guard  against  the  filtrate  running  through  turbid, 
by  adding  gradually  decreasing  quantities  of  ammonium  chloride  to 
the  wash-water  (at  last  none).  (Expt.  lN"o.  40.)  On  igniting  the 
precipitate  mixed  with  sulphur  in  a  stream  of  hydrogen  the 
anhydrous  sulphide  remains.  If  we  have  gently  ignited  during 
this  process,  the  product  is  light  green;  if  we  have  strongly 
ignited,  it  is  dark  green  to  black.  Neither  the  green  nor  the 
black  sulphide  attracts  oxygen  or  water  quickly  from  the  air 
(H.  ROSE).  The  anhydrous  sulphide  is  also  readily  soluble  in 
dilute  acids. 

COMPOSITION. 

Mn      ....     55-00  63-17 

S 32-07  36-83 


87-07  100-00 

f.  Anhydrous  manganous  sulphate,  produced  by  exposing  the 
crystallized  salt  to  the  action  of  heat,  is  a  white,  friable  mass, 
readily  soluble  in  water.  It  resists  a  very  faint  red  heat  ;  but 
upon  exposure  to  a  more  intense  red  heat,  it  suffers  more  or  less 
complete  decomposition  —  oxygen,  sulphur  dioxide,  and  sulphur 
trioxide  being  evolved,  and  protosesquioxide  of  manganese  re- 
maining behind.  Ignited  with  sulphur  in  a  stream  of  hydrogen  it 
is  transformed  into  sulphide  (H.  KOSE). 

COMPOSITION. 


n 

3       O  -  =  SO,  .     .     .     80-07 

151-07 

g.  Ammonium  manganese  phosphate.  —  GIBBS*  says  that  this 
precipitate  is  insoluble  in  boiling  water,  but  I  have  not  found  this 
to  be  the  case.  My  results  are  that  1  part  dissolves  in  32092  parts 
of  cold  water,  in  20122  parts  boiling  water,  and  17755  parts  of 
water  containing  -^  of  ammonium  chloride.  It  has  the  formula 


*8illim.  Amer.  Journ.  (n),  44,  216. 


§  79.]  BASES    OF   GROUP  IV.  189 

XH4MnPO4  +  II3O.  It  presents  pale  pink  scales  of  pearly 
lustre,  which  sometimes  turn  reddish  on  the  filter.  On  ignition  it 
is  converted  into  manganese  pyrophosphate. 

h.  Manganese  pyroplwspliate  is  the  white  residue  left  on  the 
ignition  of  the  preceding. 

COMPOSITION. 


_ 

^PO<g>Mn~P>0•  _^ 

284  100 

§  79. 

3.  NICKEL. 

Xickel  is  precipitated  as  HYDROXIDE,  and  as  SULPHIDE.  It  is 
weighed  in  the  form  of  NICKELOUS  OXIDE,  of  METALLIC  NICKEL,  or 
of  anhydrous  NICKELOUS  SULPHATE. 

a.  Nickelous    hydroxide    forms    an    apple-green    precipitate, 
almost  absolutely  insoluble  in  water.     "When  precipitated  from  a 
solution  of  the  chloride  or  sulphate,  it  retains  some  of  the  acid 
even  after  long  washing  (TEICHMANN*).     It  is  also  very  difficult  to 
remove  the  last  traces  of  alkali.     It  dissolves  with  some  difficulty 
in  ammonia  and  ammonium  carbonate,  far  more  readily  in  the 
presence  of  an  ammonium  salt.     From  these  solutions  it  is  com- 
pletely precipitated  by  excess  of  potassa  or  soda  ;  application  of 
heat  promotes  the  precipitation.     It  is  unalterable  in  the  air  ;  on 
ignition,  it  passes  into  nickelous  oxide. 

b.  Nickelous  oxide  is  a  dirty  grayish-green   powder.     When 
obtained  by  heating  the  nitrate  to  redness,  it  always  contains  some 
nickelic  oxide,  and  requires  very  strong  and  protracted  ignition  for 
conversion  into  the  pure  green  nickelous  oxide  (W.  J.  RUSSELL). 
It  is  insoluble  in  water,  but  readily  soluble  in  hydrochloric  acid. 
It  does  not  affect  vegetable  colors.     It  suffers   no  variation   of 
weight  upon  ignition  with  free  access  of  air.     Mixed  with  am- 
monium  chloride   and   ignited,  it   is   reduced   to  metallic  nickel 
(II.  ROSE);  it  is  also  easily  reduced  by  ignition  in  hydrogen  or 
carbon  monoxide. 

COMPOSITION. 

Ni     .     .     .     58-7  78-58 

O  16-0  21-4:2 


74-7  100-00 


*  Annal.  d.  Chem.  u.  Pharm.,  CLVI,  17. 


190  FORMS.  [§  79. 

c.  Metallic  nickel  obtained  by  the  reduction  of  nickelous  oxide 
with  hydrogen  lias  the  form  of  a  gray  powder,  or  if  the  heat  has 
been  very  strong,  and  it  lias  melted,  it  is  lustrous  and  white  like 
silver.     It  is  unaltered  in  weight  by  ignition  in  hydrogen ;  when 
ignited  in  the  air  it  is  superficially  oxidized.     It  is  attracted  by 
the  magnet.    It  is  dissolved  slowly  by  hydrochloric  acid  and  dilute 
sulphuric  acid,  and  readily  by  moderately  strong  nitric  acid. 

d.  Anhydrous  nickelous  sulphate  obtained   by  evaporating  a 
solution  of  the  chloride,  nitrate,  &c.,  with  sulphuric  acid  is  yellow, 
soluble   in   water   to    a   green  fluid.     The  hydrous  salt  may  be 
rendered  anhydrous  without  loss  of  acid  by  cautious  heating  in  a 
platinum  dish,  but  at  low  redness  it  begins  to  blacken  at  the  edges 
and  loses  acid  (F.  GAUHE*).' 

e.  Ilydrated  nickelous  sulphide,  prepared   in   the  wet   way, 
forms  a  black  precipitate,  insoluble  in  water.     I  must  make  some 
observations  on  its  precipitation.!    In  order  to  precitate  the  nickel 
from  a  pure  solution  completely  and  with  ease,  ammonium  chloride 
must  be  present ;  it  is  not  enough  to  add  ammonium  sulphide 
alone.     A  large  quantity  even  of  ammonium  chloride  produces  no 
injurious  effect.     In  the  presence  of  free  ammonia,  on  the  con- 
trary, some  nickel  remains  in  solution.     In  this  case,  the  super- 
natant fluid   appears  brown.     As   precipitant,  colorless   or   light- 
yellow  ammonium  sulphide  containing  no  free  ammonia  should  be 
used,  a  large  excess  must  be  avoided.     If  the  directions  given  are 
adhered  to — allowing  to  stand  48  hours — the  nickel  may  be  pre- 
cipitated by   means  of  ammonium  sulphide,   from  solutions  con- 
taining only  ^nroTrro  °^  ^ie  ox^e*     ^s  ^ne  precipitate  is  liable  to 
take  up  oxygen  from  the  air,  being  transformed  into  sulphate,  a 
little  ammonium  sulphide  is  mixed  with  the  wash-water,  to  which 
also  it  is  advisable  to  add  ammonium  chloride  (less  and  less — at 
last  none);   the  filter  should  be  kept  full  (Expt.  Xo.  41).    Brown 
filtrates,  containing  nickel  sulphide  in  solution,  may  be  freed  from 
the  latter  by  acidulation  with  acetic  acid,  and  boiling  some  time. 
The  sulphide  falls  down,  and  may  now  be  filtered  off.     It  is  very 
sparingly  soluble   in  concentrated  acetic    acid,    somewhat   more 
soluble  in  hydrochloric  acid.     It  is  more  readily  soluble  still  in 
nitric  acid,  but  its  best  solvent  is  nitro-hydrochloric  acid.     It  loses 
its  water  upon  the  application  of  a  red  heat ;   when  ignited  in  the 
air,  it  is  transformed  into  a  basic  compound  of  nickelous  oxide 
with  sulphuric  acid.     Mixed  with  sulphur  and  ignited  in  a  stream 

*  Zeitschr.  f.  analyt.  Chem.,  iv,  190.        \Journ.  f.  prakt.  Chem.,  LXXXII,  257. 


§  80.]  BASES    OF   GROUP   IV.  191 

of  hydrogen,  a  fused  mass  remains,  of  pale  yellow  color  and  me- 
tallic lustre.  This  consists  of  NiaS,  but  its  composition  is  not 
perfectly  constant  (F.  GATJHE  *).  On  heating  a  solution  of  a  nickel- 
ous  salt  with  an  excess  of  sodium  thiosulphate  in  a  sealed  glass 
tube  at  120°,  all  the  nickel  will  be  precipitated  in  the  course  of 
half  an  hour  as  a  sulphide  (2N1C1,  +  21Sa2S2O,  =  ]STi2S  +  2NaCl 
+  Na2S3O6).  The  sulphide  so  obtained  is  black,  and  unchange- 
able in  air;  it  may  be  readily  washed,  is  almost  unaffected  by 
hydrochloric  or  dilute  sulphuric  acid,  and  it  may  be  converted 
into  nickelous  sulphate  by  dissolving  it  in  nitric  acid  and 
evaporating  the  solution  with  sulphuric  acid  (W.  GIBBS  f). 
[Xiekel  may  be  precipitated  as  a  sulphide,  dense  in  form,  easy 
to  wash,  and  not  readily  oxidizing  by  contact  with  air,  by  proceed- 
ing as  follows  :  To  the  solution,  which  should  be  concentrated  and 
contain  a  liberal  quantity  of  ammonium  salts,  add  ammonia  (if 
necessary)  to  alkaline  reaction,  then  acetic  acid  to  slight  acid  reac- 
tion, also  ammonium  or  sodium  acetate,  and  heat  to  boiling. 
Transmit  II2S  gas  through  the  boiling  solution.  Since  much  free 
acetic  acid  prevents  complete  precipitation,  it  is  necessary  some- 
times when  much  nickel  is  present  to  partially  neutralize  once  or 
twice  the  acid  set  free  during  the  process.] 

§  80. 
4.  COBALT. 

Cobalt  is  weighed  in  the  PUKE  METALLIC  state,  or  as  COBALTOTJS 
SULPHATE.  Besides  the  properties  of  these  substances,  we  have  to 
study  also  those  of  COBALTOUS  HYDROXIDE,  of  the  SULPHIDE,  and  of 

the  TRIPOTASSIUM    COBALTIC    NITRITE. 

a.  Cobaltous  hydroxide. — Upon  precipitating  a  solution  of  a 
cobaltous  salt  with  potassa,  a  blue  precipitate  (a  basic  salt)  is 
formed  at  first,  which,  upon  boiling  with  potassa  in  excess,  exclud- 
ed from  contact  of  air,  changes  to  light  red  cobaltous  hydroxide ; 
if,  on  the  contrary,  this  process  is  conducted  with  free  access  of 
air,  the  precipitate  becomes  discolored,  and  finally  black,  part  of 
the  cobaltous  hydroxide  being  converted  into  cobaltic  hydroxide. 
But  the  hydroxide  prepared  in  this  way,  retains  always  a  certain 
quantity  of  the  acid,  and,  even  after  the  most  thorough  washing 

*  Zeitschr.  f.  analyt.  Chem.,  iv,  191. 
t/6.,  in,  389. 


192  roRMS.  [§  80. 

with  hot  water,  also  a  small  amount  of  the  alkaline  precipitant. 
The  latter,  however,  is  not  enough  to  spoil  the  accuracy  of  the 
results  (H.  ROSE,  F.  GAUIIE*).  Cobaltous  hydroxide  is  insoluble 
in  water,  and  also  in  dilute  potassa  ;  it  is  somewhat  soluble  in  very 
concentrated  potassa,  and  readily  in  ammonium  salts.  When  dried 
in  the  air,  it  absorbs  oxygen,  and  acquires  a  brownish  color.  By 
strong  ignition  it  is  converted  into  cobaltous  oxide  (even  if  some 
higher  oxide  had  formed  from  boiling  or  drying  in  the  air) ;  if 
cooled  with  exclusion  of  air,  as  in  a  current  of  carbon  dioxide, 
pure  light.brown  cobaltous  oxide  will  be  left ;  if  cooled,  on  the 
contrary,  with  access  of  air,  it  is  more  or  less  changed  to  black 
protosesquioxide  (cobaltoso-cobaltic  oxide)  (W.  J.  RUSSELL-)-).  By 
ignition  in  a  current  of  hydrogen,  metallic  cobalt  is  left,  from 
which  any  traces  of  alkali  may  now  be  almost  completely  removed 
by  boiling  water. 

b.  The  metallic  cobalt  obtained  according  to  a,  or  by  igniting 
the  chloride  or  the  protosesquioxide  (produced  by  igniting  the 
nitrate)  in  hydrogen  is  a  grayish-black  powder,  which  is  attracted 
by  the  magnet,  and  is  more  difficultly  fusible  than  gold.     If  the 
reduction  has  been  effected  at  a  faint  heat,  the  finely  divided  metal 
burns  in  the  air  to  protosesquioxide  of  cobalt,  which  is  not  the 
case  if  the  reduction  has  been  effected  at  an  intense  heat.     Cobalt 
does  not  decompose  water,  either   at   the   common   temperature, 
or  upon   ebullition— ^-except  sulphuric  acid  be  present,  in  which 
case  decomposition  will  ensue.     Heated   with    concentrated   sul- 
phuric acid,  it  forms  cobaltous  sulphate,  with  evolution  of  sulphur 
dioxide.     In  nitric  acid  it  dissolves  readily  to  cobaltous  nitrate. 

c.  Cobalt  sulphide,  produced  in  the  wet  way,  forms  a  black 
precipitate,  insoluble  in  water,  alkalies,  and  alkali  sulphides.    With 
regard  to  its  precipitation,^: — this  is  effected  but  slowly  and  im- 
perfectly by  ammonium  sulphide  alone ;  in  the  presence  of  am- 
monium chloride  however,  it  takes  place  quickly  and  completely. 
Free  ammonia  is  not  injurious  ;  it  is  all  one,  whether  colorless  or 
yellow  ammonium  sulphide  is  employed.     If  the  directions  given 
are  observed,  cobalt  may  be  precipitated  from  a  solution  contain- 
ing no  more  than   TrooVo"o    °f  ^ie  protoxide.     In  the  moist  con- 
dition, exposed  to  the  air,  it  oxidizes  to  sulphate.     In  washing  it, 
therefore,  water  containing  ammonium  sulphide  is  employed,  and 
the  niter  is  kept  full.     It  is  advisable  also  to  mix  a  little  ammo- 

*  Zeitschr.f.  analyt.  Chem.,  iv,  54.  f  H>.,  n,  471. 

\  Journ.  f.  prakt.  Chem.,  LXXXII,  262. 


§  80.]  BASES    OF   GROUP   IY.  193 

ilium  chloride  with  the  wash-water,  but  its  quantity  should  be 
gradually  decreased,  and  the  last  water  used  must  contain  none. 
It  is  but  sparingly  soluble  in  acetic  acid  and  in  dilute  mineral 
acids,  more  readily  in  concentrated  mineral  acids,  and  most  readily 
in  warm  nitro-hydrochloric  acid.  Mixed  with  sulphur  and  ignited 
in  a  stream  of  hydrogen,  we  obtain  a  product  which  varies  in 
composition  according  to  the  temperature  employed.  The  residue 
is  therefore  not  suited  for  the  determination  of  cobalt  (H.  ROSE). 
Cobalt  can  be  precipitated  as  sulphide  completely  in  the  presence 
of  a  very  small  amount  of  free  acetic  acid  by  hydrogen  sulphide, 
or  by  heating  with  an  excess  of  sodium  thiosulphate  in  a 
sealed  tube  (\Y.  GIBBS,  Zeitschr.f.  analyt.  Chem.,  in,  390),  in 
the  same  manner  as  nickel  (see  §  79,  e).  Cobalt  sulphide  may  be 
converted  into  cobaltous  sulphate  by  heating  in  the  air,  moistening 
with  nitric  acid,  evaporating  with  sulphuric  acid  and  igniting. 

d.  Cobaltous  sulphate  crystallizes,  in  combination  with  7  aq., 
slowly  in  oblique  rhombic  prisms  of  a  fine  red  color.     The  crystals 
yield  the  whole  of  the  water,  at  a  moderate  heat,  and  are  con- 
verted into  a  rose-colored  anhydrous  salt,  which  bears  the  applica- 
tion of  a  low  red  heat  without  losing  acid.     At  a  stronger  heat  the 
edges  become  black  and  some  sulphuric  acid  escapes  (F.  GAUHE*). 
It  dissolves  rather  difficultly  in  cold,  but  more  readily  in  hot  water. 

COMPOSITION. 

an'  >  O  i          _  CoO  .     75-00        48-37 

°'<0-  ~S03      ....      80-07         51-63 

155-07       100-00 

e.  Tripotassium  cobaltic  nitrite. — If  a  solution  of  a  cobalt  salt 
(not  too  dilute)  is  mixed  with  excess  of  potassa  and  then  with 
acetic  acid  till  the  precipitate  is  redissolved,  and  a  concentrated 
solution  of  potassium  nitrite  previously  acidified  with  acetic  acid  is 
added,  first  a  dirty,  brownish  precipitate  forms  which  gradually 
turns  yellow  and  crystalline,  especially  on   the  application   of  a 
gentle  heat  (X.  "W".  FISCHER-)-).     The  composition  of  this  precipi- 
tate   corresponds    to    the    formula    (KXO,)6Coa(XO,)6  +  aq  = 
CoK8(XO2)6  +  aq.(SADTLER).-     Dried  at  100°  its  composition  is 
somewhat   variable   (STROMEYER,   ERDMANN^:).      It  is  decidedly 

*  Zeitschr.f.  analyt.  Cliem.,  iv,  55.  \Pogg.  Ann.,  LXXII,  477. 

\Journ.f.  prakt.  Chem.,  cxvu,  385. 


194  FORMS.  [§  81. 

soluble  in  water,  less  in  potassium  acetate  whether  neutral  or 
acidified  with  acetic  acid,  not  in  potassium  acetate  to  which  some 
potassium  nitrite  has  been  added,  not  in  potassium  nitrite,  nor  in 
80-per  cent,  alcohol.  On  washing  with  water  or  solution  of  potas- 
sium acetate,  unless  potassium  nitrite  is  added,  nitric  oxide  is  con- 
stantly evolved  in  small  quantities.  It  is  decomposed  with  separa- 
tion of  brown  cobaltic  hydroxide,  with  difficulty  by  solution  of 
potassa,  with  ease  by  soda  or  baryta.  On  being  moistened  with 
sulphuric  acid  and  ignited  (finally  with  addition  of  ammonium 
carbonate)  it  leaves  2(CoSO4)  +  3(K2SO4),  but  there  is  a  diffi- 
culty in  driving  off  all  the  excess  of  acid  without  decomposing  the 
cobaltous  sulphate.  The  yellow  salt  is  soluble  in  hydrochloric 
acid  ;  potassa  precipitates  the  whole  of  the  cobalt  from  this  solu- 
tion as  hydroxide. 


5.  FERROUS  IRON  ;    and  6.'  FERRIC  IRON. 

Iron  is  usually  weighed  in  the  form  of  FERRIC  OXIDE,  occasion- 
ally as  SULPHIDE.  We  have  to  study  also  the  FERRIC  HYDROXIDE, 

the  FERRIC  SUCCINATE,  the  FERRIC  ACETATE,  and  the  FERRIC  FORMATE. 

a.  Ferric  hydroxide,  recently  prepared,  is  a  reddish-brown 
precipitate,  insoluble  in  water,  in  dilute  alkalies,  and  in  ammonium 
salts,  but  readily  soluble  in  acids  ;  it  shrinks  very  greatly  on 
drying.  When  dry,  it  presents  a  brown,  hard  mass,  with  shining 
conchoidal  fracture.  If  the  precipitant  alkali  is  not  used  in  excess, 
the  precipitate  contains  basic  salt  ;  on  the  other  hand,  if  the  alkali 
has  been  used  in  excess,  a  portion  of  it  is  invariably  carried  down 
in  combination  with  the  ferric  hydroxide,  —  on  which  account 
ammonia  alone  can  properly  be  used  in  analysis  for  this  purpose. 
Under  certain  circumstances,  for  instance,  by  protracted  heating  of 
a  solution  of  ferric  acetate  on  the  water-bath  (which  turns  the 
solution  from  blood-red  to  brick-red,  and  makes  it  appear  turbid 
by  reflected  light),  and  subsequent  addition  of  some  sulphuric  acid 
or  salt  of  an  alkali,  a  reddish-brown  hydrated  ferric  oxide  is  pro- 
duced, which  is  insoluble  in  cold  acids,  even  though  concentrated, 
and  is  not  attacked  even  by  boiling  nitric  acid  (L.  PEAN  DE  ST. 
GILLES*). 

Closely  allied  to  ferric  hydroxide  are  the  highly  basic  salts 
obtained  by  mixing  dilute  cold  solutions  of  ferric  salts,  best  ferric 
chloride,  with  much  ammonium  chloride,  cautiously  adding  am- 

*  Journ.f  prakt.  C'/iem  ,  i.xvi,  137. 


§  81.]  BASES    OF   GROUP   IV.  195 

nionium  carbonate  till  the  fluid  on  standing  in  the  cold  instead  of 
becoming  clear  turns  more  turbid  if  anything,  and  then  boiling. 
The  precipitates,  thus  produced  in  the  fluid  which  still  retains  its 
acid  reaction,  contain  the  whole  of  the  iron  present  and  play  an 
important  part  in  analytical  separations.  They  should  be  washed 
with  boiling  water  containing  ammonium  chloride,  being  soluble  to 
a  slight  extent  in  pure  water.  They  are  not  suitable  for  ignition, 
as  ferric  chloride  might  occasionally  escape  from  them. 

b.  Ferric  hydroxide  is,  upon  ignition,  converted  into  ferric 
oxide.  If  the  hydroxide  has  been  superficially  dried  only,  the 
violent  escape  of  steam  from  the  lumps  is  likely  to  occasion  loss ; 
but  if  it  has  been  dried  as  much  as  possible  by  suction  and  still 
remains  moist,  it  may  be  ignited  without  fear  of  loss.  Pure  ferric 
oxide,  when  placed  upon  moist  reddened  litmus-paper,  does  not 
change  the  color  to  blue.  It  dissolves  slowly  in  dilute,  but  more 
rapidly  in  concentrated  hydrochloric  acid ;  the  application  of  a 
moderate  degree  of  heat  effects  this  solution  more  readily  than 
boiling.  With  a  mixture  of  8  parts  concentrated  sulphuric  acid 
and  3  parts  water,  it  behaves  in  the  same  manner  as  alumina.  The 
weight  of  ferric  oxide  does  not  vary  upon  ignition  in  the  air; 
when  ignited  with  ammonium  chloride,  ferric  chloride  escapes. 
Ignition  with  charcoal,  in  a  closed  vessel,  reduces  it  more  or  less. 
Strongly  ignited  with  sulphur  in  a  stream  of  hydrogen,  it  is  trans- 
formed into  ferrous  sulphide. 

COMPOSITION. 

Fea 111-8  69-96 

O3 48-0  30-04: 


159-8  100-00 

c.  Ferrous -sulphide,  produced  in  the  wet  way,  forms  a  black 
precipitate.  The  following  facts  are  to  be  noticed  with  regard 
to  its  precipitation.*  Ammonium  sulphide  used  alone,  whether 
colorless  or  yellow,  precipitates  pure  neutral  solutions  of  ferrous 
salts,  but  slowly  and  imperfectly.  Ammonium  chloride  acts  very 
favorably ;  a  large  excess  even  is  not  attended  with  inconvenience. 
Ammonia  has  no  injurious  action.  It  is  all  the  same  whether  the 
ammonium  sulphide  be  colorless  or  light  yellow.  If  the  direc- 

*Jou/n.f.  praki.  them.,  LXXXII,  268. 


196  FORMS.  [§   81. 

tions  given  are  observed,  iron  may  be  precipitated  by  means  of 
ammonium  sulphide,  from  solutions  containing  only  y^ffnnj-  of 
ferrous,  oxide.  In  such  a  case,  howeve'r,  it  is  necessary  to  allow  to 
stand  forty-eight  hours.  Since  the  precipitate  rapidly  oxidizes  in 
contact  with  air,  ammonium  sulphide  is  to  be  added  to  the  wash- 
water,  and  the  filter  kept  full.  It  is  well  also  to  mix  a  little 
ammonium  chloride  with  the  wash-water,  but  the  quantity  should 
be  continually  reduced,  and  the  last  water  used  should  contain 
none.  In  mineral  acids,  even  when  very  dilute,  the  hydrated 
sulphide  dissolves  readily.  Mixed  with  sulphur,  and  strongly 
ignited  in  a  stream  of  hydrogen,  anhydrous  ferrous  sulphide  re- 
mains (H.  ROSE). 

COMPOSITION.     x 

Fe 55-90  63-54 

S  32-07  36-46 


87-97  100-00 

d.  When  a  neutral  solution  of  a  ferric  salt  is  mixed  with  a 
neutral  solution  of  an  alkali  succinate,  a  cinnamon-colored  pre- 
cipitate of  a  brighter  or  darker  tint  of  a  basic  ferric  succinate  is 
formed,  Fe(OH)C4H4O4 ,  succinic  acid  being  set  free.  From  the 
nature  of  this  precipitate  it  must  follow  that,  in  its  formation, 
one  equivalent  of  succinic  acid  (using  an  excess  of  ammonium 
succinate)  must  be  liberated,  as  follows : 

Fe,(S04),  +  3(NH.),C,H40,  +  m,O 

=  2Fe(OH)C.H,0,  +  3(NH,),SO,  +  H3-0,H4O.. 

The  free  succinic  acid  does  not  exercise  any  perceptible  sol- 
vent action  upon  the  precipitate  in  a  cold  and  highly  dilute  solu- 
tion, but  it  redissolves  the  precipitate  a  little  more  readily  in  a 
warm  solution.  The  precipitate  must  therefore  be  filtered  cold, 
if  we  want  to  guard  against  re- solution.  Formerly  the  precipi- 
tate was  erroneously  supposed  to  consist  of  a  normal  salt,  de- 
composable by  hot  water  into  an  insoluble  basic  and  a  soluble  acid 
compound.  Basic  ferric  succinate  is  insoluble  in  cold,  and  but 
sparingly  soluble  in  hot,  water,  containing  for  every  equivalent  of 


§  81.]  BASES    OF    GROUP    IV.  197 

succinic  acid,  IIa'C4H4O4,  from  18  to  30  equivalents  of 
FeaO3.  It  dissolves  readily  in  mineral  acids.  Ammonia, 
especially  if  warm,  deprives  it  of  the  greater  portion  of  its  acid, 
leaving  compounds  which  are  highly  basic  ferric  succinates 


e.  If  to  a  solution  of  a  ferric  salt,  sodium  carbonate  be  added 
in  the  cold,  till  the  fluid  contains  no  more  free  acid,  and  in  con- 
sequence of  the  formation  of  basic  salt  has  become  deep  red,  but 
remains  still  perfectly  clear,  and  then  sodium  acetate  be  poured 
in  and  the  mixture  boiled,  the  whole  of  the  iron  will  be  precipi- 
tated as  basic  ferric  acetate. 

The  precipitation  is  successfully  effected  in  this  operation  by 
having  the  ferric  salt  solution  sufficiently  dilute,  that  the  free 
acid  be  properly  neutralized,  and  that  sufficient  sodium  acetate  be 
added.  The  duration  of  boiling  is  of  little  consequence  ;  when 
proper  proportions  have  been  used,  one  boiling-up  suffices.  Of 
course  it  is  understood  that  all  the  iron  must  be  ferric.  Instead 
of  sodium  carbonate  or  acetate  the  corresponding  ammonium  salts 
will  answer  also.  The  precipitates  may,  as  a  rule,  be  filtered  off 
and  well  washed  without  any  ferric  oxide  passing  into  the  nitrate  ; 
at  times,  however,  the  reverse  may  happen.  I  should  advise  not 
to  boil  longer  than  is  necessary  to  precipitate,  then  filter  while 
hot,  and  to  add  to  the  boiling  wash-water  some  sodium-  or  ammo- 
nium acetate  ;  this  can  cause  no  inconvenience,  because  the  pre- 
cipitate is  usually  redissolved  in  hydrochloric  acid,  and  reprecipi- 
tated  with  ammonia  water. 

f.  Instead  of  the  sodium-  or  ammonium  acetate  used  in  £,  the 
corresponding  formates  may  be  used.     The  basic  ferric  formate 
obtained  in  this  case  is  more  easily  washed  than  the  basic  acetate 
(F.  SCHULZE).* 

*Chem.  Centralblatt,  1861,  3. 


198  FORMS.  [§  82. 

BASIC   RADICALS   OF   THE   FIFTH   GROUP. 

§  82. 

1.    SILVER. 

Silver  may  be  weighed  in  the  METALLIC  state,  as  CHLORIDE,  SUL- 
PHIDE, or  CYANIDE. 

a.  Metallic  silver,  obtained  by  the  ignition  of  salts  of  silver 
with  organic  acids,  &c.,  is  a  loose,  white,  glittering  mass  of  metallic 
lustre ;  but,  when  obtained  by  reducing  silver  chloride,  &c.,  in  the 
wet  way,  by  zinc,  it  is  a  dull-gray  powder.  It  fuses  at  about  1000°. 
Its  weight  is  not  altered  by  moderate  ignition.  It  may,  however, 
be  distilled  by  the  heat  of  the  oxy hydrogen  name  (CHRISTOMANOS*). 
It  dissolves  readily  and  completely  in  dilute  nitric  acid. 

1).  Silver  chloride,  recently  precipitated,  is  white  and  curdy. 
On  shaking,  the  large  spongy  flocks  combine  with  the  smaller 
particles,  so  that  the  fluid  becomes  perfectly  clear.  This  result  is, 
however,  only  satisfactorily  effected  when  the  flocks  have  been 
recently  precipitated  in  presence  of  excess  of  silver  solution  (com- 
pare Gr.  J.  MULDER  t).  Silver  chloride  is  in  a  very  high  degree 
insoluble  in  water,  and  in  dilute  nitric  acid  ;  strong  nitric  acid,  on 
the  contrary,  does  dissolve  a  trace.  Hydrochloric  acid,  especially 
if  concentrated  and  boiling,  dissolves  it  very  perceptibly.  Accord- 
ing to  PIERRE,  1  part  of  silver  chloride  requires  for  solution  200 
parts  of  strong  hydrochloric  acid  and  600  parts  of  a  dilute  acid, 
composed  of  1  part  strong  acid  and  2  parts  water.  On  sufficiently 
diluting  such  a  solution  with  cold  water  the  silver  chloride  is  pre- 
cipitated so  completely  that  the  filtrate  is  not  colored  by  hydrogen 
sulphide.  Silver  chloride  is  insoluble,  or  very  nearly  so,  in  con- 
centrated sulphuric  acid ;  in  the  dilute  acid  it  is  as  insoluble  as  in 
water.  In  a  solution  of  tartaric  acid  silver  chloride  dissolves  per- 
ceptibly on  warming ;  on  cooling,  however,  the  solution  deposits 
the  whole,  or,  at  all  events,  the  greater  part  of  it.  Aqueous  solu- 
tions of  chlorides  (of  sodium,  potassium,  ammonium,  calcium,  zinc, 

*  Zeitschr.f.  analyt.  Chem.,  vn,  299. 

f  Die  Silberprobirmethode,  translated  into  German  by  D.  CHR.  GRIMM,  pp. 
19  and  311.     Leipzig  :  J.  J.  Weber.     1859  . 


§  82.]  BASES    OF   GROUP   V.  199 

<fcc.)  all  dissolve  appreciable  quantities  of  silver  chloride,  especially 
if  they  are  hot  and  concentrated.  On  sufficient  dilution  with  cold 
water  the  dissolved  portion  separates  so  completely  that  the  filtrate 
is  not  colored  by  hydrogen  sulphide.  The  solutions  of  alkali  and 
alkali-earth  nitrates  also  dissolve  a  little  silver  chloride.  The  solu- 
bility in  the  cold  is  trifling ;  in  the  heat,  on  the  contrary,  it  is  very 
perceptible.  A  strong  solution  of  silver  nitrate  dissolves  it  slightly, 
especially  in  the  heat ;  but  I  have  found  it  insoluble  in  a  moder- 
ately dilute  cold  solution  of  lead  nitrate.  The  action  of  mercuric 
salts  upon  it  is  remarkable.  When  well  washed  and  treated  with  a 
very  dilute  solution  of  mercuric  chloride  it  becomes  white  if  pre- 
viously a  little  blackened  by- light,  is  easily  diffused  in  the  fluid, 
and  is  but  tardily  deposited.  This  depends  upon  the  mercuric  salt 
being  taken  up  ;  if  the  silver  salt  is  washed  the  mercuric  salt  will 
be  removed.  Mercuric  nitrate  acts  in  a  similar  way,  but  a  certain 
quantity  of  silver  passes  at  the  -same  time  into  solution.  Silver 
chloride  is  much  more  difficultly  dissolved  by  mercuric  acetate  than 
by  mercuric  nitrate ;  therefore,  if  you  have  a  solution  of  mercuric 
nitrate  containing  silver  chloride,  if  the  mercuric  salt  is  not  present 
in  enormous  quantity,  the  silver  may  be  almost  absolutely  thrown 
down  by  addition  of  an  alkali  acetate  (H.  DEBRAY*).  Solutions 
of  potash  and  soda  decompose  silver  chloride,  even  at  the  ordinary 
temperature,  more  readily  on  boiling ;  silver  oxide  separates,  and 
chloride  of  the  alkali  metal  is  formed.  Solution  of  sodium  or 
potassium  carbonate  decomposes  silver  chloride  only  very  imper- 
fectly even  on  boiling ;  after  long  boiling  decided  traces  of  chlorine 
are  found  in  the  filtrate.  Silver  chloride  dissolves  readily  in  aque- 
ous ammonia,  and  also  in  the  solution  of  potassium  cyanide  and 
that  of  sodium  thiosulphate.  According  to  WALLACE  and  LAMxxNrf 
1  part  of  silver  chloride  dissolves  in  12' 88  parts  of  strong  aqueous 
ammonia  of  O'SO  sp.  gr.  Under  the  influence  of  light  silver  chlo- 
ride soon  changes  to  violet,  finally  black,  losing  chlorine,  and  pass- 
ing partly  into  AgaCl.  The  change  is  quite  superficial,  but  the 
loss  of  weight  resulting  is  very  appreciable  (MULDER,  op.  cit.,  p. 
21).  If  silver  chloride  that  has  become  violet  or  black  from  the 
Influence  of  light  be  treated  with  aqueous  ammonia,  it  dissolves 
with  separation  of  a  very  small  quantity  of  metallic  silver,  Ag3Cl 
gives  AgCl  and  Ag  (WITTSTEIN).  On  long  contact  (say  for  24 

*  Zeitschr.f.  Chem.,  xui,  348.  f  Chem.  Gaz.  1859,  137. 


200  FORMS.  [§  82. 

hours)  with  water,  especially  at  75°,  silver  chloride,  although 
removed  from  the  influence  of  light,  becomes  gray,  and,  it  appears, 
decomposed ;  the  precipitate  is  found  to  contain  silver  oxide,  and 
the  water  hydrochloric  acid  (MULDER).  On  digestion  with  excess 
of  solution  of  potassium  bromide  or  iodide,  silver  chloride  is  com- 
pletely transf ormed  into  silver  bromide  or  iodide,  as  the  case  may 
be  (FIELD*).  On  drying,  silver  chloride  becomes  pulverulent ;  on 
heating  it  turns  yellow ;  at  260°  it  fuses  to  a  transparent  yellow 
fluid ;  at  a  very  high  heat  it  volatilizes  without  decomposition. 
On  cooling  after  fusion  it  presents  a  colorless  or  pale  yellowish 
mass.  Fused  in  chlorine  gas,  it  absorbs  some  chlorine  ;  on  cooling, 
this  escapes,  but  not  completely.  If  it  is  to  be  completely  expelled, 
and,  in  very  delicate  experiments  this  must  be  done,  we  pass  car- 
bon dioxide  before  allowing  to  cool  (STAS  f).  Ignition  with  char- 
coal fails  to  effect  its  reduction  to  the  metallic  state  ;  but  it  may 
be  readily  so  reduced  in  a  current  of  hydrogen,  carburetted  hydro- 
gen, or  carbon  monoxide. 

COMPOSITION. 

Ag 107-92  75-27 

01  35-45  24-73 


143-37  100-00 

c.  Silver  sulphide,  prepared  in  the  wet  way,  is  a  black  precipi- 
tate, insoluble  in  water,  dilute  acids,  alkalies,  and  alkali  sulphides. 
It  is  unalterable  in  the  air ;  after  being  allowed  to  subside,  it  is 
filtered  and  washed  with  ease,  and  may  be  dried  at  100°  without 
decomposition.  It  dissolves  in  concentrated  nitric  acid,  with 
separation  of  sulphur.  Solution  of  potassium  cyanide  dissolves 
it  with  difficulty,  if  it  has  been  precipitated  from  a  very  dilute 
solution  with  less  difficulty ;  the  quantity  of  potassium  cyanide, 
too,  has  great  influence  on  the  effect.  For  instance,  if  silver  cya- 
nide, is  dissolved  in  a  bare  sufficiency  of  potassium  cyanide  and 
hydrogen  sulphide,  or  ammonium  sulphide  is  added,  silver  sulphide 
is  thrown  down ;  if,  on  the  other  hand,  a  large  excess  of  potassium 

*  Quart.  Journ.  Chem.  Soc.,  x,  234;  Journ.  f.  prakt.  Chem.,  LXXIII,  404. 

f  liecherches  sur  les  rapports  reciproques  des  poids  atom iq ties,  p.  37.  Brux- 
elles,  1860.  The  loss  of  weight  which  about  100  grin,  silver  chloride  suffered, 
by  the  expulsion  of  the  absorbed  chlorine,  was  froui  7  to  13  ingnn. 


§  83.]  BASES   OF   GROUP   V.  201 

cyanide  is  present,  no  precipitate  will  be  produced.  If  silver 
sulphide  is  dissolved  in  a  concentrated  solution  of  potassium  cya- 
nide, it  will  generally  separate  at  once  on  addition  of  much  water 
(BECHAMP*).  Ignited  in  a  current  of  hydrogen,  it  passes  readily 
and  completely  into  the  metallic  state  (H.  ROSE). 

COMPOSITION. 

Ag2 215-84  87-06 

S 32-07  12-94: 

247-91  100-00 

d.  Silver  cyanide,  recently  thrown  down,  forms  a  white  curdy 
precipitate  insoluble  in  water  and  dilute  nitric  acid,  soluble  in 
potassium  cyanide  and  also  in  ammonia ;  exposure  to  light  fails 
to  impart  the  slightest  tinge  of  black  to  it ;  it  may  be  dried  at  100° 
without  decomposition.  Upon  ignition,  it  is  decomposed  into 
cyanogen,  which  escapes,  and  metallic  silver,  which  remains,  mixed 
with  a  little  paracyanide  of  silver.  By  boiling  with  a  mixture  of 
equal  parts  of  sulphuric  acid  and  water,  it  is,  according  to  GLASS- 
FORD  and  NAPIER,  dissolved  to  silver  sulphate,  with  liberation  of 
hydrocyanic  acid. 

COMPOSITION. 

Ag  107-92  80-56 

CN  26-04  19-44 


133-96  100-00 

§83. 
2.  LEAD. 

Lead  is  weighed  as  OXIDE,  SULPHATE,  CHKOMATE,  CHLORIDE,  and 
SULPHIDE.  Besides  these  compounds,  we  have  also  to  study  the 
CARBONATE  and  the  OXALATE. 

a.  Normal  lead  carbonate  forms  a  heavy,  white,  pulverulent 
precipitate.  It  is  but  very  slightly  soluble  in  perfectly  pure  (boiled) 
water,  one  part  requiring  50550  parts  (see  Expt.  No.  42,  a) ;  but 
it  dissolves  somewhat  more  readily  in  water  containing  ammonia 
and  ammonium  salts  (comp.  Expt.  No.  42,  5  and  c),  and  also  in 

*  Journ.  f.  prakt.  Chem.,  LX,  64. 


202  FORMS.  [§  83. 

water  impregnated  with  carbonic  acid.     It  loses  its  carbonic  acid 
when  ignited. 

b.  Lead  oxalate  is  a  white  powder,  very  sparingly  soluble  in 
water.     The  presence  of  ammonium  salts  slightly  increases  its  solu- 
bility (Expt.  ~No.  43).     When  heated  in  close  vessels,  it  leaves  lead 
suboxide ;  but  when  heated  with  access  of  air,  the  yellow  oxide. 

c.  Lead  oxide,  produced  by  igniting  the  carbonate  or  oxalate, 
is  a  lemon-yellow  powder,  inclining  sometimes  to  a  reddish-yellow, 
or  to  a  pale  yellow.     When  this  yellow  lead  oxide  is  heated,  it 
assumes  a  brownish-red  color,  without  the  slightest  variation  of 
weight.     It  fuses  at  an  intense  red  heat.     Ignition  with  charcoal 
reduces  it.   When  exposed  to  a  white  heat,  it  rises  in  vapor.    Placed 
upon  moist  red  litmus  paper,  it  changes  the  color  to  blue.     When 
exposed  to  the  air,  it  slowly  absorbs  carbonic  acid.     Mixed  with 
ammonium  chloride  and  ignited,  it  is  converted  into  lead  chloride. 
Lead  oxide  in  a  state  of  fusion  readily  dissolves  silicic  acid  and  the 
earthy  bases  with  which  the  latter  may  be  combined. 

COMPOSITION. 

Pb 206*92      92-82 

0.  16-00      7-18 


222-92     100-00 

d.  Lead  sulphate  is  a  heavy  white  powder.  It  dissolves,  at 
the  common  temperature,  in  22800  parts  of  pure  water  (Expt.  ~No. 
44*) ;  it  is  less  soluble  in  water  containing  sulphuric  acid  (1  part 
requiring  36500  parts— Expt.  No.  45) ;  it  is  far  more  readily  solu- 
ble in  water  containing  ammonium  salts ;  from  this  solution  it  may 
be  precipitated  again  by  adding  sulphuric  acid  in  excess  (Expt.  No. 
46).  It  is  almost  entirely  insoluble  in  common  alcohol.  Of  the 
ammonium  salts,  the  nitrate,  acetate,  and  tartrate  are  more  espe- 
cially suited  to  serve  as  solvents  for  lead  sulphate  :  the  two  latter 
salts  are  made  strongly  alkaline  by  addition  of  ammonia,  previous 
to  use  (WACKENRODEK).  Lead  sulphate  dissolves  in  concentrated 
hydrochloric,  acid,  upon  heating.  In  nitric  acid  it  dissolves  the 
more  readily,  the  more  concentrated  and  hotter  the  acid  ;  water 
fails  to  precipitate  it  from  its  solution  in  nitric  acid ;  but  the  addi- 
tion of  a  copious  amount  of  dilute  sulphuric  acid  causes  its  precipi- 

*  According  to  G.  F.  RODWELL  1  part  dissolves  in  31696  parts  water  at  15° 
(Ckem.  News,  1866,  50;  Zeitschr.  /.  anuyt.  Chem.,  v,  403.) 


§  83.]  BASES    OF     GROUP    V.  203 

tation  from  this  solution.  The  more  nitric  acid  the  solution  con- 
tains, the  more  sulphuric  acid  is  required.  Tt  dissolves  sparingly 
in  concentrated  sulphuric  acid,  and  the  dissolved  portion  precipi- 
tates again  upon  diluting  with  water  (more  completely  upon  addi- 
tion of  alcohol).  A  moderately  concentrated  solution  of  sodium 
thiosulphate  dissolves  lead  sulphate  completely  even  if  cold,  more 
readily  if  warmed.  On  boiling,  the  solution  becomes  black,  from 
separation  of  a  small  quantity  of  lead  sulphide  (J.  LOWE*).  The 
solutions  of  alkali  carbonates  and  alkali  hydrogen  carbonates  con- 
vert lead  sulphate,  even  at  the  common  temperature,  completely 
into  lead  carbonate.  The  solutions  of  the  normal  alkali  carbonates, 
but  not  those  of  the  alkali  hydrogen  carbonates,  dissolve  some  lead 
oxide  in  this  process  (II.  Rossf).  Lead  sulphate  dissolves  readily 
in  hot  solutions  of  potassa  or  soda.  It  is  unalterable  in  the  air,  and 
at  a  gentle  red  heat ;  when  exposed  to  a  full  red  heat,  it  fuses  with- 
out decomposition  (Expt.  No.  47),  provided  always  reducing  gases 
be  completely  excluded — for,  if  this  is  not  the  case,  the  weight  will 
continually  diminish,  owing  to  reduction  to  sulphide  (EKDMANN^:). 
At  a  white  heat  the  whole  of  the  sulphuric  acid  gradually  escapes 
(BOUSSINGAULT  §).  When  it  is  ignited  with  charcoal,  lead  sulphide 
is  formed  at  first ;  if  the  heat  be  raised,  this  sulphide  reacts  on 
undecornposed  sulphate,  metallic  lead  and  sulphur  dioxide  being 
produced.  Fusion  with  potassium  cyanide  reduces  the  whole  of 
the  lead  to  the  metallic  state.  Lead  sulphate  mixed  with  sulphur 
and  exposed  to  intense  ignition  in  a  current  of  hydrogen  yields 
the  sulphide,  but  loss  can  scarcely  be  avoided  (compare  f). 

COMPOSITION. 

/Ck™    _PbO  222-92  73-57 

<J2<0>        "SO, 80-07  26-43 

302-99         10000 

e.  Lead  chloride  obtained  by  precipitation  is  a  white  crystalline 
powder.  It  separates  in  needles  from  a  hot  solution  containing  a 
certain  quantity  of  hydrochloric  acid ;  occasionally  it  presents 
wedge-shaped  crystals,  or  when  separated  from  a  strong  hydro- 
chloric solution,  hexagonal  tables.  At  17°-7  water  dissolves  0*946 

*  Journ.  /.  prakt.  Ghem.,  LXXIV,  348.        \Pogg.  AnnaL,  xcv,  426. 

\  Journ.  f.  prakt.  Chem.,  LXII,  381.  §  Zeitschr.f.  analyt.  Chem.,  vn,  244. 


204  FORMS.  [§  83. 

per  cent. ;  a  fluid  containing  15  per  cent,  of  hydrochloric  acid  of 
1-162  sp.  gr.  dissolves  0*09;  a  fluid  containing  20  per  cent,  acid 
dissolves  0*111  per  cent.  ;  a  fluid  containing  80  per  cent,  acid  dis- 
solves 1  '498  per  cent.  Pure  hydrochloric  acid  of  the  above  strength 
dissolves  2-9  per  cent.  (J.  CARTER  BELL*).  Lead  chloride  is 
less  soluble  in  water  containing  nitric  acid  than  in  water  (1  part 
requires  1636  parts,  BISCHOF).  It  is  extremely  sparingly  soluble 
in  alcohol  of  70  to  80  per  cent.,  and  altogether  insoluble  in  absolute 
alcohol.  It  is  unalterable  in  the  air.  It  fuses  at  a  temperature 
below  red  heat,  without  loss  of  weight.  When  exposed  to  a  higher 
temperature,  with  access  of  air,  it  volatilizes  slowly,  being  partially 
decomposed :  chlorine  gas  escapes,  and  a  mixture  of  lead  oxide  and 
chloride  remains. 


COMPOSITION. 


Pb 206-92      74-47 

CL  70-90  '     25-53 


277-82     100-00 

f.  Lead  sulphide,  prepared  in  the  wet  way,  is  a  black  precipi- 
tate, insoluble  in  water,  dilute  acids,  alkalies,  and  alkali  sulphides. 
In  precipitating  it  from  a  solution  containing  free  hydrochloric 
acid,  it  is  necessary  to  dilute  plentifully,  otherwise  the  precipitation, 
will  be  incomplete.  Even  if  a  fluid  only  contain  2*5  per  cent.  HC1, 
the  whole  of  the  lead  will  not  be  precipitated  (M.  MARTIN  f).  It 
is  unalterable  in  the  air ;  it  cannot  be  dried  at  100°  without 
decomposition.  According  to  H.  ROSE  it  increases  perceptibly  in 
weight  by  oxidation  ;  in  the  case  of  long-protracted  drying  even 
becoming  a  few  per-cents  heavier.^  I  have  confirmed  his  state- 
ment (see  Expt.  No.  48).  If  lead  sulphide  mixed  with  sulphur  is 
heated  gently  in  a  current  of  hydrogen,  so  that  the  lower  quarter 
of  the  crucible  is  red  hot,  lead  sulphide  is  left  without  loss  of 
weight.  By  continuing  a  gentle  heat  the  weight  gradually  dimin- 
ishes; by  strong  ignition  the  loss  is  rapid.  This  loss  is  partly 
owing  to  volatilization  of  lead  sulphide,  but  mainly  to  escape  of 
sulphur  in  the  form  of  hydrogen  sulphide  and  formation  of  Pb2S, 
or  even  of  lead  (A.  SOUCIIAY  §).  It  dissolves  in  concentrated  hot 

*  Journ.f.  prakt.  Chem  ,  cv,  188;  Jour.  Chem.  Soc.  (2),  vi,  355. 
\Journ.f.  prakt.  Chem.,  LXVII,  374.          \  Pogg.  Annal.,  xci,  110,  and  ex,  134,. 

.  /.  analyt.  Chem.,  rv,  63. 


§  84.]  .BASES   OF   GROUP  V.  205 

hydrochloric  acid,  with  evolution  of  hydrogen  sulphide.  In  mod- 
erately strong  nitric  acid  lead  sulphide  dissolves,  upon  the  applica- 
tion of  heat,  with  separation  of  sulphur  ;-•— if  the  acid  is  rather  con- 
centrated, a  small  portion  of  lead  sulphate  is  also  formed.  Fuming 
nitric  acid  acts  energetically  upon  lead  sulphide,  and  converts  it 
into  sulphate  without  separation  of  sulphur. 

COMPOSITION. 

Pb 206-92  86-58 

S 32-07  13-42 

238-99         100-00 

g.  For  the  composition  and  properties  of  lead  chromate,  see 
Chromic  acid,  §  93. 

§84. 

3.  MERCURY   IN   MERCUROUS   COMPOUNDS  ;    and   4.    MERCURY   IN 
MERCURIC   COMPOUNDS. 

Mercury  is  weighed  either  in  the  METALLIC  STATE,  as  MERCUROUS 
CHLORIDE,  or  as  SULPHIDE,  or  occasionally  as  MERCURIC  OXIDE. 

a.  Metallic  mercury  is  liquid  at  .the  common  temperature ;  it 
has  a  tin- white  color.  When  pure,  it  presents  a  perfectly  bright 
surface.  It  is  quite  unalterable  in  the  air  at  the  common  tempera- 
ture. It  boils  at  360°.  It  evaporates,  but  very  slowly,  at  the 
ordinary  temperature  of  summer.  Upon  long-continued  boiling 
with  water,  a  small  portion  of  mercury  volatilizes,  and  traces  escape 
along  with  the  aqueous  vapor,  whilst  a  very  minute  proportion 
remains  suspended  (not  dissolved)  in  the  water  (comp.  Expt.  No. 
49).  This  suspended  portion  of  mercury  subsides  completely  after 
long  standing.  When  mercury  is  precipitated  from  a  fluid,  in  a 
very  minutely  divided  state,  the  small  globules  will  readily  unite 
to  a  large  one  if  the  mercury  be  perfectly  pure ;  but  even  the 
slightest  trace  of  extraneous  matter,  such  as  fat,  etc.,  adhering  to 
the  mercury  will  prevent  the  union  of  the  globules.  Mercury 
does  not  dissolve  in  hydrochloric  acid,  even  in  concentrated ;  it  is 
barely  soluble  in  dilute  cold  sulphuric  acid,  but  dissolves  readily  in 
nitric  acid. 

J.  Mercurous  chloride,  prepared  in  the  wet  way,  is  a  heavy 


206  FORMS.  [§  84. 

white  powder.  It  is  almost  absolutely  insoluble  in  cold  water ;  in 
boiling  water  it  is  gradually  decomposed,  the  water  taking  up 
chlorine  and  mercury ;  upon  continued  boiling,  the  residue  acquires 
a  gray  color.  Highly  dilute  hydrochloric  acid  fails  to  dissolve  it 
at  the  common  temperature,  but  dissolves  it  slowly  at  a  higher 
temperature ;  upon  ebullition,  with  access  of  air,  the  whole  of  the 
mercurous  chloride  is  gradually  dissolved ;  the  solution  contains  mer- 
curic chloride  (Hg2Cl2  +  2HC1  +  O  =  2HgCl2  +  H2O).  When  acted 
upon  by  boiling  concentrated  hydrochloric  acid,  it  is  rather  speedily 
decomposed  into  mercury,  which  remains  undissolved,  and  mer- 
curic chloride,  which  dissolves.  Boiling  nitric  acid  dissolves  it  to 
mercuric  chloride  and  nitrate.  Chlorine  water  and  nitrohydrochlo- 
ric  acid  dissolve  it  to  mercuric  chloride,  even  in  the  cold.  Solutions 
of  ammonium  chloride,  sodium  chloride,  and  potassium  chloride, 
decompose  it  into  metallic  mercury  and  mercuric  chloride,  which 
latter  dissolves  ;  in  the  cold,  this  decomposition  is  but  slight ;  heat 
promotes  the  action.  It  is  soluble  in  hot  solution  of  mercurous 
nitrate,  and  still  more  in  that  of  mercuric  nitrate;  on  cooling  it 
crystallizes  out  almost  completely  (DERRAY*).  It  does  not  affect 
vegetable  colors ;  it  is  unalterable  in  the  air,  and  may  be  dried  at 
100°,  without  loss  of  weight ;  when  exposed  to  a  higher  degree  of 
heat,  though  still  below  redness,  it  volatilizes  completely,  without 
previous  fusion. 

COMPOSITION. 

Hg, 400-0  84-94 

C13 70-9  15-06 


470- 9  100-00 

c.  Mercuric  sulphide,  prepared  in  the  wet  way,  is  a  black  pow- 
der, insoluble  in  water.  Dilute  hydrochloric  acid  and  dilute  nitric 
acid  fail  to  dissolve  it,  hot  concentrated  nitric  acid  scarcely  attacks 
it,  boiling  hydrochloric  acid  has  no  action  on  it.  By  prolonged 
heating  with  red  fuming  nitric  acid  it  is  finally  converted  into  a 
white  compound,  2HgS  +  Hg(NO3)2,  which  is  insoluble,  or  barely 
soluble,  in  nitric  acid.  It  dissolves  readily  in  nitrohydrochloric 
acid.  From  a  solution  of  mercuric  chloride  containing  much  free 
hydrochloric  acid,  the  whole  of  the  metal  cannot  be  precipitated  aa 


*  Compt.  Rend  ,  LXX,  995. 


§  84.]  BASES   OF   GROUP  V.  207 

sulphide  by  means  of  hydrogen  sulphide,  until  the  solution  is  prop- 
erly diluted.  Should  such  a  solution  be  very  concentrated,  mer- 
curous  chloride  and  sulphur  are  precipitated  (M.  MARTIN*).  Solu- 
tion of  potassa,  even  boiling,  fails  to  dissolve  it.  It  dissolves  in 
potassium  sulphide,  but  readily  only  in  presence  of  free  alkali.  It 
is  insoluble  in  potassium  hydrosulphide  and  in  the  corresponding 
sodium  compound,  and  is  therefore  precipitated  from  its  solution 
in  potassium  or  sodium  sulphide  by  hydrogen  sulphide  or  by 
ammonium  hydrosulphide  (C.  BAKFOEDf).  Small  but  distinctly 
perceptible  traces  dissolve  on  cold  digestion  with  yellowish  or  yel- 
low ammonium  sulphide,  but  after  hot  digestion  it  is  scarcely  possi- 
ble to  detect  any  traces  in  solution.;):  Potassium  cyanide  and  sodium 
sulphite  do  not  dissolve  it.  On  account  of  the  solubility  of  mer- 
curic sulphide  in  potassium  sulphide,  it  is  impossible  to  precipitate 
mercury  by  means  of  ammonium  sulphide  completely  from  solutions 
containing  potassium  or  sodium  hydroxides  or  carbonates.  Such 
solutions  may  occur,  for  instance,  when  a  solution  of  mercuric 
chloride  contains  much  potassium  chloride,  or  sodium  chloride,  for, 
in  this  case,  no  mercuric  oxide  would  be  precipitated  on  the  addi- 
tion of  potassa  or  soda  (H.  ROSE§).  In  the  air  it  is  unalterable, 
even  in  the  moist  state,  and  at  100°.  When  exposed  to  a  higher 
temperature,  it  sublimes  completely  and  unaltered. 

COMPOSITION. 

Hg 200-00  8618 

S  32-07  13-82 


232-07         100-00 

d.  Mercuric  oxide,  prepared  in  the  dry  way,  is  a  crystalline 
brick-colored  powder,  which,  when  exposed  to  the  action  of  heat, 
changes  to  the  color  of  cinnabar,  and  subsequently  to  a  violet-black 
tint.  It  bears  a  tolerably  strong  heat  without  decomposition  ;  but 
when  heated  to  incipient  redness,  it  is  decomposed  into  mercury 
and  oxygen  ;  perfectly  pure  mercuric  oxide  leaves  no  residue  upon 
ignition.  Its  escaping  fumes  also  should  not  redden  litmus-paper. 
"Water  takes  up  a  trace  of  mercuric  oxide,  acquiring  thereby  a  very 
weak  alkaline  reaction.  Hydrochloric  or  nitric  acid  dissolves  it 
readily. 

*Joun.f.  prakt.  Chem.,  LXVII,  376.  f  Zeitschr.  f.  analyt.  Chem.,  iv,  436. 

%  lb.,  m,  140.  $P0ffff-  Annal.,  ex,  141. 


208  FORMS.  [§  85. 


COMPOSITION. 


Hg 200  92-59 

O  16  7-4:1 


216  100-00 


5.  COPPER. 

Copper  is  usually  weighed  in  the  METALLIC  STATE,  or  in  the 
form  of  CUPRIC  OXIDE,  or  of  CUPROUS  SULPHIDE.  Besides  these 
forms,  we  have  to  examine  CUPRIC  SULPHIDE,  CUPROUS  OXIDE,  and 

CUPROUS    SULPHOCYANATE. 

a.  Copper,  in  the  pure  state,  is  a  metal  of   a  peculiar  well- 
known  color.     It  fuses  only  at  a  white  heat.     Exposure  to  dry  air,, 
or  to  moist  air,  free  from  carbon  dioxide,  leaves  the  fused  metal 
unaltered;    but   upon   exposure   to   moist  air   impregnated   with 
carbon  dioxide,  it  becomes  gradually  tarnished  and  coated  with  a 
film,  first  of  a  blackish-gray,  finally  of  a  bluish-green  color.     Pre- 
cipitated finely  divided  copper,  in   contact  with  water  and  air, 
oxidizes  far  more  quickly,  especially  at  an  elevated  temperature. 
On  igniting  copper  in  the  air,  it  oxidizes  superficially  to  a  varying 
mixture  of  cuprous  and  cupric  oxide.     In  hydrochloric  acid,  in  the 
cold,  it  does  not  dissolve  if  air  be  excluded  ;  in  the  heat  it  dissolves 
but  slightly  if  the  metal  is  in  a  compact  state.     Finely  divided 
copper  on  the  contrary  dissolves  slowly  when  heated  with  strong 
hydrochloric  acid,  hydrogen  being  evolved  and  cuprous  chloride 
being  formed  (WELTZIEN*).     Copper  dissolves   readily  in    nitric 
acid.     In  ammonia  it  dissolves  slowly  if  free  access  is  given  to  the 
air  ;  but  it  remains   insoluble   if   the   air   is   excluded.     Metallic 
copper  brought  into  contact  in  a  closed  vessel  with  solution  of 
cupric  chloride  in  hydrochloric  acid,  reduces  the  cupric  to  cuprous 
chloride,  an  atom  of  metal  being  dissolved  for  every  molecule  of 
chloride. 

b.  Cupric  oxide.  —  If  a  dilute,  cold,  aqueous  solution  of  a  cupric 
salt  is  mixed  with  solution  of  potassa  or  soda  in  excess,  a  light  blue 
precipitate  of  cupric  hydroxide,  Cu(OH)2,  is  formed,  which  it  is 
found  difficult  to  wash.     If  the  precipitate  be  left  in  the  fluid 

*  Ann.  d.  Chem.  u.  Pkarm.,  cxxxvi,  109. 


§  85.]  BASES    OF  GROUP   V.  209 

from  wliicli  it  lias  been  precipitated,  it  will,  even  at  a  summer 
lieat,  gradually  change  to  brownish-black,  passing,  with  separation 
of  water,  into  6CuO  -f-  HaO  (SOUCHAY).  This  transformation  is 
immediate  upon  heating  the  fluid  nearly  to  boiling.  The  fluid 
iiltered  off  from  the  black  precipitate  is  free  from  copper.  It 
follows  from  this  that  the  black  precipitate  is  insoluble  in  dilute 
potassa.  Concentrated  potassa  or  soda  on  the  contrary  dissolves  the 
hydroxide,  and  on  long  warming  even  the  black  oxide  (O.  Low*). 
The  resulting  blue  solutions  remain  clear  on  boiling,  even  if  mixed 
with  some  water ;  but  if  boiled  after  being  much  diluted  the  whole 
of  the  copper  will  separate  as.  black  oxide.  If  a  solution  of  a 
cnpric  salt  contains  non-volatile  organic  substances,  the  addition  of 
alkali  in  excess  will,  even  upon  boiling,  fall  to  precipitate  the 
whole  of  the  copper.  The  hyd rated  cupric  oxide,  6CuO  -f-  H2O, 
precipitated  with  potassa  or  soda  from  hot  dilute  solutions  obsti- 
nately retains  a  portion  of  the  precipitant ;  it  may,  however,  be 
completely  freed  from  this  by  washing  with  boiling  water.  The 
precipitated  oxide  after  ignition,  or  the  oxide  prepared  by  decom- 
posing cupric  carbonate  or  nitrate  by  heat,  is  a  brownish-black,  or 
black  powder,  the  weight  of  which  remains  unaltered  even  upon 
strong  ignition  over  the  gas-  or  spirit-lamp,  provided  all  reducing 
gases  be  excluded  (Expt.  No.  50).  If  cupric  oxide  is  exposed  to  a 
heat  approaching  the  fusing  point  of  metallic  copper,  it  fuses, 
yields  oxygen,  and  becomes  CusO,  (FAVKE  and  MAUMENE).  It  is 
very  readily  reduced  by  ignition  with  charcoal,  or  under  the  in- 
fluence of  reducing  gases ;  heated  in  the  air  for  a  long  time,  the 
reduced  metallic  copper  re-oxidizes.  Mixed  with  sulphur  and 
ignited  in  a  current  of  hydrogen,  towards  the  end  strongly,  cupric 
oxide  passes  into  cuprous  sulphide  (CuaS — H.  ROSE).  Cupric  oxide, 
in  contact  with  the  atmosphere,  absorbs  water ;  less  rapidly  after 
being  strongly  ignited  (Expt.  No.  51).  It  is  nearly  insoluble  in 
water;  but  it  dissolves  readily  in  hydrochloric  acid,  nitric  acid, 
&Q.  ;  less  readily  in  ammonia.  It  does  not  affect  vegetable  colors. 

COMPOSITION. 

Cu     .     .     .     .     63-6  80-00 

O       .     .     .     .     16-0  20-00 

79-6  100-00 

*  Zeitschr.  /.  analyt.  Chem. ,  ix,  463. 


210  FOKMS.  [§  85. 

c.  Cupric  sulphide,  prepared  in  the  wet  way,  is  a  brownish- 
black,  or  black  precipitate,  almost  absolutely  insoluble  in  water.* 
When  exposed  to  the  air  in  a  moist  state,  it  acquires  a  greenish 
tint  and  the  property  of  reddening  litmus  paper,  cupric  sulphate 
being  formed.     Hence  the  sulphide  must  be  washed  with  water 
containing   hydrogen   sulphide.      It    dissolves   readily   in   boiling 
nitric  acid,  with  separation   of  sulphur.     Hydrochloric  acid   dis- 
solves it  with  difficulty.    This  is  the  reason  why  hydrogen  sulphide 
'precipitates  copper  entirely  from  solutions  which  contain   even  a 
very  large  amount  of  free  hydrochloric  acid  (GuuNDMANNf).    Only 
when  we  dissolve  a  copper  salt  directly  in  pure  hydrochloric  acid 
of  1*1  sp.  gr.  does  any  copper  remain  unprecipitated  (M.  MARTINA). 
It  does  not   dissolve   in   solutions    of   potassa   and   of   potassium 
sulphide,  particularly  if   these   solutions  be  boiling;  it  dissolves 
perceptibly   in  colorless,  and  much  more   readily  in   hot  yellow 
ammonium  sulphide.    Potassium  cyanide  dissolves  the  freshly  pre-' 
cipitated  sulphide  readily  and  completely.     Upon  intense  ignition 
in  a  current  of  hydrogen  it  is  converted  into  pure  Cu2S. 

d.  If  the  blue  solution  which  is  obtained  upon  adding  to  solu- 
tion of  copper  tartaric  acid  and  then  soda  in  excess,  is  mixed  with 
solution  of  grape  sugar  or  sugar  of  milk,  and  heat  applied,  an 
orange-yellow  precipitate  of  cuprous  hydroxide  is  formed,  which 
contains  the  whole  of  the  copper  originally  present  in  the  solu- 
tion, and  after  a  short  time,  more  particularly  upon  the  applica- 
tion of  a  stronger  heat,  turns  red,  owing  to  the  conversion  of  the 
hydroxide  into  anhydrous  cuprous  oxide  (Cu2O).     The  precipitate, 
which  is  insoluble  in  water,  retains  a  portion  of  alkali  with  con- 
siderable  tenacity.     When  treated  with  dilute  sulphuric  acid,  it 
gives  cupric  sulphate  which  dissolves,  and  metallic  copper  which 
separates. 

e.  Cuprous  sulphocyanate,  Cu2(CNS)2,  which  is  always  formed 
when  potassium  sulphocyanate  is  added  to  a  solution  of  copper,  mixed 
with  sulphurous  or  hypophosphorous  acid,  is  a  white  precipitate  in- 
soluble in  water,  as  well   as  in  dilute  hydrochloric  or  sulphuric 
acid.   Dried  at  115°,  the  salt  retains  from  1  to  3  per  cent,  of  water, 
which  is  driven  off  only  by  heating  to  incipient  decomposition ; 
it   is,  therefore,  not  well   adapted   for   direct   weighing.     When 

*  In  some  experiments  that  I  made  when  examining  the  Weilbach  water,  I 
found  that  about  9.50000  P_art_s  of.  water  are  required  to  dissolve  1  part  of  CuS. 
\Journ.f.  prakt.  Chem.,  LXXIII,  241.         .  |  Ib.,  LXVIT,  375. 


§  86.]  BASES   OF   GROUP   V.  211 

ignited  with  sulphur,  with  exclusion  of  air,  it  changes  to  CuaS 
(RivoT*).  When  heated  with  hydrochloric  acid  and  potassium 
chlorate,  or  with  sulphuric  acid  and  nitric  acid,  it  is  dissolved 
and  suffers  decomposition.^  Solutions  of  potassa  and  soda  separate 
hydrated  cuprous  oxide,  with  formation  of  sulphocyanate  of  the 
alkali  metal. 

f.  Cuprous  sulphide,  produced  by  heating  CuS  in  a  current  of 
hydrogen  or  Cua(CNS)?  with  sulphur,  is  a  grayish-black  crystalline 
mass,  which  may  be  ignited  and  fused  without  decomposition  if 
the  air  is  excluded. 

COMPOSITION. 

Cua    ....     127-20  80-00 

S 32-07  20-00 

159-27  100-00 

§  86. 
6.  BISMUTH. 

Bismuth  is  weighed  as  OXIDE,  as  METAL,  or  as  CHROMATE 
(Bi2O,2CrO4).  Besides  these  compounds,  we  have  to  study  here 

the  BASIC    CARBONATE,  the    BASIC    NITRATE,  the    BASIC    CHLORIDE,  aild 

the  SULPHIDE. 

a.  Bismuth  trioxide,  prepared  by  igniting  the  carbonate  or 
nitrate,  is  a  pale  lemon-yellow  powder  which,  under  the  influence 
of  heat,  assumes  transiently  a  dark  yellow  or  reddish-brown  color. 
When  heated  to  intense  redness,  it  fuses,  without  alteration  of 
weight.  Ignition  with  charcoal,  or  in  a  current  of  carbon  mon- 
oxide, reduces  it  to  the  metallic  state.  Fusion  with  potassium 
cyanide  also  effects  its  complete  reduction  (H.  RosEf).  It  is  in- 
soluble in  water,  and  does  not  affect  vegetable  colors.  It  dissolves 
readily  in  those  acids  which  form  soluble  salts  with  it.  When 
ignited  with  ammonium  chloride  it  gives  metallic  bismuth,  the 
reduction  being  attended  with  deflagration. 

COMPOSITION. 


Bi3     .     .     . 
0,      ... 

416-2 

48-0 

89-66 
10-34 

464-2 

100-00 

*Journ.f.  prakt.  Ghem 

.,  LXII,  252. 

jib.,  LXI,  188. 

212  FORMS.  [§  86. 

b.  Metallic  bismuth  is  white,  with  a  reddish  tinge,  moderately 
hard,  brittle,  with  a  tendency  to  crystallize.     It  fuses  at  264°,  and 
at  a  low  white  heat  volatilizes.     It  does  not  oxidize  in  the  air  at 
the  ordinary  temperature,  but  with  the  co-operation   of  water  it 
oxidizes  slowly,  more  speedily  on   fusion.     It   dissolves  in  dilute 
nitric  acid. 

c.  Bismuth  carbonate. — Upon  adding  ammonium  carbonate  in 
excess  to  a  solution  of  bismuth,  free  from  hydrochloric  acid,  a 
white  precipitate  of  basic  bismuth  carbonate  (Bi3OzCO3)  is  imme- 
diately formed;  part  of  this  precipitate,  however,  redissolves  in 
the  excess  of  the  precipitant.     But  if  the  fluid  with  the  precipitate 
be  heated  before  filtration,  the  filtrate  will  be  free  from  bismuth. 
(Potassium  carbonate   likewise   precipitates   solutions  of  bismuth 
completely  ;  but  the  precipitate  in  this  case  invariably  contains 
traces  of  potassium,  which  it  is  very  difficult  to  remove  by  wash- 
ing.    Sodium    carbonate    precipitates   solutions   of   bismuth   less 
completely.)     The  precipitate  is  easily  washed ;   it  is  practically 
insoluble   in    water,  but   dissolves  readily,  with  effervescence,  in 
hydrochloric  and  nitric  acids.     Upon  ignition  it  leaves  the  oxide. 

d.  The  basic  bismuth  nitrate,   which  is  obtained   by  mixing 
with  water  a  solution  of  the  nitrate  containing  little  or  no  free 
acid,  presents  a  white,  crystalline  powder.     It  cannot  be  washed 
with  pure  cold  water  without  suffering  a  decided  alteration.     It 
becomes  more  basic,  while  the  washings  show  an  acid  reaction,  and 
contain  bismuth.     If  the  basic  salt,  however,  be  washed  with  cold 
water  containing  -g-i^-  of   ammonium  nitrate,   no   bismuth  passes 
through  the  filter.     The  solution  of  ammonium  nitrate  must  not 
be  warm.     These  remarks  only  apply  in  the  absence  of  free  nitric 
acid  (J.  LOWE*).     On  ignition  the  basic  nitrate  passes  into  the 
oxide. 

e.  Basic  bismuth  chloride,  formed  by  adding  much  water  to 
solution   of    bismuth   containing    hydrochloric    acid    or    sodium 
chloride,  is  a  brilliant  white  powder  (BiOCl  after  drying  at  100°). 
It  is  insoluble  in  water,  but  dissolves  in  concentrated  hydrochloric 
or  nitric  acid.     Fused  with  potassium  cyanide  it   gives  metallic 
bismuth.  * 

f.  Bismuth   chromate  (Bi2O3,2CrO3),  which   is   produced    by 
adding  potassium  dichromate,  slightly  in  excess,  to  a  solution  of 

*Journ.f.  prakt.  (JJiem.,  LXXJV,  341. 


§  87.]  BASES    OF  GROUP   V.  213 

bismuth  nitrate  as  neutral  as  possible,  is  an  orange-yellow,  dense, 
readily-subsiding  precipitate,  insoluble  in  water,  even  in  presence 
of  some  free  chromic  acid,  but  soluble  in  hydrochloric  acid  and 
nitric  acid.  It  may  be  dried  at  100°-112°  without  decomposition 
(LowE*). 

COMPOSITION. 
Q 

Q  /          °'  =  Bi.0,  .  .  464-2     69-87 
x  Bi  <  °  >  Cr02  ~  2Cr°3  '  •  2QO'2 

664-4    100-00 

g.  Bismuth  trisulphide,  prepared  in  the  wet  way,  is  a  brownish  • 
black,  or  black  precipitate,  insoluble  in  water,  dilute  acids,  alkalies, 
alkali  sulphides,  sodium  sulphite,  and  potassium  cyanide.  In 
moderately  concentrated  nitric  acid  it  dissolves,  especially  on 
warming,  to  nitrate?  with  separation  of  sulphur.  Hence  in  pre- 
cipitating bismuth  from  a  nitric  acid  solution,  care  should  be 
taken  to  dilute  sufficiently.  Hydrochloric  acid  impedes  the  pre- 
cipitation by  hydrogen  sulphide  only  when  a  very  large  excess  is 
present,  and  the  fluid  is  quite  concentrated.  The  sulphide  does 
not  change  in  the  air.  Dried  at  100°,  it.  continually  takes  up 
oxygen  and  increases  slightly  in  weight ;  if  the  drying  is  protracted 
this  increase  may  be  considerable  (Expt.  No.  52).  Fused  with 
potassium  cyanide,  it  is  completely  reduced  (H.  ROSE).  Reduction 
takes  place  more  slowly  by  ignition  in  a  current  of  hydrogen. 

COMPOSITION. 

Bi2 416-20       81-22 

S3 96-21       18-78 

512-41      100-00 

§  87. 
7.  CADMIUM. 

Cadmium  is  weighed  either  as  OXIDE  or  as  SULPHIDE.  Besides 
these  substances,  we  have  to  examine  CADMIUM  CARBONATE. 

a.  Cadmium  oxide,  produced  by  igniting  the  carbonate  or 
nitrate,  is  a  yellowish-brown  or  reddish-brown  powder.  The  appli- 

*Journ.f.  prakt.  Chem.,  LXVII,  291 


214  FOEMS.  [§  87. 

cation  of  a  white  heat  fails  to  fuse,  volatilize,  or  decompose  it ;  it 
is  insoluble  in  water,  but  dissolves  readily  in  acids ;  it  does  not 
alter  vegetable  colors.  Ignition  with  charcoal,  or  in  a  current  of 
hydrogen,  carbon  monoxide,  or  carburetted  hydrogen,  reduces  it 
readily,  the  metallic  cadmium  escaping  in  the  form  of  vapor. 

COMPOSITION. 

Cd 1124  87-54 

O 16.0  12-46 

128-4          100-00 

b.  Cadmium  carbonate  is  a  white  precipitate,  insoluble  in  water 
and  the  fixed  alkali  carbonates,  and  extremely  sparingly  soluble  in 
ammonium  carbonate.     It  loses  its  water  completely  upon  drying. 
Ignition  converts  it  into  oxide. 

c.  Cadmium  sulphide,  produced  in  the  wet  way,  is  a  lemon- 
yellow  to  orange-yellow  precipitate,  insoluble  in  water,  dilute  acids, 
alkalies,  alkali  sulphides,  sodium  sulphite,  and  potassium  cyanide 
(Expt.  JTo.  53).     It  dissolves  readily  in  concentrated  hydrochloric 
acid,  with  evolution  of  hydrogen  sulphide.     In  precipitating,  there- 
fore, with   hydrogen    sulphide,  a   cadmium   solution   should   not 
contain  too  much  hydrochloric  acid,  and  should  be  sufficiently 
diluted.     The  sulphide   dissolves  readily  in  dilute  sulphuric  acid 
on  heating.     It  dissolves  in  moderately  concentrated  nitric  acid, 
with  separation  of  sulphur.     It  may  be  washed,  and  dried  at  100° 
or  105°,  without  decomposition.     Even  on  gentle  ignition  in  a 
current    of    hydrogen,  it  volatilizes   in    appreciable   amount  (II. 
ROSE*),  partially  unchanged,  partially  as  metallic  vapor. 

COMPOSITION. 

Cd     .     .     .     .     112-40  77-80 

S       ....       32-07  22-20 

144-47          100-00 
*Pogg.  Annal,  ex,  134. 


§§  88,  89.]  METALS   OF   GROUP   VI.  215 

METALS  OF  THE   SIXTH   GROUP. 

§  88. 
1.  GOLD. 

Gold  is  always  weighed  in  the  metallic  state.  Besides  METALLIC 
GOLD,  we  have  to  consider  the  TRISULPHIDE  or  AURIC  SULPHIDE. 

a.  Metallic  gold,  obtained  by  precipitation,  presents  a  blackish- 
brown  powder,  destitute  of  metallic  lustre,  which  it  assumes,  how- 
ever, upon  pressure  or  friction ;  when  coherent  in  a  compact  mass, 
it  exhibits  the  well-known  bright  yellow  color  peculiar  to  it.     It 
fuses  only  at  a  white  heat,  and  resists,  accordingly,  all  attempts  at 
fusion  over  a  spirit-lamp.     It  remains  wholly  unaltered  in  the  air 
and  at  a  red  heat,  and  is  not  in  the  slightest  degree  affected  by 
water,  nor  by  any  simple  acid.     Nitrohydrochloric  acid  dissolves 
it  to  trichloride.     Hot  concentrated  sulphuric  acid  containing  a 
little  nitric  acid  dissolves  gold,   especially  if  in  a  linely  divided 
condition,  to  a  yellow  fluid,  from  which  it  is  thrown  down  again 
by  water  (J.  SPILLEE!). 

b.  Auric  sulphide. — When  hydrogen  sulphide  is  transmitted 
through  a  cold  dilute  solution  of  auric  chloride,  the  whole  of  the 
gold  separates  as  auric  sulphide.  Au2S3,  in  form  of  a  brownish- 
black  precipitate.     If  this  precipitate  is  left  in  the  fluid,  it  is 
gradually  transformed  into  metallic  gold  and  free  sulphuric  acid. 
Upon  transmitting  hydrogen  sulphide  through  a  warm  solution 
of  auric  chloride,  aurous  sulphide  AuaS  precipitates,  with  formation 
of    sulphuric    and    hydrochloric  acids,  thus :     4AuCla  +  3HaS 
+  4H3O  =  2Au,S  +  12HC1  +  HaSO4. 

Auric  sulphide  is  insoluble  in  water,  hydrochloric  acid,  and 
nitric  acid,  but  dissolves  in  nitrohydrochloric  acid.  Colorless  am- 
monium sulphide  fails  to  dissolve  it ;  but  it  dissolves  almost 
entirely  in  yellow  ammonium  sulphide,  and  completely  upon 
addition  of  potassa.  It  dissolves  in  potassa,  with  separation  of 
gold.  Yellow  potassium  sulphide  dissolves  it  completely.  It  dis- 
solves in  potassium  cyanide.  Exposure  to  a  moderate  heat  reduces 
it  to  the  metallic  state. 


2.  PLATINUM. 

Platinum  is  invariably  weighed  in  the  METALLIC  STATE  ;  it  is 
f  Cliem.  News,  xiv,  256;  Zeitschr.f.  analyt.  Chem.,vi,  228. 


216  FORMS.  [§  90. 

generally   precipitated   as   AMMONIUM    PLATINIC    CHLORIDE,    or   as 

POTASSIUM  PLATINIC  CHLORIDE,  rarely  aS  PLATLNIC    SULPHIDE. 

a.  Metallic  platinum,  produced  by  igniting  ammonium  platinic 
chloride,  or  potassium  platinic  chloride,  presents  the  appearance  of 
a  gray,  lustreless,  porous  mass  (spongy  platinum).  The  fusion  of 
platinum  can  be  effected  only  at  the  very  highest  degrees  of  heat. 
It  remains  wholly  unaltered  in  the  air,  and  in  the  most  powerful 
furnaces.  It  is  not  attacked  by  water,  or  simple  acids,  and  scarcely 
by  aqueous  solutions  of  the  alkalies.  Nitrohydrochloric  acid  dis- 
solves it  to  platinic  chloride. 

I).  The  properties  of  potassium  platinic  chloride,  and  those  of 
ammonium  platinic  chloride,  have  been  given  already  in  §§  68 
and  TO  respectively. 

c.  Platinic  sulphide. — When  a  concentrated  solution  of  pla- 
tinic chloride  is  mixed  with  hydrogen  sulphide  water,  or  when 
hydrogen  sulphide  gas  is  transmitted  through  a  rather  dilute 
solution  of  the  chloride,  no  precipitate  forms  at  first ;  after  stand- 
ing some  time,  however,  the  solution  turns  brown,  and  finally  a 
precipitate  subsides.  But  if  the  mixture  of  solution  of  platinic 
chloride,  with  hydrogen  sulphide  in  excess,  is  gradually  heated 
(finally  to  ebullition),  the  whole  of  the  platinum  separates  as 
platinic  sulphide  (free  from  any  admixture  of  platinic  chloride). 
Platinic  sulphide  is  insoluble  in  water  and  in  simple  acids  ;  but  it 
dissolves  in  nitrohydrochloric  acid.  It  dissolves  partly  in  caustic 
alkalies,  with  separation  of  platinum,  and  completely  in  alkali 
sulphides,  especially  the  polysulphides  if  used  in  sufficient  excess. 
When  hydrogen  sulphide  is  transmitted  through  water  holding 
minutely  divided  platinic  sulphide  in  suspension,  the  sulphide, 
absorbing  hydrogen  sulphide,  acquires  a  light  grayish-brown  color  ;. 
the  hydrogen  sulphide  thus  absorbed,  separates  again  upon  exposure 
to  t^ie  air.  When  moist  platinic  sulphide  is  exposed  to  the  air,  it 
is  gradually  decomposed,  being  converted  into  metallic  platinum 
and  sulphuric  acid.  Ignition  in  the  air  reduces  platinic  sulphide  to 
metallic  platinum. 

§  90. 
3.   ANTIMONY. 

Antimony  is  weighed  as  ANTIMONOUS  SULPHIDE,  as  ANTIMONY 
TETROXIDE  (or  ANTIMONOUS  ANTiMONATE),  or  more  rarely  in  the 
METALLIC  state. 


§  90.]  METALS    OF   GROUP   VI.  217 

a.  Upon  transmitting  hydrogen  sulphide  through  a  solution  of 
antimonous  chloride  mixed  with  tartaric  acid,  an  orange  precipi- 
tate of  amorphous  antimonous  sulphide  is  obtained,  mixed  at  first 
with  a  small  portion  of  basic  antimony  chloride.  However,  if  the 
fluid  is  thoroughly  saturated  with  hydrogen  sulphide,  and  a  gentle 
heat  applied,  the  chloride  mixed  with  the  precipitate  is  decom- 
posed, and  pure  antimonous  sulphide  obtained.  Antimonous 
sulphide  is  insoluble  in  water  and  dilute  acids ;  it  dissolves  in  con- 
centrated hydrochloric  acid,  with  evolution  of  hydrogen  sulphide. 
In  precipitating  with  hydrogen  sulphide,  therefore,  antimony 
solutions  should  not  contain  too  much  free  hydrochloric  acid,  and 
should  be  sufficiently  diluted.  The  amorphous  antimonous  sul- 
phide dissolves  readily  in  dilute  potassa,  ammonium  sulphide,  and 
potassium  sulphide,  sparingly  in  ammonia,  very  slightly  in  ammo- 
nium carbonate,  and  not  at  all  in  hydrogen  potassium  sulphite. 
The  amorphous  sulphide,  dried  in  the  desiccator  at  the  ordinary 
temperature,  loses  very  little  weight  at  100°;  if  kept  for  some 
time  at  this  latter  temperature  its  weight  remains  constant.  But 
it  still  retains  a  little  water,  which  does  not  perfectly  escape  even 
at  190°,  but  at  200°  the  sulphide  becomes  anhydrous,  turning 
black  and  crystalline  (H.  KOBE*  and  Expt.  No.  54).  Ignited 
gently  in  a  stream  of  carbon  dioxide,  the  weight  of  this  anhydrous 
sulphide  remains  constant ;  at  a  stronger  heat  a  small  amount 
volatilizes.  The  amorphous  sulphide,  if  long  exposed  to  the  action 
of  air,  in  presence  of  water,  slowly  takes  up  oxygen,  so  that  on 
treatment  with  tartaric  acid  it  yields  a  filtrate  containing  anti- 
mony. 

Antimonic  sulphide  is  insoluble  in  water,  also  in  wrater  con- 
taining hydrogen  sulphide.  It  dissolves  completely  in  ammonia, 
especially  on  warming;  traces  only  dissolve  in  ammonium  car- 
bonate. On  heating  dried  antimonic  sulphide  in  a  current  of 
carbon  dioxide  2  atoms  of  sulphur  escape,  black  crystalline  anti- 
monous sulphide  remaining. 

On  treating  antimonous  or  antimonic  sulphide  with  fuming 
nitric  acid  violent  oxidation  sets  in.  We  obtain  first  antimonic 
acid  and  pulverulent  sulphur  ;  on  evaporating  to  dryness  antimonic 
acid  and  sulphuric  acid  ;  and  lastly  on  igniting  antimony  tetroxide. 
The  same  antimony  tetroxide  is  obtained  by  igniting  the  sulphide 

*Journ.f,prakt.  Chem.,  LIX.  381. 


•218  FORMS.  [§  90. 

with  30  to  50  times  its  amount  of  mercuric  oxide  (BUNSEN*). 
[According  to  later  investigations  of  BuNSEN,f  the  temperature 
necessary  to  reduce  Sb2O6  to  Sb2O4  lies  so  near  that  which  reduces 
Sb2O4  to  Sb2O3  that  it  is  not  easy  to  bring  antimony  into  Sb2O4  for 
weighing.  It  is  possible  only  by  using  a  large  covered  platinum  or 
rather  large  open  porcelain  crucible  (by  suitable  choice  of  size  of 
crucible  and  intensity  of  flame)  and  heating  with  a  gas  blast  lamp 
so  that  the  bottom  only  of  the  crucible  reaches  a  strong  red  heat, 
to  drive  off  exactly  one  atom  of  oxygen  from  Sb2O5.]  Ignition  in, 
.a  current  of  hydrogen  converts  the  sulphides  of  antimony  into  the 
metallic  state. 

COMPOSITION. 

Sba    .     .     .     .     240-80  71-45 

S8      ....       96-21  28-55 

337-01  100-00 

b.  Antimony  tetroxide  is  a  white  powder,  which,  when  heated, 
acquires  transiently  a  yellow  tint ;  it  is  infusible  ;  it  loses  weight 
when  ignited  intensely  in  a  small  platinum  crucible  with  a  gas 
blast  flame  (BuNSENf).  It  is  almost  insoluble  in  water,  and  dis- 
solves in  hydrochloric  acid  with  very  great  difficulty.  It  undergoes 
no  alteration  on  treatment  with  ammonium  sulphide.  It  manifests 
:.an  acid  reaction  when  placed  upon  moist  litmus-paper. 

COMPOSITION. 

Sb2     ......      240-8  79.00 

O4 64-0  21-00 


304-8  100-00 

c.  Metallic  antimony,  produced  in  the  wet  way,  by  precipita- 
tion, presents  a  lustreless  black  powder.  It  may  be  dried  at  100° 
without  alteration.  It  fuses  at  a  moderate  red  heat.  Upon  ignition 
in  a  current  of  gas,  e.g.,  hydrogen,  it  volatilizes,  without  formation 
of  antimonetted  hydrogen.  Hydrochloric  acid  has  very  little 
action  on  it,  even  when  concentrated  and  boiling.  Nitric  acid 
converts  it  into  antimorious  oxide,  mixed  with  more  or  less 

*  Annal.  de  Chem.  u.  Pharm.,  cvi,  3.         \Zeitschr.f.analyt.   Chem.,  1879,  268 


§  91.]  METALS   OF  GROUP  VI.  219 

antimony  tetroxide,  according  to  the  concentration  of  the  nitric 
acid. 

§91. 

4.  TIN  IN  STANNOUS  COMPOUNDS  ;   and  5.  TIN  IN  STANNIC 
COMPOUNDS. 

Tin  is  generally  weighed  in  the  form  of  STANNIC  OXIDE  ;  be- 
sides stannic  oxide,  we  have  to  examine  STANNOUS  SULPHIDE  and 

STANNIC    SULPHIDE. 

a.  Stannic  oxide. — If  a  solution  of  an  alkali,  sodium  sulphate 
or  ammonium  nitrate  is  added  to  a  solution  of  stannic  chloride, 
stannic  acid  (HaSnO3)  is  precipitated.  This  precipitate  is  soluble 
in  excess  of  soda,  and  does  not  separate  again  even  on  the  addition 
of  a  large  quantity  of  soda  (C.  F.  BAKFOED*).  It  is  also  readily 
soluble  in  hydrochloric  acid. 

By  the  action  of  nitric  acid  on  metallic  tin,  or  by  evaporating 
a  solution  of  tin  with  an  excess  of  nitric  acid,  a  white  residue  is 
obtained  which  is  metastannic  acid  (SnBH]0O1B  ?).  This  residue  is 
insoluble  in  water,  but  very  slightly  soluble  in  nitric  acid,  or 
sulphuric  acid.  By  heating  with  hydrochloric  acid  it  does  not 
dissolve,  but  is  changed  to  metastannic  chloride,  which  is  soluble 
in  water  after  removal  of  the  excess  of  hydrochloric  acid.  Soda 
added  to  a  solution  of  metastannic  chloride  precipitates  sodium 
inetastannate,  which  is  insoluble  in  excess  of  soda  and  in  weak 
alcohol,  whereas  when  added  to  ordinary  stannic-chloride  solution, 
it  affords  a  precipitate  which  is  soluble  in  excess,  and  is  not 
reprecipitated  by  even  a  very  large  excess  (C.  F.  BARFOED*). 
Upon  intense  ignition,  both  stannic  and  metastannic  acids 
are  converted  into  stannic  oxide.  Mere  heating  to  redness  is  not 
sufficient  to  expel  all  the  water  (DuMAsf). 

Stannic  oxide  is  a  straw-colored  powder,  which  under  the 
influence  of  heat,  transiently  assumes  a  different  tint,  varying  from 
bright  yellow  to  brown.  It  is  insoluble  in  water  and  acids,  and 
does  not  alter  the  color  of  litmus-paper.  Mixed  with  ammonium 
chloride  in  excess,  and  ignited,  it  volatilizes  completely  as  stannic 
chloride.  If  stannic  oxide  is  fused  with  potassium  cyanide,  all  the 
tin  is  obtained  in  form  of  metallic  globules,  which  may  be  com- 
pletely, and  without  the  least  loss  of  metal,  freed  from  the  adhering 
slag,  by  extracting  with  dilute  alcohol,  and  rapidly  decanting  the 
fluid  from  the  tin  globules  (IT.  ROSE;}:). 

*  Zeitschr.f.  analyt.  Chem.,  vu,  260.       f  Annal.  d.  Chem.  u.  Pharm.,  cv,  104. 
\Journ.f.  prakt.  Chem.,  LXI,  189. 


220  FORMS.  [§91. 


COMPOSITION. 


Sn 119  78-81 

Oa 32  21-19 


151  100-00 

b.  Hydrated  stannous  sulphide  forms   a   brown   precipitate, 
insoluble  in  water,  hydrogen  sulphide  water,  and  dilute  acids.     In 
precipitating  tin  from  stannous  solutions  by  means  of  hydrogen 
sulphide,  free  hydrochloric  acid  must  not  be  present  in  too  large 
amount,  and  the  solution  must  be  diluted  sufficiently.     Ammonia 
fails  to  dissolve  it ;  but  it  dissolves  pretty  readily  in  yellow  ammo- 
nium sulphide,   and  in  yellow  potassium  sulphide ;    it  dissolves 
readily  in  hot  concentrated  hydrochloric  acid.    Heated,  with  exclu- 
sion of  air,  it  loses  its  water,  and  is  rendered  anhydrous ;  when  ex- 
posed to  the  continued  action  of  a  gentle  heat,  with  free  access  of 
air,  it  is  converted  into  sulphur  dioxide,  which  escapes,  and  stannic 
oxide,  which  remains. 

c.  Hydrated  stannic  sulphide,  precipitated  by  acids  from  the 
solution  of  its  alkali  sulphur  salts,  is  a  light-yellow  precipitate.    In 
washing  with  pure  water,  it  is  inclined  to  yield  a  turbid  filtrate 
and  to  stop  up  the  pores  of  the  filter ;  this  annoyance  is  got  over 
by  washing  with  water  containing  sodium  chloride,   ammonium 
acetate,  or  the  like  (BUNSEN).     On  drying,  the  precipitate  assumes 
a  darker  tint.     It  is  insoluble  in  water;  it  dissolves  with  difficulty 
in  ammonia,  but  readily  in  potassa,  alkali  sulphides,  and  hot  con- 
centrated hydrochloric  acid.    It  is  insoluble  in  hydrogen  potassium 
sulphite.     In  precipitating  tin  from  stannic  solutions  by  hydrogen 
sulphide,  the  solution  should  not  contain  too  much  free  hydro- 
chloric acid,  and   should   be   sufficiently   diluted.    According   to 
C.   F.   BARFOED*  the   precipitates   thus   produced   are   not   pure 
hydrated  stannic  sulphide,  but  a  mixture  of  this  with  stannic  or 
metastannic  acid,  as  the  case   may  be.     The  precipitate  thrown 
down  from  ordinary  stannic  chloride  keeps  its  yellow  color  even 
after  long  standing  in  the  fluid,  and  dissolves  completely  in  excess 
of  soda ;  that  thrown  down  from  the  metastannic  chloride  is  first 
white  and  becomes  gradually  yellow,  it  turns  brown  on  standing 
in  the  fluid  and   dissolves  in  excess  of  soda,  leaving,  however,  a 
considerable  residue  of  sodium  metastannate.     When  heated,  with 

*  Zeitschr.  f.  analyt.  Cliem. ,  vii,  261. 


§  92.]  METALS   OF   GROUP   VI.  221 

exclusion  of  air,  stannic  sulphide  loses  its  water  of  hydration,  and, 
at  the  same  time,  according  to  the  degree  of  heat,  one-half  or  one- 
fourth  of  its  sulphur,  becoming  converted  either  into  stannous 
sulphide  or  the  sesquisulphide  of  tin ;  when  heated  very  slowly, 
with  free  access  of  air,  it  is  converted  into  stannic  oxide,  with  dis- 
engagement of  sulphur  dioxide. 

§92. 

6.  ARSENIC  OF  ARSENOUS  COMPOUNDS;  and  7.  ARSENIC  OF 
ARSENIC  COMPOUNDS. 

ARSENIC  is  weighed  either  as  LEAD  ARSENATE,   as   ARSENOUS 

SULPHIDE,  as    AMMONIUM  MAGNESIUM  ARSENATE,  as  MAGNESIUM  PYRO- 

ARSENATE,  or  as  URANYL  PYROARSENATE ;  besides  these  forms,  we 
have  here  to  examine  also  ARSENIO-MOLYBDATE  OF  AMMONIUM. 

a.  Lead  arsenate,  in  the  pure  state,  is  a  white  powder,  which 
agglutinates  when  exposed  to  a  gentle  red  heat,  at  the  same  time 
transitorily  acquiring  a  yellow  tint ;  it  fuses  wrhen  exposed  to  a 
higher  degree  of  heat.     When  strongly  ignited,  it  suffers  a  slight 
diminution   of  weight,  losing  a  small  proportion  of  arsenic  acid, 
which  escapes  as  arsenous  oxide  and  oxygen.    In  analysis  we  have 
never  occasion  to  operate  upon  the  pure  lead  arsenate,  but  upon  a 
mixture  of  it  with  lead  oxide. 

b.  Arsenous  sulphide  forms  a  precipitate  of  a  rich  yellow 
color;   it  is  insoluble  in  water,*  and  also  in  hydrogen-sulphide 
water.     When  boiled  with  water,  or  left  for  several  days  in  con- 
tact with  that  fluid,  it  undergoes  a  very  trifling  decomposition  :  a 
trace  of  arsenous  acid  dissolves  in  the  water,  and  a  minute  pro- 
portion of  hydrogen  sulphide  is  disengaged.     This  does  not  in  the 
least  interfere,  however,  with  the  washing  of  the  precipitate.    The 
precipitate  may  be   dried   at   100°,   without   decomposition ;   the 
whole  of  the  water  which  it  contains  is  expelled  at  that  tempera- 
ture.    When  exposed  to  a  stronger  heat,  it  transitorily  assumes 
a  brownish-red   color,  fuses,  and   finally  rises  in  vapor,  without 
decomposition.     It  dissolves  readily  in  alkalies,  alkali  carbonates, 

*  In  some  experiments  which  I  had  occasion  to  make,  in  the  course  of  an 
analysis  of  the  springs  of  Weilbach  (Chemische  Untersuchung  der  wichtigsten 
Nassauischen  Mineralwasser  von  Dr.  Fresenius,  V.  Schwefelquelle  zu  Weil  bach. 
Weisbaden,  Kreidel  und  Niedner.  1856),  I  found  that  one  part  of  AsaS3  dis- 
solves in  about  one  million  parts  of  water. 


222  FORMS.  [§  92. 

alkali  sulphides,   potassium-hydrogen  sulphite,    and  nitrohydro- 
chloric  acid ;  but   it  is   scarcely  soluble  in  boiling   concentrated 
hydrochloric  acid.     Red  fuming  nitric  acid  converts  it  into  arsenic 
'  acid  and  sulphuric  acid.     It  is  insoluble  in  carbon  disulphide. 

COMPOSITION. 

As, 150-00         60-92 

S3 96-21          39-08 

246-21        100-00 

c.  Ammonium  magnesium  ar  senate  forms  a  white,  somewhat 
transparent,  finely  crystalline  precipitate,  which  when  dried  in  a 
desiccator  has  the  formula  NH4MgAsO4  +  6HQO.  After  drying 
at  100°,  its  composition  is  (NH4MgAsO4)2  +  H2O.  At  a  higher 
temperature,  say  105° — 110°,  more  water  escapes,  and  at  130°  this 
loss  is  considerable  (PULLER*).  Upon  ignition  it  loses  water  and 
ammonia,  and  changes  to  magnesium  pyroarsenate,  Mg2As2O7.  On 
rapid  ignition  the  escaping  ammonia  has  a  reducing  action  on  the 
arsenic  acid,  and  a  notable  loss  is  occasioned  (H.  ROSE)  ;  by  raising 
the  heat  very  gradually  reduction  may  be  avoided  (H.  ROSE, 
WiTTSTEiN,f  PULLER),  or  by  passing  a  current  of  dry  oxygen 
during  the  ignition.  Ammonium  magnesium  arsenate  dissolves 
very  sparingly  in  water,  one  part  of  the  salt  dried  at  100°,  requir- 
ing 2656,  one  part  of  the  anhydrous  salt,  2788  parts  of  water  of 
15°.  It  is  far  less  soluble  in  amrnoniated  water,  one  part  of  the 
salt  dried  at  100°  requiring  15038,  one  part  of  the  anhydrous  salt, 
15786  parts  of  a  mixture  of  one  part  of  solution  of  ammonia 
(0'96  sp.  gr.),  and  3  parts  of  water  at  15°.  In  water  containing 
ammonium  chloride,  it  is  much  more  readily  soluble,  one  part  of 
the  anhydrous  salt  requiring  886  parts  of  a  solution  of  one  part  of 
ammonium  chloride  in  7  parts  of  water.  Presence  of  ammonia 
diminishes  the  solvent  capacity  of  the  ammonium  chloride  ;  one 
part  of  the  anhydrous  salt  requires  3014  parts  of  a  mixture  of 
60  parts  of  water,  10  of  solution  of  ammonia  (0*96  sp.  gr.)  and 
one  of  ammonium  chloride.:):  A  solution  of  ammonium  chloride, 
ammonia  and  magnesium  sulphate  dissolves  much  less  of  the  salt 
than  ammoniated  water ;  thus,  PULLER  (loc.  cit.)  found  that  one 

*Zettschr.f.analyL  Chem.,  x,  62.  \lb.,  n,  19. 

|  lb.,  in,  206.     PULLER  obtained  almost  the  same  numbers  (Ib. ,  x,  53). 


§  92.]  METALS    OF    GROUP    VI. 

part  of  the  anhydrous  salt  dissolved  in  32827  parts  of  a  fluid  con- 
taining ^  of  magnesia  mixture  (p.  113).  Excess  of  alkali  arsenate 
still  more  diminishes  the  solubility  of  the  salt  in  water  containing, 
ammonia  and  ammonium  chloride  (PULLER). 

COMPOSITION    OF    AMMONIUM    MAGNESIUM    ARSENATE     DEIED   AT     100°. 

2MgO.  80-600         21-17 

4  NI>    •     -       52-144         13-69 


K^^t   '  As2O5  .  .  .  230-000  60-41 

H,0  .  .  .   18-016  4-7S 
+H.O 

380-760  100-00 

d.  Magnesium  pyroarsenate,  obtained  by  careful  ignition  of 
the  preceding  salt,  is  white,  infusible  by  ignition  in  a  porcelain 
crucible  even  over  the  blowpipe,  but  agglutinating  at  a  still  higher 
temperature,  and  finally  fusing.  After  ignition  in  a  porcelain 
crucible  it  dissolves  readily  in  hydrochloric  acid :  ammonia  pre- 
cipitates ammonium  magnesium  arsenate  from  the  solution  in  a 
crystalline  form. 

COMPOSITION. 
O  , 


2MgO     .     .     .      80-6         25-95 

V  Q  = 

\AsO  <  o  >  Mg         As3O6     .     .     .     230-0         74-05 

310-6       100-00 

e.  Uranyl  pyroarsenate. — If  a  solution  of  arsenic  acid  is  mixed 
with  potash  in  slight  excess,  then  writh  acetic  acid  to  strongly  acid 
reaction,  and  finally  with  uranyl  acetate,  the  whole  of  the  arsenic 
is  thrown  down  as  UO2HAsO4  -|-  4H2O.  In  the  presence  of  salts 
of  ammonia  the  precipitate  also  contains  the  whole  of  the  arsenic, 
and  consists  of  UO2KH4AsO4  -f-  water.  Both  precipitates  are  pale 
yellowish-green,  slimy,  insoluble  in  water,  acetic  acid  and  saline 
solutions,  such  as  ammonium  chloride ;  soluble  in  mineral  acids. 
Boiling  favors  the  separation  of  the  precipitate,  addition  of  a  few 
drops  of  chloroform  will  help  it  to  settle,  the  washing  is  to  be 
effected  by  boiling  up  and  decanting.  Both  precipitates  give 
(UOQ)2As2O7. on  ignition.  The  latter  is  a  light  yellovy  residue ;  if 
it  has  turned  greenish  from  the  action  of  reducing  gases,  it  maybe 
restored  to  its  proper  color  by  moistening  writh  nitric  acid  and 


224  FORMS.  [§  92. 

re-igniting.  On  igniting  the  ammonium  uranyl  arsenate,  the 
ammonia  must  first  be  expelled  by  cautious  heating,  or  a  current  of 
oxygen  must  be  passed  during  the  ignition,  otherwise  the  arsenic 
acid  will  be  partially  reduced,  and  arsenic  will  be  lost  (PULLER*). 

COMPOSITION. 

/  AsO  <  °  >  U02          2U020  .     .      575-2       71-44 
0< 

N  AsO  <  Q  >  UO,          As2O5     .     .      230-0       28-56 

805-2      100-00 

f.  Ferric  Arsenate. — The  white,  slimy  precipitate  obtained 
when  ferric  chloride  is  treated  with  sodium  arsenate  has  the  follow- 
ing composition  :  2Fe2.As6Oa,  -f-  aq.,  and  it  is  formed  as  in  the  fol- 
lowing reaction  :  2Fe2Cl6  +  6NaaHAsO4  =  2Fe2.As6O21  +  12NaCl 
-f-  3HaO.     It  is  soluble  in  mineral  acids,  but  is  soluble  in  arsenic- 
acid  solution  only  when  this  is  highly  concentrated  and  cold.    On 
either  heating  or  diluting  such  a  solution  a  precipitate  of  ferric 
arsenate  occurs;   on  cooling  the  solution  the  precipitate  does  not 
again  dissolve  (LUNGE  *).     Ferric  arsenate  is  soluble  in  ammonia 
with   yellow    color.     Besides  this   neutral  compound   there  are 
others  with  higher  iron  content,  e.g.)  FeAsO4  -j-  5H2O,  precipi- 
tated  on  adding  ferric   acetate  to  arsenic  acid  (KOTSCHOUBET)  ; 
2Fe2.As2On  -f-  12H2O,    obtained  when  basic  ferrous  arsenate  is 
oxidized  with  nitric  acid  and  ammonia  added;    16Fea.As2OB3  + 
24H2O,  obtained  on  boiling  less  basic  compounds  with  excess  of 
potassium-hydroxide  solution  (BERZELIUS).      The  last  two    com- 
pounds are  insoluble  in  ammonia;   the  last  is  quite  like   ferric 
hydroxide.     In  BERTHIER'S  method  of  estimating  arsenic  acid  a 
mixture  of  these  various  salts  is  obtained.     The  more  basic  they 
are,  the  better  radapted  they  are  for  estimation,   on  account  of 
their  insolubility  in  ammonia.      They  are  also  then  more  easily 
washed.     On  being  very  gradually  heated  to  redness,  water  alone 
is  expelled;    if  the    salt   is  strongly   heated  suddenly,  however 
(before  the  adhering  ammonia  has  escaped),  a  part  of  the  arsenic 
acid  is  reduced  to  arsenous  acid  (H.  ROSE), 

g.  Arsenio-molybdate  of  ammonium. — If  a  fluid  containing 

*  ZeitscJir.  /.  analyt.  Chem.,  x,  72. 


§  93.]  ACIDS   OF   GROUP  I.  225 

arsenic  acid  is  mixed  with  excess  of  the  nitric  acid  solution  of 
ammonium  molybdate,  the  fluid  remains  clear  in  the  cold,  but  on 
heating  a  yellow  precipitate  of  arsenio  molybdate  of  ammonium 
separates.  This  precipitate  comports  itself  with  solvents  like  the 
analogous  compound  of  phosphoric  acid ;  it  is,  like  the  latter, 
insoluble  in  water,  nitric  acid,  dilute  sulphuric  acid  and  salts,  pro- 
vided an  excess  of  solution  of  ammonium  molybdate,  mixed  with 
acid  in  moderate  excess,  be  present.  Hydrochloric  acid  or  metallic 
chlorides,  when  present  in  large  quantity,  interfere  with  the 
thoroughness  of  the  precipitation.  SELIGSOHN  *  found  it  to  be 
composed  of  87*666  per  cent,  of  molybdic  acid,  6*308  arsenic  acid, 
4*258  ammonia,  and  1*768  water. 

B.  FORMS  IN  WHICH   THE  ACID  RADICALS  ARE  WEIGHED   OR 

PRECIPITATED. 

ACIDS  OF  THE  FIRST  GROUP. 
§93. 

1.  AKSENOCS  ACID  and  ARSENIC  ACID. — See  §  92. 

2.  CHROMIC  ACID. 

Chromic  acid  is  weighed  either  as  CHROMIC  OXIDE,  or  as  LEAD 
OHROMATE,  or  BARIUM  CHROMATE.  We  have  also  to  consider  MER- 

CUROUS  CHROMATE. 

a.  Chromic  oxide. — See  §  76. 

J.  Lead  chromate  obtained  by  precipitation  forms  a  bright-yel- 
low precipitate,  insoluble  in  water  and  acetic  acid,  barely  soluble  in 
dilute  nitric  acid,  readily  in  solution  of  potassa.  When  lead  chro- 
mate is  boiled  with  concentrated  hydrochloric  acid,  it  is  readily 
decomposed,  lead  chloride  and  chromic  chloride  being  formed. 
Addition  of  alcohol  tends  to  promote  this  decomposition.  Lead 
chromate  is  unalterable  in  the  air.  It  dries  thoroughly  at  100°. 
Under  the  influence  of  heat  it  transitorily  acquires  a  reddish-brown 
tint ;  it  fuses  at  a  red  heat  ;  when  heated  beyond  its  point  of 
fusion,  it  loses  oxygen,  and  is  transformed  into  a  mixture  of  chro- 
mic oxide  and  basic  lead  chromate.  Heated  in  contact  with  organic 
substances,  it  readily  yields  oxygen  to  the  latter. 

*Journ.f.  prakt.  Chem.,  LXXVII,  481. 


226  FORMS.  [§  93. 

COMPOSITION. 

°\Tn,      PbO    .     .     .  222-92  69-01 

0>         "CrO,   .     .     .  100-10  30-99 


323-02          100-00 

c.  Barium  chromate  is  obtained  as  a  light-yellow  precipitate 
on  mixing  a  solution  of  an  alkali  chromate  with  barium  chloride. 
It  dissolves  in  hydrochloric  and  in  nitric  acid,  but  not  in  acetic 
acid.  On  washing  with  pure  water,  the  latter  begins  to  dissolve  it 
slightly,  as  soon  as  all  soluble  salts  are  removed,  to  such  an  extent 
that  the  washings  run  off  yellow.  The  precipitate  is  insoluble  in 
saline  solutions.  Hence  it  is  best  to  use  a  solution  of  ammonium 
acetate  for  washing  (PEARSON  and  RICHARDS*).  It  is  not  decom- 
posed by  moderate  ignition. 

COMPOSITION, 

'     '     '     153'40  60'51 


253-50         100-00 

d.  Mercurous  chromate  obtained  by  adding  mercurous  nitrate 
to  an  alkali  chromate  is  a  brilliant-red  precipitate,  which  turns 
black  by  the  action  of  light.  It  dissolves  very  slightly  in  cold 
water,  more  in  boiling  water,  being  partially  converted  into  a  mer- 
curic salt ;  it  dissolves  slightly  in  dilute  nitric  acid.  For  washing, 
it  is  best  to  use  a  dilute  solution  of  mercurous  nitrate  containing 
but  little  free  acid ;  in  this  solution  it  is  insoluble  (H.  ROSE  f). 

3.  SULPHURIC  ACID. 

Sulphuric  acid  is  determined  best  in  the  form  of  BARIUM  SUL- 
PHATE, for  the  properties  of  which  see  §  71. 

4.  PHOSPHORIC  ACID. 

The  principal  forms  into  which  phosphoric  acid  is  converted  are 

as  follows  :— LEAD    PHOSPHATE,  MAGNESIUM  PYROPHOSPHATE,  MAGNE- 
SIUM PHOSPHATE  Mgs(PO4)a,  FERRIC    PHOSPHATE,  URANYL   PYROPHOS* 


*  Zeitschr.f.  analyt.  Chem.,  ix,  108.  \Pogg.  Ann.,  LIU,  124. 


§  93.]  ACIDS    OF    GROUP    I.  227 

PHATE,   STANNIC   PHOSPHATE,   1111(1    SILVER  PHOSPHATE.       Besides  these 

compounds,    we   have   to    examine    MERCUROUS    PHOSPHATE    and 

PHOSPHO-MOLYBDATE  OF  AMMONIUM. 

a.  The  lead  phosphate  obtained  in  the  course  of  analysis  is 
rarely  pure,  but  is  generally  mixed  with  free  lead  oxide.     In  this 
mixture  we  have  accordingly  the  normal  lead  phosphate  Pb3(PO4)2 ; 
in  the  pure  state,  this  presents  the  appearance  of  a  white  powder ; 
it  is  insoluble  in  water,  acetic  acid,  and  ammonia.     It  dissolves 
readily  in  nitric  acid.     When  heated  it  fuses  without  decomposi- 
tion. 

b.  Magnesium  pyrophosphate. — See  §  74. 

c.  Magnesium  phosfjhate  (Mg3(PO4)2).— A  mixture  of  this  com- 
pound with  excess  of  magnesia  is  produced  by  mixing  a  solution  of 
an  alkali  phosphate,  containing  ammonium  chloride,  with  magnesia, 
evaporating,  heating  until  the  ammonium  chloride  is  expelled,  and 
finally  treating  with  water.     It  is  practically  insoluble  in  water  and 
in  solutions  of  salts  of  the  alkalies  (FR.  SCHULZE  *). 

d.  Ferric  phosphate. — If  a  solution  of  phosphoric  acid  or  of 
calcium  phosphate   in  acetic  acid  is  carefully  precipitated  with  a 
solution  of  ferric  acetate,  or  with  a     mixture  of  iron-alum  and 
sodium  acetate,  so  that  the  iron  salt  may  just  predominate,  the  pre- 
cipitate always  contains  1  mol.  P2O5  to  1  mol.  Fe2O3  corresponding 
to  the  formula  of  normal  ferric  phosphate,  Fe2(PO4)2  (RAWSKY, 
WITTSTEIN,  E.  DAVY-f);  if,  on  the  other  hand,  the  ferric  acetate  is 
in  larger  excess,  the  precipitate  is  more  basic.    WITTSTEIN  obtained, 
by  using  a  considerable  excess  of  ferric  acetate,  a  precipitate  con- 
taining 3P3OB  to  4FeaO3.     Precipitates  obtained  with  a  small  excess 
of  the  precipitant  possess  a  composition  varying  between  the  above- 
mentioned  limits.     RAMMELSBERG  obtained  Fe2(PO4)2  -f-  4H2O,  and 
WITTSTEIN  subsequently  the  same  compound  (with  8H2O  instead 
of  4)  upon  mixing  ferric  sulphate  with  sodium  phosphate  in  excess ; 
with  an  insufficient  quantity  of  sodium  phosphate  the  latter  chem- 
ist obtained  a  more  yellowish  precipitate  which  had  a  composition 
corresponding  to  the  formula  3Fe2(PO4)2  +  Fe2(OH)6  +  8H2O.     If 
an  acid  fluid  containing  a  considerable  excess  of  phosphoric  acid  is 
mixed  with  a  small  quantity  of  a  ferric  solution,  and   an   alkali 

*  Journ.f.  prakt.  Chem.t  LXIII,  440.  \Phil.  Mag.,  xix,  181. 


228  FORMS.  [§  93. 

acetate  is  added,  a  precipitate  of  the  formula,  Fe2(PO4)2  -f-  water,  is 
invariably  obtained,  which  accordingly  leaves  upon  ignition  Fe, 
(PO4)2  =  Fe2O3  -|-  P2O6  (WITTSTEIN).  Fresh  experiments  which 
I  have  made  upon  this  subject  have  convinced  me  of  the  perfect 
correctness  of  this  statement.  MOHR  obtained  the  same  results.* 
The  precipitate  is  insoluble  in  a  fluid  containing  salts,  but  when 
washing,  as  soon  as  the  soluble  salts  are  nearly  remoAred,  the  pre- 
cipitate begins  to  dissolve.  The  filtrate  has  an  acid  reaction,  and 
contains  iron  and  phosphoric  acid.  The  precipitate,  under  these 
circumstances,  alters  in  composition,  and  this  explains  why  different 
results  were  obtained  in  the  analysis  of  precipitates  which  had  been. 
washing  for  different  lengths  of  time  (Fn.  MOHB). 


COMPOSITION. 


142-0  47-05 

159-8  52-95 


301-8         100-00 

If  we  dissolve  ferric  phosphate  in  hydrochloric  acid,  supersatu- 
rate the  solution  with  ammonia,  and  apply  heat,  we  obtain  more 
basic  salts,  viz.,  3Fe2O3,2PaO5  (RAMMELSBEKG)  ;  2Fe2O3,P2OB  (WITT- 
STEIN — after  long  washing).  In  WITTSTEIN'S  experiment,  the  wash- 
water  contained  phosphoric  acid.  The  white  ferric  phosphate  does 
not  dissolve  in  acetic  acid,  but  it  dissolves  in  a  solution  of  ferric 
acetate.  Upon  boiling  the  latter  solution  (of  ferric  phosphate  in 
ferric  acetate),  the  whole  of  the  phosphoric  acid  precipitates,  with 
basic  ferric  acetate,  as  hyperbasic  ferric  phosphate.  Similar 
extremely  basic  combinations  are  invariably  obtained  (often  mixed 
with  ferric  hydroxide),  upon  precipitating  with  ammonia  or  barium 
carbonate  a  solution  containing  phosphoric  acid  and  an  excess  of 
a  ferric  salt.  The  precipitate  obtained  by  barium  carbonate  can  be 
conveniently  filtered  off  and  washed,  the  filtrate  is  perfectly  free 
from  either  iron  or  phosphoric  acid ;  on  the  contrary,  the  precipi- 
tate obtained  by  ammonia,  especially  if  the  latter  were  much  in 
excess,  is  slimy,  and  therefore  difficult  to  wash,  and  the  filtrate 
always  contains  small  traces  of  both  iron  and  phosphoric  acid. 

*  ZeitscJir.f.  analyt.  Chem.,  n,  250. 


§  93.]  ACIDS   OF   GROUP  I.  229 

e.  Uranyl  pyrophosphate. — If  the  hot  aqueous  solution  of  a 
phosphate  soluble  in  water  or  acetic  acid  is  mixed,  in  presence  of 
free  acetic  acid,  with  uranyl  acetate,  a  precipitate  of  uranyl  hydro- 
gen phosphate  is  immediately  formed.  If  the  fluid  contains  much 
ammonium  salt,  the  precipitate  contains  also  uranyl  ammonium 
phosphate.  The  same  precipitate  forms  also  if  aluminium  or  ferric 
salts  are  present ;  but  in  that  case  it  is  always  mixed  with  more 
or  less  aluminium  or  ferric  phosphate.  Presence  of  potassium  or 
sodium  salts,  on  the  contrary,  or  of  salts  of  the  alkali-earth  metals, 
has  no  influence  on  the  composition  of  the  precipitate.  Aimnonium- 
uranyl  phosphate  (UO2NII4P04  -f-  a?H,O)  is  a  somewhat  gelatinous, 
whitish-yellow  precipitate,  with  a  tinge  of  green.  The  best  way 
of  washing  it,  at  least  so  far  as  the  principal  part  of  the  operation 
is  concerned,  is  by  boiling  with  water  and  decanting.  If,  after 
having  allowed  the  fluid  in  which  the  precipitate  is  suspended  to 
cool  a  little,  a  few  drops  of  chloroform  are  added,  and  the  mixture 
is  shaken  or  boiled  up,  the  precipitate  subsides  much  more  readily 
than  without  this  addition. 

The  precipitate  is  insoluble  in  water  and  in  acetic  acid ;  but  it 
dissolves  in  mineral  acids ;  ammonium  acetate,  added  in  sufficient 
excess,  completely  re-precipitates  it  from  this  solution,  upon  appli- 
cation of  heat.  Upon  igniting  the  precipitate,  no  matter  whether 
containing  ammonium  or  not,  uranyl  pyrophosphate  of  the  for- 
mula (UO2)2P2O7  is  produced.  This  has  the  color  of  the  yolk  of  an 
egg.  If  the  precipitate  is  ignited  in  presence  of  charcoal  or  of  some 
reducing  gas,  partial  reduction  to  uranous  phosphate  ensues,  owing 
to  which  the  ignited  mass  acquires  a  greenish  tint ;  however,  upon 
warming  the  greenish  residue  with  some  nitric  acid,  the  green  ura- 
nous salt  is  readily  reconverted  into  the  yellow  uranyl  salt.  Uranyl 
pyrophosphate  is  not  hygroscopic,  and  may  therefore  be  ignited 
and  weighed  in  an  open  platinum  dish  (A.  ARENDT  and  W.  KNOP*). 


2UO3O      .     .    575-2  80-20 

O  ^     w     "  — 
\_   ()  _ 

PaO5   .  .  .  142-0     19-80 


717-2    100-00 


*  Chemisches  Centralblatt,  1856,  769,  803;  and  1857,  177. 


230  FORMS.  [§  93. 

The  one-fifth  part  of  the  precipitate  may  accordingly  be  cal- 
culated as  phosphoric  anhydride  in  ordinary  analyses.* 

f.  Stannic  phosphate  is  never  obtained  in  the  pure  state  in  the 
analytical  process,  but  contains  always  an  admixture  of  hydrated 
metastannic  acid  in  excess,  which,  upon  ignition,  changes  to  meta- 
stannic acid.     It  has,  generally  speaking,  the  same  properties  as 
hydrated  metastannic  acid,  and  is  more  particularly,  like  the  latter, 
insoluble  in  nitric  acid.     Upon  heating  with  concentrated  solution 
of  potassa,  potassium  phosphate  and  metastannate  are  formed. 

g.  Normal  silver  phosphate  is  a  yellow  powder ;  it  is  insoluble 
in  water,  but  readily  soluble  in  nitric  acid,  and  also  in  ammonia. 
In  ammonium  salts,  it  is  difficultly  soluble.     It  is  unalterable  in 
the  air.      Upon  ignition,  it  acquires  transiently  a  reddish-brown 
color ;  at  an  intense  red  heat,  it  fuses  without  decomposition. 

695-52  83-04: 

»0  16-96 


837-52  10000 

h.  Mercurous  phosphate. — This  compound  is  employed  for  the 
purpose  of  effecting  the  separation  of  phosphoric  acid  from  many 
bases,  after  H.  ROSE'S  method. 

Mercurous  phosphate  presents  the  appearance  of  a  white  crys- 
talline mass,  or  of  a  white  powder.  It  is  insoluble  in  water,  but 
dissolves  in  nitric  acid.  The  action  of  a  red  heat  converts  it  into 
fused  mercuric  phosphate,  with  evolution  of  vapor  of  mercury. 
Upon  fusion  with  alkali  carbonates,  alkali  phosphates  are  pro- 
duced, and  mercury,  oxygen,  and  carbon  dioxide  escape. 

i.  Phospho-molybdate  of  ammonium. — This  compound  also 
serves  to  effect  the  separation  of  phosphoric  acid  from  other 
bodies ;  it  is  of  the  utmost  importance  in  this  respect. 

The  composition  of  ammonium  phospho-molybdate  is  variable ; 


*  The  atomic  weight  of  uranium  is  here  taken  as  239 '6,  according  to  Clark 
(0  =  16).  If  we  take  it  according  to  Peligot,  as  240,  the  ignited  phosphate 
would  contain  80-22  UO3  and  19 '78  P,O6.  W.  Knop  and  Arendt  found  in 
four  experiments  2013,  20'06,  20*04,  and  20'04  respectively  (in  another  20*77). 
It  will  be  seen  that  these  numbers  agree  better  with  the  composition  as  reck- 
oned from  Ebelmen's  atomic  weight  for  uranium,  237 -6,  than  from  Peligot's 
atomic  weight. 


§  93.]  ACIDS    OF   GEOUP   I.  231 

it  is  usually  given  as  2(KH4),P04.22MoO,  +  12HaO.  It  forms 
a  bright  yellow,  readily  subsiding  precipitate.  Dried  at  100°, 
it  lias,  according  to  SELIGSOHN,  the  following  (average)  com- 
position : 

MoO3 90-744: 

P2O6 3-142 

(XH4)20 3-570 

HnO   '  2-544 


100-000* 

Iii  the  pure  state,  it  dissolves  but  sparingly  in  cold  water  (1  in 
10000 — EGGERTZ)  ;  but  it  is  soluble  in  hot  water.  It  is  readily 
soluble  even  in  the  cold,  in  caustic  alkalies,  alkali  carbonates  and 
phosphates,  ammonium  chloride,  and  ammonium  oxalate.  It  dis- 
solves sparingly  in  ammonium  sulphate,  potassium  nitrate,  and 
potassium  chloride ;  and  very  sparingly  in  ammonium  nitrate. 

It  is  soluble  in  potassium  sulphate  and  sodium  sulphate,  sodium 
chloride  and  magnesium  chloride,  and  sulphuric,  hydrochloric  and 
nitric  acids  (concentrated  and  dilute).  Water,  containing  1  per 
cent,  of  common  nitric  acid,  dissolves  -^g^ir  (EGGERTZ).  Appli- 
cation of  heat  does  not  check  the  solvent  action  of  these  substances. 
Presence  of  ammonium  molybdate  totally  changes  its  deportment 
with  acid  fluids.  Dilute  nitric  or  sulphuric  acid  containing 
ammonium  molybdate  does  not  dissolve  it ;  but  much  hydro- 
chloric acid,  even  in  the  presence  of  ammonium  molybdate,  has 
a  solvent  action,  and  this  acid  consequently  interferes  with  the 
complete  precipitation  of  phosphoric  acid  by  nitric  acid  solution 
of  ammonium  molybdate.  The  solution  of  the  phospho-molybdate 
of  ammonium  in  acids  is  probably  attended,  in  all  cases,  with 
decomposition  and  separation  of  the  molybdic  acid,  which  cannot 
take  place  in  the  presence  of  ammonium  molybdate  (J.  CRAW)-)-. 
Tartaric  acid  and  similar  organic  substances  entirely  prevent  the 

*  From  the  varying  results  of  different  analysts  it  is  plain  that  the  precipiv 
tate,  prepared  under  apparently  the  same  circumstances,  has  not  always  exactly 
the  same  composition.  SONNENSCIIEIN  (Journ.  f.  prakt.  Client.,  LIII,  342)  found 
in  the  precipitate  dried  at  120°,  2  -93— 3  -12£  P;jO6;  LIPOWITZ  (Pogg.  Annal, 
cix,  135),' in  the  precipitate  dried  at  from  20°  to  30°,  3'607#  PaO6  ;  EGGERTZ 
(Journ.  f.  prakt.  Chem.,  LXXIX,  496),  3 -7  to  3  -8& 

t  Chem.  Gaz.  1852,  216. 


232  FORMS.  [|  93. 

precipitation  of  the  phospho-molybdate  of  ammonium  (EGGERTZ). 
In  the  presence  of  an  iodide  instead  of  a  yellow  precipitate,  a  green 
precipitate  or  a  green  fluid  is  formed,  resulting  from  the  reducing 
action  of  the  hydriodic  acid  on  the  molybdic  acid  (J.  "W.  BILL*). 
Other  substances  which  reduce  molybdic  acid  have  of  course  a 
similar  action. 

5.  BORIC  ACID. 

POTASSIUM  BOROFLUORIDE  is  the  best  form  to  convert  boric 
acid  into  for  the  purpose  of  the  direct  estimation  of  the  acid.  This 
compound  is  produced  by  mixing  the  solution  of  an  alkali  borate, 
in  presence  of  a  sufficient  quantity  of  potassa,  with  hydrofluoric 
acid  in  excess,  in  a  silver  or  platinum  dish,  and  evaporating  to  dry- 
ness.  The  gelatinous  precipitate  which  forms  in  the  cold,  dissolves 
upon  application  of  heat,  and  separates  from  the  solution  subse- 
quently, upon  evaporation,  in  small,  hard,  transparent  crystals. 
The  compound  has  the  formula  KF,BF3.  It  is  soluble  in  water 
and  also  in  dilute  alcohol ;  but  strong  alcohol  fails  to  dissolve  it ; 
it  is  insoluble  also  in  concentrated  solution  of  potassium  acetate. 
It  may  be  dried  at  100°,  without  decomposition  (AUG.  STRO- 

MEYERf). 

COMPOSITION. 

K 39-11  30-96 

B 11-00  8-71 

F4 76-20  60-33 

126-31  100-00 

6.  OXALIC  ACID. 

When  oxalic  acid  is  to  be  directly  determined  it  is  usually  pre- 
cipitated in  the  form  of  CALCIUM  OXALATE  ;  and  its  weight  is 
inferred  from  the  CALCIUM  CARBONATE  or  CALCIUM  OXIDE  produced 
from  the  oxal^e  by  ignition.  For  the  properties  of  these  bodies 
see  §  73. 

7.  HYDROFLUORIC  ACID. 

The  direct  estimation  of  hydrofluoric  acid  is  usually  effected 
by  weighing  the  acid  in  the  form  of  CALCIUM  FLUORIDE. 

Calcium  fluoride  forms  a  gelatinous  precipitate,  which  it  is 
found  difficult  to  wash.  If  digested  with  ammonia,  previous  to 

*  Sillim.  Journ.,  July,  1858.  f  Annal.  d.  CJiem.  u.  Pharm.,  c,  82. 


§  93.]  ACIDS    OF   GROUP   I. 

filtration,  it  is  rendered  denser  and  less  gelatinous.  It  is  not  alto- 
gether insoluble  in  water ;  aqueous  solutions  of  the  alkalies  fail  to 
decompose  it.  It  is  very  slightly  soluble  in  dilute,  but  more 
readily  in  concentrated  hydrochloric  acid.  When  acted  upon  by 
sulphuric  acid,  it  is  decomposed,  and  calcium  sulphate  and  hydro- 
fluoric acid  are  formed.  Calcium  fluoride  is  unalterable  in  the  air, 
and  at  a  red  heat.  Exposed  to  a  very  intense  heat,  it  fuses.  Upon 
intense  ignition  in  moist  air,  it  is  slowly  and  partially  decomposed 
into  calcium  oxide  and  hydrofluoric  acid.  Mixed  with  ammonium 
chloride,,  and  exposed  to  a  red  heat,  calcium  fluoride  suffers  a  con- 
tinual loss  of  weight ;  but  the  decomposition  is  incomplete. 

COMPOSITION. 

Ca 40.1          51-28 

Fa .    38-1          48.72 


78 -2        100-00 

We  often  determine  fluorine,  more  particularly  in  presence  of 
silicic  acid,  by  converting  it  into  silicon  fluoride  (SiF4).  This  is  a 
colorless  gas,  fuming  in  the  air,  with  suffocating  odor,  of  sp.  gr. 
3'574,  which  decomposes  when  mixed  with  water  forming  silica 
and  hydrofluosilicic  acid  thus  :  3SiF4  +  2HaO  =  2H2SiF.+  SiOa. 

8.  CARBONIC  ACID. 

The  direct  estimation  of  carbonic  ucid — which,  however,  i& 
only  rarely  resorted  to — is  usually  effected  by  weighing  the  acid  in 
the  form  of  CALCIUM  CARBONATE.  For  the  properties  of  the  latter 
substance,  see  §  73. 

9.  SILICIC  ACID.* 

When  silicic  acid  is  separated  by  acids  from  aqueous  solutions- 
of  alkali  silicates,  it  is  at  first  perfectly  soluble  in  water.  It  be- 
comes insoluble  or  rather  difficultly  soluble  when  it  coagulates. 
Coagulation  is  a  permanent  change  and  is  furthered  by  concentra- 
tion and  by  elevation  of  temperature.  Silicic  acid  solution  con- 
taining 10  or  12  per  cent,  of  SiO2  coagulates  at  the  ordinary  tem- 
perature in  a  few  hours,  and  immediately  if  heated.  A  solution  of 

*  Five  silicic  acid  in  solution  is  assumed  to  have  the  composition  expressed 
by  the  formula  Si(OH)4.  Silicic  anhydride  (SiO2)  is  usually  called  "silica." 
Compounds  of  SiOa  with  less  water  than  corresponds  to  the  formula  Si(OH)4  = 
SiO2(H2O)3  are  here  called  "  hydrates  of  silica." 


234  FORMS.  [§  93. 

5  per  cent,  may  be  preserved  without  coagulating  for  five  or  six:  days, 
one  of  2  per  cent,  for  two  or  three  months,  and  one  of  1  per  cent, 
for  several  years,  and  solutions  containing  ^  per  cent,  or  less  are 
not  appreciably  altered  by  time.  Solid  matter  in  powder  such  as 
graphite,  hastens  coagulation,  alkali  salts  induce  it  rapidly.  Aque- 
ous solutions  of  silicic  acid  may,  on  the  contrary,  be  mixed  with 
hydrochloric  acid,  nitric  acid,  acetic  acid,  tartaric  acid  and  alcohol 
without  coagulating.  The  gelatinous  silicic  acid  produced  by 
coagulation  may  contain  more  or  less  water,  and  it  appears  to  be 
the  more  difficultly  soluble  in  water,  the  less  water  it  contains ; 
thus  a  jelly  of  silicic  acid  containing  1  per  cent,  of  silica  (SiOa)  gives 
a  solution  with  cold  water  containing  1  part  of  silica  in  about  5000 
parts,  a  jelly  of  5  per  cent,  gives  a  solution  containing  1  part  of  silica 
in  about  10000  parts  of  water.  A  jelly  containing  less  water  is  still 
less  soluble,  and  when  the  jelly  is  dried  up  to  a  gummy  mass  it  is> 
barely  soluble  at  all ;  this  is  also  the  case  with  the  pulverulent 
hydrate  of  silica  obtained  in  the  analysis  of  silicates  by  drying  a 
jelly  containing  much  salts  at  100°  (GRAHAM*).  The  hydrated 
silica  dried  at  100°  dissolves  but  very  slightly  in  acids  (with  the 
exception  of  hydrofluoric  acid) ;  it  dissolves,  however,  in  solutions 
of  fixed  alkalies  and  alkali  carbonates,  especially  on  heating.  Aque- 
ous ammonia  dissolves  the  jelly  in  tolerable  quantity  and  the  dry 
hydrate  in  very  notable  quantity  (PRiBRAM)f.  Regarding  the 
amount  of  water  in  the  hydrate  dried  at  given  temperatures  chem- 
ists do  not  agree.  J 

On  ignition  all  the  hydrates  pass  into  anhydrous  silica.  As 
the  vapor  escapes  small  particles  of  the  extremely  fine  powder 
are  liable  to  whirl  up.  This  may  be  avoided  by  moistening  the 
hydrate  in  the  crucible  with  water,  evaporating  to  dryness  on  a 
water  bath,  and  then  applying  at  first  a  slight  and  then  a  gradu- 
ally increased  heat. 

The  silica  obtained  by  igniting  the  hydrate  appears  in  the 
amorphous  condition,  with  a  sp.  gr.  of  2'2  to  2*3.  It  forms  a 

*  Fogg.  AnnaL,  cxui,  529.  \Zeitschr.f.analyt.  Chem.,  vi,  119. 

\  DOVERI  (AnnaL  de  Chim.  et  de  Phys.,  xxi,  40;  AnnaL  d.  Chem.  u.  Pharm., 
LXIV,  256)  found  in  the  air-dried  hydrate  16 '9  to  17 "8#  water;  J.  FUCHS 
(AnnaL  d.  Chem.  u.  PJiarm.,  LXXXII,  119  to  123),  9'1  to  9-6;  G.  LIPPERT,  9-38 
to  9-95.  DOVERI  found  in  the  hydrate  dried  at  100°,  8'3  to  9 '4;  J.  FUCHS,  6'63 
to  6  96;  G.  LIPPERT,  4-97  to  5'52.  H.  ROSE  (Pogg.  AnnaL,  cvm,  1;  Journ.  fur 
prakt.  Chem.,  LXXXI,  227)  found  in  the  hydrate  obtained  by  digesting  stilbite 
with  concentrated  hydrochloric  acid,  and  dried  at  150°,  4'85$  water. 


§  94.]  ACIDS    OF   GROUP   II.  235 

white  powder  insoluble  in  water,  and  acids  (hydrofluoric  excepted), 
soluble  in  solutions  of  the  fixed  alkalies  and  their  carbonates, 
especially  in  the  heat.  Hydrofluoric  acid  readily  dissolves  amor- 
phous silica;  the  solution  leaves  no  residue  on  evaporation  in 
platinum,  if  the  silica  was  pure.  The  amorphous  silica,  when 
heated  with  ammonium  fluoride  in  a  platinum  crucible,  readily 
volatilizes.  The  ignited  amorphous  silica,  exposed  to  the  air, 
eagerly  absorbs  water,  which  it  will  not  give  up  at  from  100°  to 
150°  (H.  ROSE).  The  lower  the  heat  during  ignition  the  more 
hygroscopic  is  the  residue  (SOUCHAY*).  Silica  fuses  at  the  strong- 
est heat ;  the  mass  obtained  being  vitreous  and  amorphous.  Amor- 
phous silica  ignited  with  ammonium  chloride,  at  first  loses  weight, 
and  then,  when  the  ignition  has  rendered  it  denser,  the  weight 
remains  constant. 

The  amorphous  silica  must  be  distinguished  from  the  crystallized 
or  crystalline  variety,  which  occurs  as  rock  crystal,  quartz,  sand,  &c. 
This  has  a  sp.  gr.  of  2'6  (SCHAFFGOTSCH),  and  is  far  more  difficultly, 
arid  in  far  less  amount,  dissolved  by  potash  solution  or  solution  of 
fixed  alkali  carbonates ;  it  is  also  more  slowly  attacked  by  hydro- 
fluoric acid,  or  ammonium  fluoride.  Crystallized  silica  is  not  hygro- 
scopic, whether  strongly  or  gently  ignited  (SOUCHAY).  Vegetable 
colors  are  not  changed  either  by  silica  or  its  hydrates. 

COMPOSITION. 

Si 28-4  47-02 

O2     .....      32-0  52-98 


60-4  100-00 

ACID  RADICALS  OF  THE  SECOND  GROUP. 

§94. 
1.  HYDROCHLORIC  ACID. 

Hydrochloric  acid  is  almost  invariably  weighed  in  the  form  of 
SILVER  CHLORIDE — for  the  properties  of  which  see  §  82. 

2.  HYDROBROMIC  ACID. 

Hydrouromic  acid  is  always  weighed  in  the  form  of  SILVER 
BROMIDE. 

*  Zeitschr.  f.  analyt.  Chem.,  vin,  423. 


236  FORMS.  [§  94. 

Silver  bromide,  prepared  in  the  wet  way,  forms  a  yellowish- 
white  precipitate.  It  is  wholly  insoluble  in  water  and  in  nitric 
acid,  tolerably  soluble  in  ammonia,  readily  soluble  in  sodium  thio- 
sulphate  and  potassium  cyanide.  Concentrated  solutions  of  potas- 
sium, sodium,  and  ammonium  chlorides  arid  bromides  dissolve  it  to 
a  very  perceptible  amount,  while  in  very  dilute  solutions  of  these 
salts  it  is  entirely  insoluble.  Traces  only  dissolve  in  the  alkali 
nitrates.  It  dissolves  abundantly  in  a  concentrated  warm  solution 
of  mercuric  nitrate.  On  digestion  with  excess  of  potassium  iodide 
solution  it  is  completely  converted  into  silver  iodide  (FIELD).  On 
ignition  in  a  current  of  chlorine  silver  bromide  is  transformed  into 
chloride ;  on  ignition  in  a  current  of  hydrogen  it  is  converted  into 
metallic  silver.  Exposed  to  the  light  it  gradually  turns  gray,  and 
finally  black.  Under  the  influence  of  heat,  it  fuses  to  a  reddish 
liquid,  which,  upon  cooling,  solidifies  to  a  yellow,  horn-like  mass: 
Brought  into  contact  with  zinc  and  water,  it  is  decomposed ;  a 
spongy  mass  of  metallic  silver  forms,  and  the  solution  contains  zinc 
bromide. 

COMPOSITION. 

Ag    .     .     .     .     107-92  57-44 

Br  79-95  42-56 


187-87  100-00 

3.  HYDRIODIC  ACID. 

Hydriodic  acid  is  usually  determined  in  the  form  of  SILVER 
IODIDE,  and  occasionally  also  in  that  of  PALLADIOUS  IODIDE. 

a.  Silver  iodide,  produced  in  the  wet  way,  forms  a  light-yellow 
precipitate,  insoluble  in  water,  and  in  dilute  nitric  acid,  and  very 
slightly  soluble  in  ammonia.  One  part  dissolves,  according  to- 
WALLACE  and  LAMONT*  in  2493  parts  of  aqueous  ammonia  sp.  gr. 
0*89 ;  according  to  MARTINI,  in  2510  parts  of  0*96  sp.  gr.  It  is  copi- 
ously taken  up  by  concentrated  solution  of  potassium  iodide,  but  it 
is  insoluble  in  very. dilute;  it  dissolves  readily  in  sodium  .thiosul- 
phate  and  in  potassium  cyanide;  traces  only  are  dissolved  by  alkali 
nitrates.  In  concentrated  warm  solution  of  mercuric  nitrate  it  is 
copiously  soluble.  Hot  concentrated  nitric  and  sulphuric  acids- 
convert  it,  but  with  some  difficulty,  into  silver  nitrate  and  sulphate 
respectively,  with  expulsion  of  the  iodine.  Silver  iodide  acquires  a 

*  Chem.  Gaz.,  lS5Q.—Jahresbericht,  KOPP  and  WILL,  1859,  670. 


§  94.]  ACIDS    OF   GROUP   II.  237 

black  color  when  exposed  to  the  light.  When  heated,  it  fusea 
without  decomposition  to  a  reddish  fluid,  which,  upon  cooling, 
solidifies  to  a  yellow  mass,  that  may  be  cut  with  a  knife.  Under 
the  influence  of  excess  of  chlorine  in  the  heat  it  is  completely  con- 
verted into  silver  chloride ;  ignition  in  hydrogen  reduces  it  but 
incompletely  to  the  metallic  state.  When  brought  into  contact 
with  zinc  and  water,  it  is  decomposed  but  incompletley ;  zinc  iodide 
is  formed,  and  metallic  silver  separates. 

COMPOSITION. 

Ag     .     .     .     .     107-92  45-97 

I  126-85  54-03 


234-77  100-00 

J.  Palladious  iodide,  produced  by  mixing  an  alkali  iodide 
with  palladious  chloride,  is  a  deep  brownish-black,  flocculent  pre- 
cipitate, insoluble  in  water  and  in  dilute  hydrochloric  acid,  but 
slightly  soluble  in  saline  solutions  (sodium  chloride,  magnesium 
chloride,  calcium  chloride,  &c.).  It  is  unalterable  in  the  air.  Dried 
simply  in  the  air  it  retains  one  molecule  of  water=5'05  per  cent. 
Dried  long  in  vacuo,  or  at  a  rather  high  temperature  (70°  to  80°), 
it  yields  the  whole  of  this  water,  without  the  least  loss  of  iodine. 
Dried  at  100°,  it  loses  a  trace  of  iodine ;  at  from  300°  to  400°,  the 
whole  of  the  iodine  is  expelled.  It  may  be  washed  with  hot  water, 
without  loss  of  iodine. 

COMPOSITION. 

Pd 107-00       29-66 

I3 253-70      70-34 


360-70      100-00 

4.  HYDROCYANIC  ACID. 

Hydrocyanic  acid,  if  determined  gravimetrically  and  directly,  is 
always  converted  into  SILVER  CYANIDE — for  the  properties  of  which 
compound  see  §  82. 

5.  HYDROSULPHURIC  ACID. 

The  forms  into  which  the  sulphur  in  hydrogen  sulphide  or 
metallic  sulphides,  is  converted  for  the  purpose  of  being  weighed, 


238  FOKMS.  [§  95. 

are  ARSENOUS  SULPHIDE,  SILVER  SULPHIDE,  COPPER  SULPHIDE,  and 

BARIUM  SULPHATE. 

For  the  properties  of  the  sulphides  named,  see  §§  82,  85,  92 ; 
for  those  of  barium  sulphate,  see  §  71. 


ACID  RADICALS  OF  THE  THIRD  GROUP. 

§95. 
1.  NITRIC  ACID  ;  and  2.  CHLORIC  ACID. 

These  two  acids  are  never  determined  directly — that  is  to  say, 
in  compounds  containing  them,  but  always  in  an  indirect  way ; 
generally  volumetrically. 


SECTION    IV. 

THE   DETERMINATION  (OR  ESTIMATION)  OF 
RADICALS. 

§96. 

IN  the  preceding  Section  we  have  examined  the  composition 
and  properties  of  the  various  forms  and  combinations  in  which 
radicals  are  separated  from  each  other,  or  in  which  they  are  weighed. 
"We  have  now  to  consider  the  special  means  and  methods  of  con- 
verting them  into  such  forms  and  combinations. 

For  the  sake  of  greater  clearness  and  simplicity,  we  shall,  in 
the  present  Section,  confine  our  attention  to  the  various  methods 
applied  to  effect  the  determination  of  single  radicals,  deferring  to 
the  next  Section  the  consideration  of  the  means  adopted  for  sepa- 
rating them  from  each  other. 

We  shall  here  deal  with  the  estimations  of  substances  in  the 
free  state,  or  compounds  consisting  of  one  base  and  one  acid,  or 
containing  one  metal  and  one  metalloid. 

As  in  the  c '  Qualitative  Analysis, ' '  the  acids  of  arsenic  will  be 
treated  of  among  the  bases,  on  account  of  their  behavior  to  hydro- 
gen sulphide ;  and  those  elements  that  form  acids  with  hydrogen 
will  be  treated  of  under  their  respective  hydrogen  acids. 

In  the  quantitative  analysis  of  a  compound  we  have  to  study 
first,  the  most  appropriate  method  of  dissolving  it ;  and,  secondly, 
the  modes  of  determining  the  quantity  of  one  or  more  of  its  con- 
stituents. 

"With  regard  to  the  latter  point,  we  have  to  turn  our  attention, 
first,  to  the  performance  •  and  secondly,  to  the  accuracy  of  the 
methods. 

It  happens  very  rarely  in  quantitative  analyses  that  the  amount 
of  a  substance,  as  determined  by  the  analytical  process,  corresponds 
exactly  with  the  amount  theoretically  calculated  or  actually  pres- 
ent ;  and  if  it  does  happen,  it  is  merely  by  chance. 

It  is  of  importance  to  inquire  what  is  the  reason  of  this  fact, 
and  what  are  the  limits  of  inaccuracy  in  the  several  methods. 

The  cartse  of  this  almost  invariably  occurring  discrepancy 
between  the  quantity  present  and  that  actually  found,  is  to  be 
ascribed  either  exclusively  to  the  execution,  or  it  lies  partly  in  the 
method  itself. 

239 


240  DETERMINATION.  [§  96. 

The  execution  of  tlie  analytical  processes  and  operations  can 
never  be  absolutely  accurate,  even  though  the  greatest  care  and 
attention  be  bestowed  on  the  most  trifling  minutiae.  To  account 
for  this,  we  need  only  bear  in  mind  that  our  weights  and  measures 
are  never  absolutely  correct,  nor  our  balances  absolutely  accurate, 
nor  our  reagents  absolutely  pure ;  and,  moreover,  that  we  do  not 
weigh  in  vacuo ;  and  that,  even  if  we  deduce  the  weight  in  vacua 
from  the  weight  we  actually  obtain  by  weighing  in  the  air,  the 
very  volumes  on  which  the  calculation  is  based  are  but  approxi- 
mately known ; — that  the  hygroscopic  state  of  the  air  is  liable  to 
vary  between  the  weighing  of  the  empty  crucible  and  of  the  cru- 
cible -f-  the  substance ; — that  we  know  the  weight  of  a  filter  ash 
only  approximately  / — that  we  can  never  succeed  in  completely 
keeping  off  dust,  &c. 

With  regard  to  the  methods,  many  of  them  are  not  entirely 
free  from  certain  unavoidable  sources  of  error ; — precipitates  are 
not  absolutely  insoluble;  compounds  which  require  ignition  are 
not  absolutely  fixed ;  others,  which  require  drying,  have  a  slight 
tendency  to  volatilize ;  the  final  reaction  in  volumetric  analyses  is 
usually  produced  only  by  a  small  excess  of  the  standard  fluid, 
which  is  occasionally  liable  to  vary  with  the  degree  of  dilution,  the 
temperature,  &c. 

Strictly  speaking,  no  method  can  be  pronounced  quite  free 
from  defect ;  it  should  be  borne  in  mind,  for  example,  that  even 
barium  sulphate  is  not  absolutely  insoluble  in  water.  Whenever 
we  describe  any  method  as  free  from  sources  of  error,  we  mean, 
that  no  causes  of  considerable  inaccuracy  are  inherent  in  it. 

We  have,  therefore,  in  our  analytical  processes,  invariably  to 
contend  against  certain  sources  of  inaccuracy  which  it  is  impossi- 
ble to  overcome  entirely,  even  though  our  operations  be  conducted 
with  the  most  scrupulous  care  and  with  the  utmost  attention  to 
established  rules.  It  will  be  readily  understood  that  several  defects 
and  sources  of  error  may,  in  some  cases,  combine  to  vitiate  the 
results  ;  whereas,  in  other  cases,  they  may  compensate  one  another, 
and  thus  enable  us  to  attain  a  higher  degree  of  accuracy.  The 
comparative  accuracy  of  the  results  attainable  by  an  analytical 
method  oscillates  between  two  points — these  points  are  called  the 
limits  of  error.  In  the  case  of  methods  free  from  sources  of  error, 
these  limits  will  closely  approach  each  other ;  thus,  for  instance,  in 


§90.]  DETERMINATION.  241 

the  determination  of  chlorine,  with  great  care  one  will  always  be 
able  to  obtain  between  99'9and  lOO'l  for  the  100  parts  of  chlorine 
actually  present. 

Less  perfect  methods  will,  of  course,  exhibit  far  greater  dis- 
crepancies ;  thus,  in  the  estimation  of  strontium  by  means  of  sul- 
phuric acid,  the  most  attentive  and  skilful  operator  may  not  be 
able  to  obtain  more  than  99  (and  even  less)  for  the  100  parts 
of  strontium  actually  present.  I  may  here  incidentally  state  that 
the  numbers  occasionally  given  in  this  manner,  in  the  course  of  the 
present  work,  to  denote  the  degree  of  accuracy  of  certain  methods, 
refer  invariably  to  the  substance  estimated  (chlorine,  nitrogen, 
baryta,  for  instance),  and  not  to  the  combination  in  which  that 
substance  may  be  weighed  (silver  chloride,  ammonium  platinic 
chloride,  barium  sulphate,  for  instance) ;  otherwise  the  accuracy  of 
various  methods  would  not  be  comparable. 

The  occasional  attainment  of  results  exactly  corresponding  witli 
the  numbers  calculated  does  not  always  justify  the  assumption,  on 
the  part  of  the  student,  that  his  operations,  to  have  led  to  such  a 
result,  must  have  been  conducted  with  the  utmost  precision  and 
accuracy.  It  may  sometimes  happen,  in  the  course  of  the  analyti- 
cal process,  that  one  error  serves  to  compensate  another ;  thus,  for 
instance,  the  analyst  may,  at  the  commencement  of  his  operations, 
spill  a  minute  portion  of  the  substance  to  be  analyzed ;  whilst,  at  a 
later  stage  of  the  process,  he  may  recover  the  loss  by  an  imperfect 
washing  of  the  precipitate.  As  a  general  rule,  results  showing  a 
trifling  deficiency  of  substance  may  be  looked  upon  as  better  proof 
of  accurate  performance  of  the  analytical  process  than  results 
exhibiting  an  excess  of  substance. 

As  not  the  least  effective  means  of  guarding  against  error  and 
inaccuracies  in  gravimetric  analyses,  I  would  most  strongly  recom- 
mend the  analyst,  after  weighing  a  precipitate,  <&c.,  to  compare 
its  properties  (color,  solubility,  reaction,  dec.)  with  those  which  it 
should  possess,  and  which  have  been  amply  described  in  .the  pre- 
ceding Section. 

In  my  own  laboratory,  I  insist  upon  all  substances  that  are 
weighed  in  the  course  of  an  analysis  being  kept  between  watch- 
glasses,  until  the  whole  affair  is  concluded.  This  affords  always  a 
chance  of*  testing  them  once  more  for  some  impurity,  the  presence 
of  which  may  become  suspected  in  the  after-course  of  the  process. 


242  DETERMINATION.  [§  97. 

I.   DETERMINATION  OF  BASIC  RADICALS  IN  SIMPLE  SALTS. 

First  Group. 

POTASSIUM SODIUM AMMONIUM (LITHIUM). 

§97. 

1.  POTASSIUM. 

a.  Solution. 

Potassa  and  potassium  salts  of  those  inorganic  acids  which  we 
have  to  consider  here,  are  dissolved  in  water,  in  which  menstruum 
they  dissolve  readily,  or  at  all  events,  pretty  readily. 

Potassium  salts  of  organic  acids  it  is  most  convenient  to  convert 
into  potassium  carbonate  by  long-continued,  gentle  ignition  in  a 
covered  crucible.  Heated  to  fusion,  the  carbon  separated  acts  on 
the  potassium  carbonate ;  carbonic  oxide  escapes,  and  some  potas- 
sium hydroxide  is  formed.  On  simple  carbonization  a  slight  loss 
is  caused ;  on  fusing,  which  must  be  avoided,  a  further  loss  occurs. 

b.  Determination. 

Potassium  is  weighed  either  as  potassium  sulphate,  ^potas- 
sium chloride,  or  as  potassium -platinic  chloride  (see  §  68).  It 
may  also  be  determined  volumetrically.  For  the  alkalimetric 
estimation  of  potassa  or  potassium  carbonate,  see  §§  219  and  220. 
For  estimating  potassium  as  potassium  hydrogen  tartrate,  and 
which  only  gives  approximate  results,  a  chapter  will  be  given  in 
the  Special  Part. 

"We  may  convert  into 

1.  POTASSIUM  SULPHATE. 

Potassium  salts  of  strong  volatile  acids ;  e.  g. ,  potassium  chlo- 
ride, potassium  bromide,  potassium  nitrate,  etc.,  and  salts  of 
organic  acids. 

2.  POTASSIUM  NITRATE. 

Potassium  hydroxide  and  compounds  of  potassium  witli  weak, 
volatile  acids  not  decomposable  by  nitric  acid,  e.g.,  potassium 
carbonate  (potassium  salts  with  organic  acids). 

3.  POTASSIUM  CHLORIDE. 

In  general,  caustic  potassa  and  potassium  salts  of  weak  volatile 
acids;  also,  and  more  particularly,  such  as  are  decomposed  by 


§  97.]  POTASSIUM. 

nitric  acid,  e.g.,  potassium  sulphide,  potassium  sulphate,  chromate, 
chlorate,  and  silicate. 

4.  POTASSIUM  PLATINIC  CHLORIDE. 

Potassium  Baits  of  non-volatile  acids  soluble  in  alcohol.  This 
method  is  particularly  important  for  salts  of  the  non-volatile  acids ; 
e.g.,  potassium  phosphate,  potassium  borate;  also  for  separating 
potassium  from  sodium. 

The  potassium  in  potassium  borate  may  be  determined  also  as 
sulphate  (§  136)  ;  and  the  potassium  in  the  phosphate,  as  potas- 
sium chloride  (§  135). 

The  form  of  potassium  platinic  chloride  may  also  be  resorted 
to  in  general,  for  the  estimation  of  potassium  in  all  potassium  salts 
of  those  acids  which  are  soluble  in  alcohol.  This  form  is,  more- 
over, of  especial  importance,  as  that  in  which  the  separation  of 
potassium  from  sodium,  etc.,  is  effected. 

5.  POTASSIUM  SILICOFLUOKIDE. 

Potassium  salts  of  those  acids  which  are  soluble  in  weak  alcohol, 
except  borate. 

1.  Determination  as  Potassium  Sulphate. 

Evaporate  the  aqueous  solution  of  the  potassium  sulphate  to 
dryness,  ignite  the  residue  in  a  platinum  crucible  or  dish,  and 
weigh  (§  42).  The  residue  must  be  thoroughly  dried  before  you 
proceed  to  ignite  it ;  the  heat  applied  for  the  latter  purpose  must 
be  moderate  at  first,  and  very  gradually  increased  to  the  requisite 
degree  ;  the  crucible  or  dish  must  be  kept  well  covered — neglect  of 
those  precautionary  rules  involves  always  a  loss  of  substance  from 
decrepitation.  If  free  sulphuric  acid  is  present,  we  obtain,  upon 
evaporation,  acid  potassium  sulphate ;  in  such  cases  the  acid  salt  is 
to  be  converted  into  the  normal  by  igniting  first  alone  (here  it  is 
best  to  place  the  lamp  so  that  the  flame  may  strike  the  dish-cover 
obliquely  from  above),  then  with  ammonium  carbonate.  See  §  68. 

For  properties  of  the  residue,  see  §  68.  Observe  more  particu- 
larly that  the  residue  must  dissolve  to  a  clear  fluid,  and  that  the 
solution  must  be  neutral.  Should  traces  of  platinum  remain  behind 
(the  dish  rfot  having  been  previously  weighed),  these  must  be  care- 
fully determined,  and  their  weight  subtracted  from  that  of  the 
ignited  residue. 


244  DETERMINATION.  [§  97. 

With  proper  care  and  attention,  this  method  gives  accurate 
results. 

To  convert  the  above-mentioned  salts  (potassium  chloride,  &c.) 
into  potassium  sulphate,  add  to  their  aqueous  solution  a  quantity 
of  pure  sulphuric  acid  more  than  sufficient  to  form  normal  sulphate 
with  the  whole  of  the  potassium,  evaporate  the  solution  to  dry- 
ness,  ignite  the  residue,  and  convert  the  resulting  acid  potassium 
sulphate  into  the  normal,  by  treating  with  ammonium  carbonate 
(§  68). 

As  the  expulsion  of  a  large  quantity  of  sulphuric  acid  is  a  very 
disagreeable  process,  avoid  adding  too  great  an  excess.  Should  too 
little  of  the  acid  have  been  used,  which  you  may  infer  from  the 
non-evolution  of  sulphuric  acid  fumes  on  ignition,  moisten  the 
residue  with  dilute  sulphuric  acid,  evaporate,  and  again  ignite.  If 
you  have  to  deal  with  a  small  quantity  only  of  potassium  chloride, 
&c.,  proceed  at  once  to  treat  the  dry  salt,  cautiously,  with  dilute 
sulphuric  acid  in  the  platinum  crucible ;  provided  the  latter  be 
capacious  enough.  In  the  case  of  potassium  bromide  and  iodide, 
the  use  of  platinum  vessels  must  be  avoided. 

Potassium  salts  of  organic .  acids  are  directly  converted  into 
potassium  sulphate  by  first  carbonizing  them  at  the  lowest  possible 
temperature,  and  after  cooling  adding  some  crystals  of  pure  ammo- 
nium sulphate  and  a  little  water  to  the  mass.  The  crucible  being 
covered,  the  water  is  evaporated  by  heating  the  crucible  cover,  and 
the  whole  is  afterwards  heated  to  dull  redness,  until  the  excess  of 
ammonium  sulphate  is  destroyed.  If  the  carbon  is  not  fully  con- 
sumed by  this  operation,  add  a  little  ammonium  nitrate  and  repeat 
the  ignition.  The  potassium  sulphate  is  then  weighed.  (KAM- 
MEREK.  *)  It  is  usually  advisable  to  ignite  finally  in  an  atmosphere 
of  ammonium  carbonate.  The  results  are  accurate. 

2.   Determination  as  Potassium  Nitrate. 

The  general  method  is  the  same  as  in  1 .  The  potassium  nitrate 
must  be  heated  gently  to  the  melting-point,  otherwise  loss  will 
arise  from  evolution  of  oxygen.  For  properties  of  the  residue 
see  §  68.  The  process  is  easily  carried  out,  and  the  results  are 
accurate.  In  converting  potassium  carbonate  into  the  nitrate, 
consult  §  38. 

[*Fres.  Zeit.,  vn,  222.] 


§  97.]  POTASSIUM.  245 

3.  Determination  as  Potassium  Chloride. 
General  method  the  same  as  described  in  1.  The  residue  of 
potassium  chloride  must,  previously  to  ignition,  be  treated  in  the 
same  way  as  potassium  sulphate,  and  for  the  same  reason.  The 
salt  must  be  heated  in  a  well-covered  crucible  or  dish,  and  only  to 
dull  redness,  as  the  application  of  a  higher  degree  of  heat  is  likely 
to  cause  some  loss  by  volatilization.  No  particular  regard  need  be 
had  to  the  presence  of  free  acid.  For  properties  of  the  residue, 
see  §  68.  This  method,  if  properly  and  carefully  executed,  gives 
very  accurate  results.  The  potassium  chloride  may,  instead  of 
being  weighed,  be  determined  volumetrically  by  §  141,  b.  This 
method,  however,  has  no  advantage  in  the  case  of  single  estima- 
tions, but  saves  time  when  a  series  of  estimations  has  to  be 
made. 

In  determining  potassium  in  the  carbonate  it  is  sometimes 
desirable  to  avoid  the  effervescence  occasioned  by  treatment  with 
hydrochloric  acid,  as,  for  instance,  in  the  case  of  the  ignited  resi- 
due of  a  potassium  salt  of  an  organic  acid,  which  is  contained  in 
the  crucible.  This  may  be  effected  by  treating  the  carbonate  with 
solution  of  ammonium  chloride  in  excess,  evaporating  and  igniting, 
when  ammonium  carbonate  and  the  excess  of  ammonium  chloride 
will  escape,  leaving  potassium  chloride  behind. 

.  The  methods  of  converting  the  potassium  compounds  specified 
above  into  potassium  chloride,  will  he  found  in  Part  II.  of  this 
Section,  under  the  respective  hea ,i  of  the  acids  which  they  con- 
tain. 

4.  Determination  as  Potassium- Platinie  Chloride, 
a.  Potassium  salts  of  volatile  acids  (nitric  acid,  acetic  acid,  &c.). 
Mix  the  solution  with  hydrochloric  acid,  evaporate  to  dryness, 
dissolve  the  residue  in  a  little  water,  add  a  concentrated  solution  of 
platinic  chloride,  as  neutral  as  possible,  in  excess,  and  evaporate  in 
a  porcelain  dish,  on  the  water-bath,  nearly  to  dryness,  taking  care 
not  to  heat  the  water-bath  quite  to  boiling.  If  the  platinum  con- 
tent of  the  platinum- chloride  solution  is  known,  the  proper  quan- 
tity of  the  latter  to  add  is  more  readily  used  (§63,  8).  Add 
alcohol  of  about  80  per  cent,  by  volume  to  the  residue  and  let  it 
stand  for  some  time,  pour  the  alcoholic  solution  through  a  small 
iilter,  and  treat  the  residue  if  necessary  a  few  times  with  small 
quantities  of  alcohol  of  the  same  strength,  until  it  appears  to  be 
pure  potassium-platinic  chloride.  Bring  this  upon  the  filter  and 


246  DETERMINATION.  [§  97. 

wash  completely  by  applying  repeatedly  small  quantities  of  the 
same  alcohol.  Dry  next  the  filter  and  its  contents  in  the  funnel, 
for  it  is  necessary  that  the  alcohol  should  be  completely  volatilized. 
Transfer  the  contents  of  the  filter  carefully  to  a  watch-glass,  and 
place  the  filter  back  into  the  funnel  and  dissolve  and  wash  out  the 
small  quantity  of  adhering  potassium-platinic  chloride  with  hot 
water.  Evaporate  the  yellow  solution  thus  obtained  to  dryness  in 
a  weighed  platinum  vessel.  Then  bring  the  chief  quantity  of  the 
precipitate  into  the  platinum  dish  and  dry  the  whole  to  a  constant 
weight  at  130°  C. 

If  the  quantity  of  potassium-platinic  chloride  obtained  is  very 
small,  the  whole  may  be  dissolved  from  the  filter,  evaporated  and 
dried  in  the  same  manner.* 

The  asbestos  filtering- tube  described  on  page  108,  Fig.  68,  is 
generally  to  be  recommended  for  filtering.  The  tube  to  be  dried 
is  freed  from  water  so  far  as  possible  by  suction,  and  then  inserted 
into  another,  wider  tube  about  4  cm.  shorter,  which  is  fixed  in  the 
air-bath  shown  on  page  64,  Fig.  38.  Air  is  then  slowly  drawn 
through  the  tube,  while  the  air-bath  is  heated,  towards  the  end  at 
130°,  for  a  long  time.  The  air-current  should  enter  the  tube  at  the 
wide  end,  and  should  be  dried  by  concentrated  sulphuric  acid.  After 
the  drying  is  effected  and  the  tube  weighed,  the  results  may  be  readily 
controlled  by  converting  the  potassium-platinic  chloride  into  plati- 
num. For  this*  purpose  dry  hydrogen  is  passed  through  the  tube 
while  it  is  moderately  heated.  After  the  decomposition  is  com- 
plete, the  potassium  chloride  is  leached  out  with  water,  all  the 
water  is  removed  by  suction  while  the  tube  is  being  heated,  and  the 
residual  platinum  weighed,  1  equivalent  being  equal  to  2  equivalents 
of  potassium. 

If  a  paper  filter  is  used,  this  must  first  be  dried  at  100°,  weighed, 
and  then  the-  loss  of  weight  of  an  aliquot  portion  dried  at  130° 
determined,  from  which  the  loss  of  weight  of  the  entire  filter  at 
130°  may  be  calculated. 

ft.  Potassium  salts  of  non-volatile  acids  (phosphoric  acid,  boracic 
acid,  &c.). 


*  When  many  successive  determinations  are  to  be  made,  especially  in  technical 
analyses,  much  time  can  be  saved  by  using  Goocn's  apparatus  (see  pp.  120,  121) 
for  washing  and  weighing  the  K2PtCl6. 


§  97.]  POTASSIUM.  247 

Make  a  concentrated  solution  of  the  salt  in  water,  add  some 
hydrochloric  acid,  and  platinic  chloride  in  excess,  mix  with  a  toler- 
able quantity  of  the  strongest  alcohol,  let  the  mixture  stand  24 
hours,  after  which  filter,  and  proceed  as  directed  in  a. 

For  properties  of  the  precipitate  see  §  68.  This  method,  prop- 
erly executed,  gives  satisfactory  results.  Still  there  is  generally  a 
trifling  loss  of  substance,  potassium-platinic  chloride  not  being 
absolutely  insoluble  even  in  strong  alcohol.  In  accurate  analyses, 
therefore,  the  alcoholic  washings  must  be  evaporated,  with  addition 
of  a  little  pure  sodium  chloride,  at  a  temperature  not  exceeding  75°, 
nearly  to  dryness,  and  the  residue  treated  once  more  with  80-per 
•cent,  alcohol.  A  trifling  additional  amount  of  potassium-platinic 
chloride  is  thus  obtained,  which  is  either  added  to  the  principal 
precipitate  or  collected  on  a  separate  small  filter,  and  weighed  by 
dissolving  from  the  filter  arid  evaporating  to  dryness  as  above  de- 
scribed. The  object  of  the  addition  of  a  little  sodium  chloride  to 
the  platinic  chloride  is  to  obviate  the  decomposition  to  which  pure 
platinic  chloride  is  more  liable  upon  evaporation  in  alcoholic  solu- 
tion alone,  than  it  is  when  mixed  with  sodium-platinic  chloride. 
The  atmosphere  of  a  laboratory  often  contains  ammonia,  which 
might  give  rise  to  the  formation  of  some  ammonium-platinic  chlo- 
ride, and  to  a  consequent  increase  of  weight  in  the  potassium  salt. 

As  the  collection  of  a  precipitate  on  a  weighed  filter-paper  is  very 
tedious,  and,  where  small  quantities  are  operated  upon,  inaccurate 
as  well,  it  is  better,  where  the  filter-tube  is  not  used,  to  collect 
small  quantities  of  potassium-platinic  chloride  (up  to  0*03  grm.)  in 
a  very  small,  un weighed  filter,  dry,  transfer  the  filter,  wrapped  up 
around  the  precipitate,  to  a  small  covered  porcelain  crucible,  and 
slowly  carbonize.  The  cover  is  then  removed,  the  carbon  burned, 
and  the  crucible  allowed  to  cool.  A  very  small  quantity  of  pure 
oxalic  acid  is  next  added,  the  cover  placed  on,  and  heat  applied,  at 
first  gently,  finally  strongly.  By  the  addition  of  the  oxalic  acid  the 
complete  decomposition  of  the  potassium-platinic  chloride  is  greatly 
facilitated,  and  which  is  not  so  readily  accomplished  by  simple 
ignition.  Of  course  the  oxalic  acid  may  be  replaced  by  a  current 
of  hydrogen.  The  cooled  contents  of  the  crucible  are  treated  with 
water,  the'  residual  platinum  washed  out  until  the  washings  give  no 
cloudiness  with  silver-nitrate  solution,  then  dried  and  weighed.  As 
a  rule  the  washing  may  be  accomplished  by  simple  decantation. 


248  DETERMINATION.  [§  98. 

5.  Volumetric  determination  as  Potassium  Silicofluoride. 

To  the  moderately  concentrated  solution  of  the  potassium  salt 
in  a  beaker  add  a  sufficiency  of  hydrofluosilicic  acid,*  and  then  an 
equal  volume  of  pure  strong  alcohol.  If  the  potassium  salt  was 
difficultly  soluble  (such  as  potassium  platinic  chloride),  warm  it 
with  the  hydrofluosilicic  acid  before  adding  the  spirit.  The  potas- 
sium silicofluoride  will  separate  as  a  translucent  precipitate ;  when 
it  has  settled,  filter,  wash  out  the  beaker  with  a  mixture  of  equal 
parts  strong  alcohol  and  water,  and  wash  the  precipitate  with  the 
same  mixture  till  the  washings  are  no  longer  acid  to  litmus  paper. 
Put  the  filter  and  precipitate  into  the  beaker  previously  used,  treat 
with  water,  add  some  tincture  of  litmus,  heat  to  boiling,  and  add 
standard,  or,  in  the  case  of  very  small  quantities,  decinornial, 
potassa  or  soda  solution  (§  215)  till  the  fluid  is  just  blue,  and 
remains  so  after  continued  boiling.  The  reaction  is  as  follows : 
(KF),SiF4  +  4KOH  =  6KF  +  Si(OH)4,  consequently  2  atoms 
potassium  in  the  standard  solution  correspond  to  1  at.  potassium 
originally  present  and  precipitated  as  potassium  silicofluoride  (FR. 
STOLBA|). 

If  the  solution  of  the  potassium  salt  contains  much  free  acid, 
particularly  sulphuric  acid,  this  is  to  be  removed  by  heat  before 
adding  the  hydrofluosilicic  acid.  Small  quantities  of  ammonium 
salts  are  of  no  influence,  but  large  quantities  should  be  removed. 
It  need  hardly  be  mentioned  that  other  metals  precipitable  by 
hydrofluosilicic  acid  must  be  absent.  The  results  are  satisfactory. 
STOLBA  obtained  99 -2  to  100  per  cent.  Potassium-platinic  chloride 
may  be  easily  converted  into  potassium  silicofluoride;  hence,  in 
technical  analyses,  the  potassium  may  be  separated  in  the  first 
form,  and  then  titrated  as  the  latter  (STOLBA,  loc.  cit.). 

§98. 

2.    SODIUM. 
a.  Solution. 

See  §  97,  a,  all  the  directions  given  in  that  place  applying 
equally  to  the  solution  of  NaOH  and  sodium  salts. 

*  W.  KNOP  and  W.  WOLF  use  hydrofluosilicate  of  aniline  instead. — Zeitschr. 
f.  analyt.  Chem.,  i,  471. 

f  Zeitschr.  f.  analyt.  Chem.,  in,  298. 


§  98.]  SODIUM.  .  249 

5 .  Determination . 

Sodium  is  determined  either  as  sodium  sulphate,  as  sodium 
chloride,  or  as  sodium  carbonate  (§  69).    For  the  alkalirnetric  esti- 
mation of  caustic  soda  and  sodium  carbonate,  see  §§219  and  220. 
We  may  convert  into 

1.    SODIUM       SULPHATE;         2.    SODIUM      NITRATE; 

3.  SODIUM    CHLORIDE. 

In  general  the  sodium  salts  corresponding  to  the  potassium  salts 
specified  under  the  analogous  potassium  compounds,  §  97. 

4.  SODIUM  CARBONATE. 

Caustic  soda,  sodium  hydrogen  carbonate,  and  sodium  salts  of 
organic  acids,  also  sodium  nitrate  and  sodium  chlorate. 

5.  SODIUM  SILICOFLUORIDE.  „ 
Sodium  salts  of  acids  soluble  in  dilute  alcohol,  excepting  sodium 

borate. 

In  sodium  borate  the  sodium  is  estimated  best  as  sodium  sul- 
phate (§  136) ;  in  the  phosphate,  as  sodium  chloride,  or  sodium 
carbonate  (§  135). 

Sodium  salts  of  organic  acids  are  determined  either,  like  the 
corresponding  potassium  compounds,  as  chloride,  or — by  preference 
—as  carbonate.  (This  latter  method  is  not  so  well  adapted  for 
potassium  salts.)  The  analyst  must  here  bear  in  mind,  that  when 
carbon  acts  on  fusing  sodium  carbonate,  carbon  monoxide  escapes, 
and  caustic  soda  in  not  inconsiderable  quantity  is  formed. 

1.  Determination  as  Sodium,  Sulphate. 

If  alone  and  in  aqueous  solution,  evaporate  to  dryness,  ignite 
and  weigh  the  residue  in  a  covered  platinum  crucible  (§  42).  The 
process  does  not  involve  any  risk  of  loss  by  decrepitation,  as  in  the 
case  of  potassium  sulphate.  If  free  sulphuric  acid  happens  to  be 
present,  this  is  removed  in  the  same  way  as  in  the  case  of  potas- 
sium sulphate. 

With  regard  to  the  conversion  of  sodium  chloride,  &c. ,  into 
sodium  sulphate,  see  §  97,  £,  1.     For  properties  of  the  residue, 
see  §  69.     The  method  is  easy,  and  gives  accurate  results. 
2.   Determination  as  Sodium  Nitrate. 

Same  method  as  described  in  1 .  The  rules  and  observations 
are  the  same  as  those  given  under  the  estimation  of  potassium 
nitrate  (§  97).  For  properties  of  the  residue,  see  §  69. 


250  DETERMINATION.  [§  98. 

3.  Determination  as  Sodium  Chloride. 

Same  method  as  described  in  1.  The  rules  given  and  the 
observations  made  in  §  97,  &,  2,  apply  equally  here.  The  fact 
that  sodium  chloride  is  more  difficultly  volatilizable  than  potassium 
chloride  favors  greater  accuracy  of  results.  For  properties  of  the 
residue,  see  §  69. 

The  methods  of  converting  sodium  sulphate,  chromate,  chlorate, 
and  silicate  into  sodium  chloride  will  be  found  in  Part  II.  of  this 
Section,  under  the  respective  heads  of  the  acids  which  these  salts 
contain. 

4.  Determination  as  Sodium  Carbonate. 
Evaporate  the  aqueous  solution,  ignite  moderately,  and  weigh. 

TJie  results  are  perfectly  accurate.  For  properties  of  the  residue, 
see  §  69. 

Caustic  soda  is  converted  into  the  carbonate  by  adding  to  its 
aqueous  solution  ammonium  carbonate  in  excess,  evaporating  at  a 
gentle  heat,  and  igniting  the  residue. 

Sodium  hydrogen  carbonate,  if  in  the  dry  state,  is  converted 
into  the  normal  carbonate  by  ignition.  The  heat  must  be  very 
gradually  increased,  and  the  crucible  kept  well  covered.  If  iu 
aqueous  solution,  it  is  evaporated  to  dryness,  in  a  capacious  silver 
or  platinum  dish,  and  the  residue  ignited. 

Sodium  salts  of  organic  acids  are  converted  into  the  carbonate 
by  ignition  in  a  covered  platinum  crucible,  from  which  the  lid  is 
removed  after  a  time.  The  heat  must  be  increased  very  gradually. 
When  the  mass  has  ceased  to  swell,  the  crucible  is  placed  obliquely, 
•with  the  lid  leaning  against  it  (see  §  52,  fig.  42),  and  a  dull  red 
heat  applied  until  the  carbon  is  consumed  as  far  as  practicable. 
The  contents  of  the  crucible  are  then  warmed  with  water,  and  the 
fluid  is  filtered  off  from  the  residuary  carbon,  which  is  carefully 
washed.  The  filtrate  and  rinsings  are  evaporated  to  dryness  with 
the  addition  of  a  little  ammonium  carbonate,  and  the  residue  is 
ignited  and  weighed.  The  ammonium  carbonate  is  added,  to  con- 
vert any  caustic  soda  that  may  have  been  formed  into  carbonate. 
The  method,  if  carefully  conducted,  gives  accurate  results ;  how- 
ever, a  small  loss  of  soda  on  carbonization  is  not  to  be  avoided. 
If  any  residue  soluble  in  water  remains,  dissolve  it  and  add  it  to 
the  principal  solution. 


§  99.]  AMMONIUM.  251 

Sodium  nitrate,  or  sodium  chloride,  may  be  converted  into  car- 
bonate, by  adding  to  its  aqueous  solution  perfectly  pure  oxalic 
iicid  in  moderate  excess,  and  evaporating  several  times  to  dryne.-^, 
with  repeated  renewal  of  the  water.  All  the  nitric  acid  of  the 
sodium  nitrate  escapes  in  this  process  (partly  decomposed,  partly 
undecomposed)  ;  and  equally  so  all  the  hydrochloric  acid  in  the 
case  of  sodium  chloride.  If  the  residue  is  now  ignited  until  the 
excess  of  oxalic  acid  is  removed,  sodium  carbonate  is  left. 

§  99. 

3.  AMMONIUM. 

a.  Solution. 

Ammonia  is  soluble  in  water,  as  are  all  ammonium  salts  of  those 
acids  which  claim  our  attention  here.  It  is  not  always  necessary, 
however,  to  dissolve  ammonium  salts  for  the  purpose  of  determin- 
ing the  amount  of  ammonium  contained  in  them. 

b.  Determination. 

Ammonium  is  weighed,  as  stated  §  TO,  either  in  the  form  of 
ammonium  chloride,  or  in  that  of  ammonium  platinic  chloride, 
Into  these  forms  it  may  be  converted  either  directly  or  indirectly 
(i.e.,  after  expulsion  as  ammonia,  and  re-combination  with  an  acid). 
Ammonium  is  also  frequently  determined  by  volumetric  analysis, 
and  its  quantity  is  sometimes  inferred,  from  the  volume  of  nitrogen. 

We  convert  directly  into 

1.  AMMONIUM  CHLORIDE. 

Ammonia  gas  and  its  aqueous  solution,  and  also  ammonium  salts 
of  weak  volatile  acids  (ammonium  carbonate,  ammonium  sulphide, 


2.  AMMONIUM  PLATINIC  CHLORIDE. 

Ammonium  salts  of  acids  soluble  in  alcohol,  such  as  ammonium 
sulphate,  ammonium  phosphate,  &c. 

3.  The  methods  based  on  the  EXPULSION  OF  AMMONIA  from 
ammonium  compounds,  and  also  that  of  inferring  the  amount  of 
ammonium  from  the  volume  of  nitrogen  eliminated  in  the  dry 
way,  are  equally  applicable  to  all  ammonium  salts. 

The  expulsion  of  ammonia  in  the  dry  way  (by  ignition  with 
soda-lime),  and  its  estimation  from  the  volume  of  nitrogen  elimi- 
nated in  the  dry  way,  being  effected  in  the  same  manner  as  the 


DETERMINATION.  [§  99. 

estimation  of  the  nitrogen  in  organic  compounds,  I  refer  the  stu- 
dent to  the  Section  on  Organic  Analysis.  The  process  of  estimating 
ammonia  by  decomposing  with  a  bromized  solution  of  sodium  hypo- 
chlorite  will  be  given  under  the  Analysis  of  Soils,  in  the  Special 
Part.  For  the  alkali  metric  estimation  of  free  ammonia,  see  §§219 
and  220 ;  and  for  the  calorimetric  method  based  on  the  use  of 
KESSLEK'S  solution,  see  under  the  Analysis  of  Waters,  §  205. 

1.  Determination  as  Ammonium  Chloride. 
Evaporate  the  aqueous  solution  of  the  ammonium  chloride  on 
the  water-bath,  and  dry  the  residue  at  100°  until  the  weight  re- 
mains constant  (§  42).  The  results  are  accurate.  The  volatiliza- 
tion of  the  chloride  is  very  trifling.  A  direct  experiment  gave 
99-94  instead  of  100.  (See  Expt.  15.)  The  presence  of  free 
hydrochloric  acid  makes  no  difference ;  the  conversion  of  caustic 
ammonia  into  ammonium  chloride  may  accordingly  be  effected  by 
supersaturating  with  hydrochloric  acid.*  The  same  applies  to  the 
conversion  of  the  carbonate,  with  this  addition  only,  that  the  process 
of  supersaturation  mu^t  bo  conducted  in  an  obliquely-placed  flask, 
and  the  mixture  heated  in  ihe  samo,  till  the  carbonic  acid  is  driven 
off.  In  the  analysis  of  ammonium  gulphide  we  proceed  in  the  same 
way,  taking  care  simply,  after  the  expulsion  of  the  hydrogen 
sulphide,  and  before  proceeding  to  evaporate,  to  filter  off  the  sul- 
phur which  may  have  separated.  Instead  of  weighing  the  ammo- 
nium chloride,  its  quantity  may  be  inferred  by  the  determination 
of  its  chlorine  according  to  §  141,  ~b  (Comp.  potassium  chloride, 
§  W,  5,  3.) 

2.   Determination  as  Ammonium- Platinic  Chloride. 
a.   Ammoniacal  salts  with  volatile  acids. 

Same  method  as  described  in   §  97,  4,  a.  *  (potassium -platinic 
chloride). 

ft.   Ammonium  salts  of  non-volatile  acids. 

Same  method  as  described  §  97,  4,  fi  (potassium-platinic  chlo- 
ride).     The  results  obtained  by  these  methods  are  accurate. 

If  you  wish  to  control  the  results,  f  ignite  the  double  chloride, 

-GUNNING  (Zeitschr.  /.  analyt.  Chem.,  YII,  480)  has  pointed  out  that  fluids 
during  evaporation  may  take  up  ammonia  because  of  the  presence  of  this  in  illu- 
minating gas. 

flf  the  ammonium-platinic  chloride  is  pure,  which  maybe  known  by  its  color 
and  general  appearance,  this  control  may  be  dispensed  with. 


§  99.]  AMMONIUM.  253 

wrapped  up  in  the  filter,  in  a  covered  crucible,  and  calculate  the 
amount  of  ammonium  from  that  of  the  residuary  platinum.  The 
results  must  agree.  If  the  double  salt  is  in  a  filtering-tube,  slowly 
pass  a  current  of  air  through  it,  while  heating  very  carefully.  If, 
however,  the  salt  is  in  a  paper  filter,  it  is  best  to  transfer  the  pre- 
cipitate, wrapped  up  in  the  filter,  to  the  crucible,  and  continue  the 
application  of  a  moderate  heat  for  a  long  time,  then  to  remove  the 
lid,  place  the  crucible  obliquely,  with  the  lid  leaning  against  it,  and 
burn  the  charred  filter  at  a  gradually-increased  heat  (H.  ROSE). 

When  the  salt  is  pure,  which  may  be  known  from  its  color  and 
general  appearance,  this  control  may  be  omitted.  Want  of  due 
caution  in  respect  to  heating  is  apt  to  lead  to  loss,  from  particles  of 
the  double  salt  being  carried  away  with  the  ammonium  chloride. 
Very  small  quantities  of  ammonium-platinic  chloride  are  collected 
on  an  unweighed  filter,  dried,  and  at  once  reduced  to  platinum  by 
ignition .  * 

3.  Estimation  by  Expulsion  of  Ammonia  in  the  Wet  Way. 

This  method,  which  is  applicable  in  all  cases,  may  be  effected 
in  different  ways,  viz.  : 

a.  EXPULSION  OF  THE  AMMONIA  BY  DISTILLATION  WITH  SOLUTION 
OF  POTASSA,  SODA,  MILK  OF  LIME,  OR  MAGNESIA. — Applicable  in 
all  cases  where  no  nitrogenous  organic  matters  from  which  ammonia 
might  be  evolved  upon  boiling  with  solution  of  potassa,  etc.,  are 
present  with  the  ammonium  salts.  Magnesia  is  used  in  cases  where 
nitrogenous  substances  capable  of  yielding  ammonia  on  boiling,  are 
present. 

Weigh  the  substance  under  examination  in  a  small  glass  tube, 
three  centimetres  long  and  one  wide,  and  put  the  tube,  with  the 
substance  in  it,  into  a  small,  tubulated  retort,  a,  Fig.  81,  contain- 
ing a  suitable  quantity  of  moderately  concentrated  solution  of 
potassa  or  soda,  milk  of  lime,  or  magnesia  mixed  with  water,  from 
which  every  trace  of  ammonia  has  been  removed  by  protracted 
ebullition,  but  which  has  been  allowed  to  get  thoroughly  cold  again. 
The  further  arrangement  of  the  apparatus  is  shown  by  the  cut.  As 
will  be  seen,  the  ammoniacal  distillate  does  not  come  into  contact  with 

*In  a  series  of  experiments  to  get  the  platinum  from  pure  and  perfectly 
anhydrous  ammonium-platinic  chloride,  by  very  cautious  ignition,  Mr.  Lucius, 
one  of  my  pupils,  obtained  from  44 -1  to  44 '3  per  cent,  of  the  metal,  instead  of 
44-3. 


254 


DETERMINATION. 


[§99. 


cork  or  rubber — and  this  is  quite  important,  since  otherwise  thsee 
might  easily  retain  some  of  the  ammoniacal  fluid. 


Fig.  81. 

If  you  wish  to  determine  volumetrically  the  quantity  of  ammo- 
nia expelled,  introduce  the  larger  portion  of  a  measured  quantity 
of  standard  solution  of  acid  (sulphuric,  hydrochloric,  or  oxalic, 
§  215)  into  the  receiver,  the  remainder  into  the  U-tube;  add  to 
the  portion  of  fluid  in  the  latter  a  little  water,  and  color  the  liquids 
in  the  receiver  and  U-tube  red  with  1  or  2  c.  c.  of  tincture  of  lit- 
mus. The  cooling-tube  must  not  dip  into  the  fluid  in  the  receiver ; 
the  fluid  in  the  U-tube  must  completely  fill  the  lower  part,  but  it 
must  not  rise  high,  as  otherwise  the  passage  of  air-bubbles  might 
easily  occasion  loss  by  spirting.  The  quantity  of  acid  used  must 
of  course  be  more  than  sufficient  to  fix  the  whole  of  the  ammonia 
expelled. 

When  the  apparatus  is  fully  arranged,  and  yoii  have  ascertained 
that  all  the  joints  are  perfectly  tight,  heat  the  contents  of  the 
retort  to  gentle  ebullition,  and  continue  the  application  of  the  same 
degree  of  heat  until  the  drops,  as  they  fall  into  the  receiver,  have 
for  some  time  altogether  ceased  to  impart  the  least  tint  of  blue  to 
the  portion  of  the  fluid  with  which  they  first  come  in  contact. 
Before  removing  the  heat,  a  strip  of  turmeric  paper  is  fixed  in 
the  tubulure  of  the  retort;  it  must  not  turn  brown.  Then  loosen 
the  stopper  of  the  retort,  allow  to  stand  half  an  hour,  pour  the  con- 
tents of  the  receiver  and  U-tube  into  a  beaker,  rinse  out  with  small 
quantities  of  water,  and  determine  finally  with  a  standard  solution 


§99.]  AMMONIUM.  255 

of  alkali  the  quantity  of  acid  still  free,  which,  by  simple  subtrac- 
tion, will  give  the  amount  of  acid  which  has  combined  with  the 
ammonia;  and  from  this  you  may  now  calculate  the  amount  of  the 
latter  (§  220).  Results  accurate.*  (Expt.  ISTo.  55.) 

If  you  wish  to  determine  by  the  gravimetric  method  the  qua/n- 
tity  of  ammonia  expelled,  receive  the  ammonia  evolved  in  a  quan- 
tity of  hydrochloric  acid  more  than  sufficient  to  fix  the  whole  of  it, 
and  determine  the  ammonium  chloride  formed,  either  by  simple 
evaporation,  after  the  directions  of  1,  or,  far  preferably,  as  ammo- 
nium-platinic  chloride,  after  the  directions  of  2. 

1).  EXPULSION  OF  THE  AMMONIA  BY  MILK  OF  LIME,  WITHOUT 
APPLICATION  OF  HEAT. — This  method,  recommended  byScHLosiNG, 
is  based  upon  the  fact  that  an  aqueous  solution  containing  free 
ammonia  gives  off  the  latter  completely,  and  in  a  comparatively 
short  time,  when  exposed  in  a  shallow  vessel  to  the  air,  at  the  com- 
mon temperature.  It  finds  application  in  cases  where  the  presence 
of  organic  nitrogenous  substances,  decomposable  by  boiling  alkalies, 
forbids  the  use  of  the  method  described  in  3,  a\  thus,  for  instance, 
in  the  estimation  of  the  ammonia  in  urine,  manures,  etc. 

The  fluid  containing  the  ammonia,  the  volume  of  which  must 
not  exceed  35  c.  c.,  is  introduced  into  a  shallow  flat-bottomed  ves- 
sel from  10  to  12  centimetres  in  diameter;  this  vessel  is  put  on  a 
plate  filled  with  mercury.  A  tripod,  made  of  a  massive  glass  rod, 
is  placed  in  the  vessel  which  contains  the  solution  of  the  ammonium 
salt,  and  a  saucer  or  shallow  dish  with  10  c.  c.  of  the  normal  solu- 
tion of  oxalic  or  sulphuric  acid  (§  215)  put  on  it.  A  beaker  is  now 
inverted  over  the  whole.  The  beaker  is  lifted  up  on  one  side  as 
far  as  is  required,  and  a  sufficient  quantity  of  milk  of  lime  added 
by  means  of  a  pipette  (which  should  not  be  drawn  out  at  the  lower 
end).  The  beaker  is  then  rapidly  pressed  down,  and  weighted 
with  a  stone  slab.  After  forty-eight  hours  the  glass  is  lifted  up, 
and  a  slip  of  moist  reddened  litmus  paper  placed  in  it ;  if  no  change 
of  color  is  observable,  this  is  a  sign  that  the  expulsion  of  the  ammo- 
nia is  complete ;  in  the  contrary  case,  the  glass  must  be  replaced. 
Instead  of  the  beaker  and  plate  with  mercury,  a  bell- jar,  with  a 
ground  and  greased  rim,  placed  air-tight  on  a  level  glass  plate,  may 
be  used.  A  bell-jar,  having  at  the  top  a  tubular  opening  furnished 
with  a  close-fitting  glass  stopper,  answers  the  purpose  best,  as  it 

*  [In  thus  estimating  minute  quantities  of  ammonia,  the  condensing  tube 
must  "be  of  tin,  since  glass  yields  a  sensible  amount  of  alkali  to  hot- water  vapor.]) 


256  DETERMINATION.  [§  99. 

permits  the  introduction  of  a  slip  of  red  litmus  paper  suspended 
from  a  thread,  thus  enabling  the  operator  to  see  whether  the  com- 
bination of  the  ammonia  with  the  acid  is  completed,  without  the 
necessity  of  removing  the  bell- jar.  According  to  SCHLOSING,  forty- 
eight  hours  are  always  sufficient  to  expel  O'l  to  1  gramme  of  ammo- 
nia from  25  to  35  c.  c.  of  solution.  However,  I  can  admit  this 
statement  only  as  regards  quantities  up  to  0'3  grin.  ;  quantities 
above  this  often  require  a  longer  time.  I  therefore  always  prefer 
operating  with  quantities  of  substance  containing  no  more  than  0*3 
grin,  ammonia  at  the  most. 

When  all  the  ammonia  has  been  expelled,  and  has  entered  into 
combination  with  the  acid,  the  quantity  of  acid  left  free  is  deter- 
mined by  means  of  standard  solution  of  alkali,  and  the  amount  of 
the  ammonia  calculated  from  the  result  (§  220). 

c.  INDIRECT  METHOD  ACCORDING  TO  FR.  MOHR.* — In  this  method 
a  known  quantity  of  alkali  in  excess,  e.g.,  sodium  carbonate,  is 
heated  in  aqueous  solution  with  the  ammonium  salt  until  all  the 
ammonia  has  been  expelled ;  the  residual  alkali  is  then  volumetri- 
cally  estimated,  and  from  the  difference  the  equivalent  quantity  of 
ammonia  estimated.  This  method  is  of  limited  application,  because 
it  can  only  be  used  with  neutral  ammoniacal  salts  in  the  absence 
of  organic  matter,  f  It  is,  however,  convenient  and  exact,  and 
may  be  conducted  in  an  obliquely  supported  flask.  As  alkali,  a 
normal  solution  of  soda  or  sodium  carbonate  (53 '05  grm.  anhy- 
drous salt  per  litre)  may  be  used.  The  boiling  is  stopped  when  the 
escaping  vapors  cease  to  act  on  red  litmus  paper  or  turmeric  paper. 
4.  Estimation  by  Expulsion  of  the  Nitrogen  in  the  Wet  Way. 

A  process  for  determining  ammonium  by  means  of  the  azo- 
tometer  has  been  given  by  "W.  KNOP.  $  It  depends  on  the  sepa- 
ration of  the  nitrogen  by  a  bromized  and  strongly  alkaline  solution 
of  sodium  hypochlorite.  § 

The  simplest  azotometer  is    that    described   by  KUMPF.||     It 

*  Lehrbuch  der  Titrirmethode. 

\  Even  organic  matter  free  from  nitrogen  has  a  disturbing  action,  as  when 
boiled  with  alkali  it  forms  humus-like  acid  decomposition  products,  which  neu- 
tralize the  alkali.  J  Hiem.  CentralbL,  1860,  244. 

§  This  is  prepared  as  follows  :  Dissolve  1  part  of  sodium  carbonate  in  15 
parts  of  water,  cooi  the  fluid  with  ice,  saturate  perfectly  with  chlorine,  keeping 
cold  all  the  while,  and  add  strong  soda  solution  (of  25  per  cent.)  till  the  mixture 
on  rubbing  between  the  fingers  makes  the  skin  slippery.  Before  using,  add  to 
the  quantity  required  for  the  series  of  experiments  bromine  in  the  proportion  of 
2-3  grm.  to  the  litre,  and  shake.  ||  Fres.  Zeit.,  vi,  398. 


99.] 


AMMONIUM. 


257 


consists  of  a  burette  of  50  or  100  c.  c.  stationed  in  a  glass  cylinder 
nearly  filled  with  mercury,  and  connected  by  a  stout  caoutchouc 
tube  with  a  small  bottle,  a,  Fig.  82,  to  which  is  fitted  a  soft,  thrice- 
perforated  caoutchouc  stopper.  The  stopper  carries  a  thermometer 
and  two  short  glass  tubes,  one  of  which  joins  it  to  the  burette,  and 
the  other  has  attached  a  short  bit  of  caoutchouc  tubing  arid  a  pinch- 
cock,  e.  The  weighed  ammonium  salt  (not  more  than  0'4  grm.)  is 
placed  in  the  tube,  /',  with  10  C,  c.  of  water,  and  50  c.  c.  of  the 
bromized  hypochlorite  solution  are  brought  into  the  bottle,  a. 
The  cock,  0,  being  open,  the  stopper  is  firmly  fixed  in  its  place,  and 
the  burette  is  depressed  in  the  mercury  un- 
til its  uppermost  degree  exactly  coincides 
with  the  surface  of  the  metal.  The  cock  is 
then  closed,  and  the  bottle  is  inclined  to 
bring  the  two  substances  in  contact.  The 
ammonium  salt  is  speedily  decomposed. 
When  no  further  evolution  of  gas  takes 
place  the  burette  is  so  adjusted  that  the 
level  of  the  mercury  without  and  within  it 
shall  nearly  coincide,  and  the  operator  waits 
10-20  minutes,  or  until  the  thermometer  in 
a  indicates  the  same  temperature  as  the  sur- 
rounding air.  Then  the  adjustment  of  the 
burette  to  exact  coincidence  of  the  mercury 
level,  within  and  without,  is  effected,  and 
the  volume  of  the  gas  is  read  off.  The  stand 
of  the  thermometer  and  barometer  are  also 
noted,  and  the  recorded  volume  of  nitrogen 
is  corrected  by  use  of  the  tables  on  pp.  259- 
261,  arranged  by  DIETRICH. 

The  first  table  gives  a  correction  for  the 
nitrogen  which  is  absorbed  by  the  60  c.  c.  of  liquid  in  the  bottle  a. 
The  amount  varies  with  the  relative  volumes  of  air  and  nitrogen, 
and  is  determined  empirically  by  decomposing  known  quantities  of 
ammonia  and  noting  the  difference  between  the  obtained  and  the 
theoretical^ volume  of  nitrogen.  The  correction  holds  strictly,  of 
rourse,  only  for  a  solution  of  such  strength  as  that  employed  by 
DIETRICH  and  at  the  mean  temperatures. 

The  second  table  serves  to  spare  the  labor  of  calculation.     The 
weight  of  1  c.  c.  of  nitrogen,  measured  e.  g.  at  751  mm.  of  barome- 


Fig.  82. 


258  DETERMINATION.  [§  10(X 

ter  and  15°,  is  found  at  the  intersection  of  the  vertical  column 
754  with  the  horizontal  column  15°,  is,  viz.,  1-16187. 

To  the  observed  volume  of  nitrogen  add  the  amount  absorbed 
as  per  Table  I.,  and  correct  the  total  by  Table  II.  It  scarcely 
requires  to  be  mentioned  that  good  results  can  only  be  obtained  in 
an  apartment  where  the  temperature  is  uniform,  and  when  care  is 
exercised  to  avoid  warming  the  apparatus  in  handling.  See  DIET- 
RICH'S papers.* 

§100. 

Supplement  to  the  First  Group. 
LITHIUM. 

In  the  absence  of  other  bases,  lithium  may,  like  potassium 
and  sodium,  be  converted  into  anhydrous  SULPHATE,  and  weighed' 
in  that  form  (Li2SO4).  As  lithium  forms  no  acid  sulphate,  the 
excess  of  sulphuric  acid  may  be  readily  removed  by  simple  igni- 
tion. LITHIUM  CARBONATE  also,  which  is  difficultly  soluble  in 
water,  and  fuses  at  a  red  heat  without  suffering  decomposition,  is 
well  suited  for  weighing  ;  whilst  lithium  chloride,  which  deliquesces 
in  the  air,  and  is  by  ignition  in  moist  air  converted  into  hydro- 
chloric acid  and  lithium  oxide,  is  unfit  for  the  estimation  of 
lithium. 

In  presence  of  other  alkali  metals,  lithium  is  best  converted 
into  LITHIUM  PHOSPHATE  (Li3PO4),  and  weighed  in  that  form.  This  is 
effected  by  the  following  process  :  add  to  the  solution  a  sufficient 
quantity  of  sodium  phosphate  (which  must  be  perfectly  free  from 
phosphates  of  the  alkali-earth  metals),  and  enough  soda  to  keep  the 
reaction  alkaline,  and  evaporate  the  mixture  to  dryness  ;  pour 
water  over  the  residue,  in  sufficient  quantity  to  dissolve  the  soluble 
salts  with  the  aid  of  a  gentle  heat,  add  an  equal  volume  of  solution 
of  ammonia,  digest  at  a  gentle  heat,  filter  after  twelve  hours,  and 
wash  the  precipitate  with  a  mixture  of  equal  volumes  of  water  and 
solution  of  ammonia.  Evaporate  the  filtrate  and  first  washings  to 
dryness,  and  treat  the  residue  in  the  same  way  as  before.  If  some 
more  lithium  phosphate  is  thereby  obtained,  add  this  to  the  prin- 
cipal quantity.  The  process  gives,  on  an  average,  99'61  for  100 
parts  of  lithium  oxide  ( 


*  Fres.  Zeit.,  in,  162;   iv,  141,  and  v,  36. 
\  Ann.  d.  Chem.  u.  Pharm.,  xcvni,  212. 


§99.] 


TABLE   OF  ABSORPTION   OF   NITROGEN   GAS. 


259 


8  I 

s  s 

8  8 

S  8 

§  S? 

TH     (^ 

s  s 

0 

3  S 

1-H 

§  S 

TH 

s  9 

<M 

03   o' 

00   $ 

*"  S 

8  1 

§  3 

*  2 

8  2 

-  I 

»  1 

JO   ""*. 

TH 

«=  2 

6  1 

-  1 

«o   § 

50  S 

8  3 

g  § 

8  3 

3  3 

0 

TH 

CO   ^j 

ia  g 
10  - 

10    05 

t-    . 

TH 

8  3 

.s  1 

s  1 

a  S 

TH 

s  | 

a  | 

co  3B 

CO    ^ 

8  5 

CO 
00    GO 

TH 

8  1 

S  8. 

0 

O?   on 

CO    ~j 

8  5 

Oi   oo 

TH 

8  » 

oi 

0 

o 

s  2 

c  a. 

s  § 

s  a 

0 

CO 

8  2 

o  °° 

TH 

g  s 

C5J 

A    « 
0 

a  1 

9  * 

§  ^ 

g  8 

CO 

00   « 

0 

8  '3 

2  §3 

TH 

TH 

8  1 

TH 

0 

£  s 

o 

*  2 

T-! 

£  1 

00 
0 

8  1 

oo 

w  g 

"— 

s  i 

10   * 

0 

§  8. 

0 

i—i 

CD 

JO   i*^ 

8  i 

CO 
0 

CO 
«i    «0 
<*   0 

^    rH 

S  8 

1—  i 

^      TH 

-  s 

8  1 

TH 

@   » 

TH 

CO   _ll 
00     . 

«  1 

8  | 

9  ? 

TH 

8  5 

GO 
S?   O 

oi 

«  s 

o 

a  1 

'  s 

«  S 

s  1 

Evolved  
Absorbed  

Evolved  
Absorbed  

Evolved  
Absorbed  

Evolved  
Absorbed  

-d  1 

0    -2 

t*        *^ 

3     ° 

t>         (^ 

W    <^ 

260 


TABLE   OF   WEIGHTS. 


[§99. 


II.  TABLE   OF  THE  WEIGHT  OF   A 

In  Milligrammes  for  Pressures  from  720  to  770  mm. 

MILLIMETRES. 


720 

722 

724 

726 

728 

730 

732 

734 

736 

738 

740 

742 

744 

10° 

1.13380 

1.13699 

1.14018 

1.14337 

1.14656 

1.14975 

1.15294 

1.15613 

1.15932 

1.16251 

1.16570 

1.16889 

1.17208 

!!• 

1.12881 

1.131991.13517 

1.13835 

1.14153 

1.14471  1.14789 

1.15107 

1.15424 

1.15742 

1.16060 

1.16378 

1.16696 

lr>° 

1.12376 

1.12693 

1.13010 

1.13326 

1.13643 

1.13960 

1.142771.145931.14910 

1.152271.15543 

1.15860 

1.16177 

13° 

1.11875 

1.12191 

1.12506 

1.12822 

1.13138 

1.13454 

1.137691.140851.14401 

1.14716 

1.15032 

1.15348 

1.15663 

14° 

1.11369 

1.11684 

1.11999 

1.12313 

1.12628 

1.12942 

1.13257 

1.13572 

1.13886 

1.14201 

1.14515 

1.14830 

1.15145 

15° 
16° 

1.10859 
1.10346 

1.11172  1.11486 
1.106581-10971 

1.11799 
1.11283 

1.12113 
1.11596 

1.12426 
1  11908 

1.12739 
1  12220 

1.13053 
1  12533 

1.13366 
1  12845 

1.13680 
1  13158 

1.13993  '1.14306 
1  13470  1  13782 

1.14620 
1  14095 

1.09828 

1.10139 

1.10450 

1.10761 

1.11073 

1.11384 

1.11695 

1.12006 

1.12317 

1.126291.12940 

1.13251 

1.13562 

18° 
19° 
20° 

1.09304 

1.08744 

1.09614 
1.09083 

1.09924 
1.09392 

1.10234 
1  09702 

1.10544 
1  10011 

1.10854 
1  10320 

1.11165 
1.10629 

1.114751.11785 
1  10938  1  1124ft 

1.120951.12405 
1  11557  1  11866 

1.12715 
1  12175 
1.11035 

1.13025 
1.12484 
1.11943 

1.08246 

1.08554 

1.08862 

1.09170 

1.09478 

1.097861.100941.104021.10710 

1.110181.11327 

21° 

1.07708 

1.08015 

1.08322 

1.08629 

1.08936 

1.092431.09550 

1.09857  1.10165 

1.104721.107791.11086 

1.11393 

22°  1.07166 

I 

23°  1.06616 

1.07472 
1.06921 

1.07778 
1.07226 

1.08084 
1.07531 

1.083901.08696 
1.078361.08141 

1.090021.093081.09614 
1.08446  1.08751  1.09056 

1.09921 
1.09361 

1.102271.10533 
1.09666  1.09971 

1.10839 
1.10276 

24° 

1.06061 

1.06365 

1.06669 

1.06973 

1.072771.07581 

1.07885 

1.081891.08493 

1.08796 

1.091001.09404 

1.09708 

25° 

1.05499 

- 

1.05801 

1.06104 

1.06407 

1.06710  1.07013  1.07316  1.07619  1.07922  1.08225  1.08538  1.08881 

1.09134 

720 

722 

724 

726 

728 

730 

732 

734 

736 

738 

740 

742 

744 

MILLIMETRES. 


TABLE    OF    WEIGHTS. 


CUBIC   CENTIMETRE  OF  NITROGEN. 

of  Mercury,  and  for  Temperatures  from  10°  to  25°  C. 

MILLIMETRES. 


746 

748 

750 

752 

754 

756 

758 

760 

762 

764 

766 

768 

770 

1.17527  1.1784G 

j 

1.18165  1.18484  1  18803  1.19122 

1  19441 

1  19760 

1  20079  1.20398 

1.20717 

1.21036 

1  21355 
1.20829 

10° 
11* 

1.17014 

1.17332 

1.17650 

1.17168 

1.18286,1.18603 

1.18921 

1.192391.19557 

1.19875 

1.20193 

1.20511 

1.16493 

1.16810  1.17127  1.17444!1.  17760 

1.18077 

1.18394 

1.18710 

1.19027 

1.19344 

1.19660 

1.19977 

1.20294 

12° 

1.15979 

1.16295 

1.16611  1.16926 

1.17242 

1.17558 

1.17873 

1.18189 

1.18505 

1.18820 

1.19136 

1.19452 

1.19768 

13° 

1.15459 

1.15774 

1.160881.16403 

1.16718 

1.17032 

1.17347 

1.17661 

1.17976 

1.18291 

1.18605 

1.18920 

1.19234 

14° 

1.14933 

1.15247 

1.155601.15873 

1.16187 

1.16500 

1.16814 

1.17127 

1.17440 

1.17754 

1.18067 

1.18381 

1.18694 

15° 

1.14407 

1.14720 

1.15032 

1.15344 

1.15657 

1.15969 

1.16282 

1.16594 

1.16906 

1.17219 

1.17531 

1.17844 

1.18156 

16° 

1.13873 

1.14185 

1.14496 

1.14807 

1.15118 

1.15429 

1.15741 

1.16052 

1.16363 

1.16674 

1.16985 

1.17297 

1.17608 

17° 

1.13335 

1.13645 

1.13955 

1.14266 

1.14576 

1.14886 

1.15196 

1.15506 

1.15816 

1.16126 

1.16436 

1.16746 

1.17056 

18° 

1.12794 

1.13103 

1.13412 

1.13721  1.14030 

1.14340 

1.14649 

1.14958 

1.15267 

1.15576 

1.15886 

1.16195 

1.16504 

19° 

1.12251 

112559 

1.12867 

1.13175 

1.13483 

1.13791 

1.14099 

1.14408 

1.14716 

1.15024 

1.15332 

1.15640 

1.15948 

20° 

1.11700 

1.12007 

1.12314 

1.12621 

1.12928 

1.13236 

1.13543 

1.13850 

1.14157 

1.14464 

1.14771 

1.15078 

1.15385 

21° 

1.11145 
1.10581 

1.11451 

1.10886 

1.11757  1.12063 
1.11191:1.11496 

1.12369 
1.11801 

1.12675 
1.12106 

1.12982 
1,12411 

1.13288 
1.12716 

1.13594 
1.13021 

1.13900 
1.13326 

1.14206 
1.13631 

1.14512 
1.13936 

1.14818 
1.14241 

22° 
23* 

1.10012 

1.10316 

1.106201.10924 

1.11228 

1.11532 

1.11835 

1.12139 

1.12443 

1.12747 

1.13051 

1.13355 

1.13659 

24° 

1.09437 

1.09740 

1.10043 

1.10346  1.10649 

1.10952 

1.11255 

1.11558 

1.11861 

1.12164 

1.12467 

1.12770 

1.13073 

25° 

746 

748 

750 

752 

754 

756 

758 

760 

762 

764 

766 

768 

770 

MILLIMETRES. 


262  DETERMINATION.  [§101. 

If  the  quantity  of  lithium  present  is  relatively  very  small, 
the  larger  portion  of  the  potassa  or  soda  compounds  should  first  be 
removed  by  addition  of  absolute  alcohol  to  the  most  highly  con- 
centrated solution  of  the  salts  (chlorides,  bromides,  iodides,  or 
nitrates,  but  not  sulphates) ;  since  this,  by  lessening  the  amount  of 
water  required  to  effect  the  separation  of  the  lithium  phosphate 
from  the  soluble  salts,  will  prevent  loss  of  lithium  (W.  MAYER). * 

The  precipitated  normal  lithium  phosphate  has  the  formula 
2Li3PO4  +  H2O.  It  dissolves  in  2539  parts  of  pure,  and 
3920  parts  of  ammoniated  water ;  at  100°,  it  completely  loses  its 
water ;  if  pure,  it  does  not  cake  at  a  moderate  red  heat  (MAYER). 

The  objections  raised  by  RAMMXLSBEBaf  to  MAYER'S  method 
of  estimating  lithia  I  find  to  be  ungrounded.  According  to  my 
own  experience,  it  appears  that  the  filtrate  and  wa&'ji- water  must 
be  evaporated  in  a  platinum  dish  not  only  once,  but  at  least  twice 
— in  fact,  till  a  residue  is  obtained  which  is  completely  soluble  in 
dilute  ammonia.  Lithium  phosphate  may  be  dried  at  100°,  or 
ignited  according  to  §  53,  before  being  weighed.  In  the  latter 
case,  care  must  be  taken  to  free  the  filter  as  much  as  possible  from 
the  precipitate  before  proceeding  to  incinerate  it.  I  have  thus 
obtained,  £  instead  of  100  parts  lithium  carbonate,  by  drying  at 
100°,  99-84,  99-89,  100-41,— by  igniting  99*66  and  100-05.  The 
lithium  phosphate  obtained  was  free  from  sodium. 

Second  Group. 

BARIUM STRONTIUM CALCIUM MAGNESIUM. 

§  101. 

1.  BARIUM. 

••     a.  Solution. 

Caustic  baryta  is  soluble  in  water,  as  are  many  barium  salts. 
Barium  salts  which  are  insoluble  in  water  are,  with  almost  the 
single  exception  of  the  sulphate,  readily  dissolved  by  dilute  hydro- 
chloric acid.  The  solution  of  the  sulphate  is  effected  by  fusion 
with  sodium  carbonate,  &c.  (See  §  132.)  Barium  silico-fluoride 

*  Ann.  der  Chem.  u.  Pharm.,  xcvin,  193,  where  Mayer  has  also  demonstrated 
the  non-existence  of  a  sodium  lithium  phosphate  of  fixed  composition  (Berzelius), 
or  of  varying  composition  (Rammelsberg). 

\Pogg.  Annal.,  en,  443.  \Zeitschr.f.  analyt.  Chem.,  1,  42. 


§  101.]  BARIUM.  263 

may  be  readily  converted  into  barium  sulphate  by  heating  and 
evaporating  with  moderately  dilute  sulphuric  acid  in  a  platinum 
vessel.  It  may  also  be  readily  decomposed  by  fusing  it  with  po- 
tassium-sodium carbonate. 

~b.   Determination, 

Barium  is  weighed  either  as  sulphate  or  as  carbonate,  rarely  (in 
the  separation  from  strontia)  as  barium  silico-fluoride  or  barium 
chromote  (§  71).  Barium  oxide  or  hydroxide,  also  barium  car- 
bonate, may  also  be  determined  by  the  volumetric  (alkalimetric) 
method.  Comp.  §  223. 

We  may  convert  into 

1.  BARIUM  STJLPHATE. 
a.   By  Precipitation. 

All  barium  compounds  without  exception. 
I.  By  Evaporation. 

All  barium  salts  of  volatile  acids,  if  no  other  non-volatile  body 
is  present. 

2.  BARIUM  CARBONATE. 

a.  All  barium  salts  soluble  in  water. 

5.  Barium  salts  of  organic  acids. 

Barium  is  both  precipitated  and  weighed,  by  far  the  most  fre- 
quently as  sulphate,  the  more  so  as  this  is  the  form  in  which  it  is 
most  conveniently  separated  from  other  bases.  The  determination 
by  means  of  evaporation  (1,  5)  is,  in  cases  where  it  can  be  applied, 
and  where  we  are  not  obliged  to  evaporate  large  quantities  of  fluid, 
very  exact  and  convenient.  Barium  is  determined  as  carbonate  in 
the  wet  way,  when  from  any  reason  it  is  not  possible  or  not  desir- 
able to  precipitate  it  as  sulphate.  If  a  fluid  or  dry  substance  con- 
tains bodies  which  impede  the  precipitation  of  barium  as  sulphate 
or  carbonate  (alkali  citrates,  metaphosphoric  acid,  see  §  71,  a  and 
Z>),  such  bodies  must  of  course  be  got  rid  of,  before  proceeding  to 
precipitation.  The  precipitation  of  barium  as  a  silico-fluoride  or 
chromate  will  be  treated  of  under  the  separation  of  barium  from 
strontium,  §  154. 

1 .   Determination  as  Barium  Sulphate. 

a.  By  Precipitation. 

Heat  the  moderately  dilute  solution  of  barium,  which  must  not 
contain  too  much  free  acid  (and  must,  therefore,  if  necessary,  first 


264  DETERMINATION.  [§  102. 

be  freed  therefrom  by  evaporation  or  addition  of  sodium  carbon- 
ate), in  a  platinum  or  porcelain  dish,  or  in  a  glass  vessel,  to  incipi- 
ent ebullition,  add  dilute  sulphuric  acid  so  long  as  a  precipitate 
forms,  keep  the  mixture  for  some  time  at  a  temperature  very  hear 
the  boiling  point,  stirring  if  not  on  a  water- bath,  and  allow  the 
precipitate  to  subside ;  decant  the  almost  clear  supernatant  fluid  on 
a  filter,  boil  the  precipitate  once  with  water  and  a  little  dilute  sul- 
phuric acid,  then  three  or  four  times  with  water,  then  transfer  it 
to  the  filter,  and  wash  with  boiling  water,  until  the  filtrate  is  no 
longer  rendered  turbid  by  barium  chloride.  Dry  the  precipitate, 
and  treat  it  as  directed  in  §  53,  employing  only  a  moderate  heat. 
If  the  precipitate  has  been  properly  washed  in  the  manner 
here  directed,  it  is  perfectly  pure.  In  the  presence  of  alkali  salts, 
however,  the  precipitate  will  still  contain  small  quantities  of  alkali 
sulphate.  Comp.  §  153. 

J.   By  Evaporation. 

Add  to  the  solution,  in  a  weighed  platinum  dish,  pure  sul- 
phuric acid  very  slightly  in  excess,  and  evaporate  on  the  water- 
bath  ;  expel  the  excess  of  sulphuric  acid  by  cautious  application  of 
heat,  and  ignite  the  residue  moderately. 

For  the  properties  of  barium  sulphate,  see  §  71. 

Both  methods,  if  properly  and  carefully  executed,  give  almost 
absolutely  accurate  results. 

2.   Determination  as  Barium  Carbonate, 
a.   In  Solutions. 

Mix  the  moderately  dilute  solution  of  the  barium  salt  in  a 
beaker  with  ammonia,  add  ammonium  carbonate  in  slight  excess, 
and  let  the  mixture  stand  several  hours  in  a  warm  place.  Filter, 
wash  the  precipitate  with  water  mixed  with  a  little  ammonia,  dry? 
and  ignite  moderately  (§  53). 

For  the  properties  of  the  precipitate,  see  §  71.  This  method 
involves  a  trifling  loss  of  substance,  as  barium  carbonate  is  not 
absolutely  insoluble  in  water.  The  direct  experiment,  No.  56, 
gave  99*79  instead  of  100. 

If  the  solution  contains  a  notable  quantity  of  ammonium  salts, 
the  loss  incurred  is  much  more  considerable,  since  the  presence  of 
such  salts  greatly  increases  the  solubility  of  barium  carbonate. 


§102.]  STRONTIUM.  265 

b.  In  Barium  Salts  of  Organic  Acids. 

Heat  the  salt  slowly  in  a  covered  platinum  crucible,  until  no 
more  fumes  are  evolved ;  place  the  crucible  obliquely,  with  the 
lid  leaning  against  it,  and  ignite,  until  the  whole  of  the  carbon  is 
consumed,  and  the  residue  presents  a  perfectly  white  appear- 
ance ;  moisten  the  residue  with  a  concentrated  solution  of  ammo- 
nium carbonate,  evaporate,  ignite  gently,  and  weigh.  The  results 
obtained  by  this  method  are  quite  satisfactory.  A  direct  experi- 
ment, No.  57,  gave  99.61  instead  of  100.  The  loss  of  substance 
which  almost  invariably  attends  this  method  is  owing  to  particles 
'of  the  salt  being  carried  away  with  the  fumes  evolved  upon  igni- 
tion, and  is  accordingly  the  less  considerable,  the  more  slowly  and* 
gradually  the  heat  is  increased.  Omission  of  the  moistening  of 
the  residue  with  ammonium  carbonate  would  involve  a  further  loss- 
of  substance,  as  the  ignition  of  barium  carbonate  in  contact  with 
carbon  is  attended  with  formation  of  some  caustic  baryta,  carbon, 
monoxide  gas  being  evolved. 

§102. 

2.  STRONTIUM. 

a.  Solution. 

See  the  preceding  paragraph  (§  101,  a. — Solution  of  baryta  and! 
barium  salts) ;   the  directions   there   given   apply  equally  here. 
Strontium  silico-fluoride  is  readily  and  completely  soluble  in  water- 
acidulated  with  hydrochloric  acid. 

J.  Determination. 

Strontium  is  weighed  either  as  strontium  sulphate  or  as  stron- 
tium carbonate  (§  72).  Strontium  in  the  form  of  oxide,  hydrox- 
ide, or  carbonate,  may  be  determined  also  by  the  volumetric-- 
^alkalimetric)  method.  Comp.  §  223. 

We  may  convert  into 

1.  STRONTIUM  SULPHATE. 

a.  By  Precipitation. 

All  compounds  of  strontium  without  exception. 

b.  By  Evaporation. 

All  strontium  salts  of  volatile  acids,  if  no  other  non-volatile 
body  is  present. 


266  DETERMINATION.  [§  102. 

2.  STRONTIUM  CARBONATE. 

a.  All  strontium  compounds  soluble  in  water. 

/?.  Strontium  salts  of  organic  acids. 

The  method  based  on  the  precipitation  of  strontium  with  sul- 
phuric acid  yields  accurate  results  only  in  cases  where  the  fluid 
from  which  the  strontium  is  to  be  precipitated  may  be  mixed, 
without  injury,  with  alcohol.  Where  this  cannot  be  done,  and 
where  the  method  based  on  the  evaporation  of  the  solution  of 
strontium  with  sulphuric  acid  is  equally  inapplicable,  the  conver- 
sion into  the  carbonate  ought  to  be  resorted  to  in  preference,  if 
admissible.  As  in  the  case  of  barium,  so  here,  we  have  to  be  on- 
our  guard  against  the  presence  of  substances  which  would  impede 
precipitation  (citrates  of  the  alkalies,  metaphosphoric  acid,  etc.), 
and  if  necessary,  these  must  first  be  removed. 

1.  Determination  as  Strontium  Sulphate. 

a.  By  Precipitation. 

Mix  the  solution  of  the  strontium  salt  (which  must  not  be  too 
dilute,  nor  contain  much  free  hydrochloric  or  nitjic  acid)  with 
dilute  sulphuric  acid  in  excess,  in  a  beaker,  and  add  at  least  an 
equal  volume  of  alcohol;  let  the  mixture  stand  twelve  hours, 
and  filter ;  wash  the  precipitate  with  dilute  alcohol,  dry  and  ignite 
(§  53). 

If  the  circumstances  of  the  case  prevent  the  use  of  alcohol,  the 
fluid  must  be  precipitated  in  a  tolerably  concentrated  state,  and  a 
fairly  large  excess  of  sulphuric  acid  added  (this  is  particularly  neces- 
sary if  large  quantities  of  potassium  chloride,  sodium  chloride,  or 
magnesium  chloride  are  present).  The  mixture  is  then  allowed 
to  stand  in  the  cold  for  at  least  twenty-four  hours,  filtered,  and  the 
precipitate  washed  with  cold  water,  until  the  last  rinsings  manifest 
no  longer  an  acid  reaction,  and  leave  no  perceptible  residue  upon 
evaporation.  If  traces  of  free  sulphuric,  acid  remain  adhering  to 
the  filter,  the  latter  turns  black  on  drying,  and  crumbles  to  pieces; 
too  protracted  washing  of  the  precipitate,  on  the  other  hand,  tends 
to  increase  the  loss  of  substance. 

Care  must  be  taken  that  the  precipitate  be  thoroughly  dry, 
before  proceeding  to  ignite  it ;  otherwise  it  will  be  apt  to  throw 
off  fine  particles  during  the  latter  process.  The  filter,  which  is  to 
be  burnt  apart  from  the  precipitate,  must  be  as  clean  as  possible, 
or  some  loss  of  substance  will  be  incurred ;  as  may  be  clearly  seen 


§  102.]  STRONTIUM.  267 

from  the  depth  of  the  carmine  tint  of  the  flame  with  which  the 
filter  burns  if  the  precipitate  has  not  been  properly  removed. 

For  the  properties  of  the  precipitate,  see  §  72.  When  alcohol 
is  used  and  the  directions  given  are  properly  adhered  to,  the  results 
are  very  accurate ;  when  the  sulphate  of  strontium  is  precipitated 
from  an  aqueous  solution,  on  the  contrary,  a  certain  amount  of  loss 
is  unavoidable,  as  strontium  sulphate  is  not  absolutely  insoluble  in 
water.  The  direct  experiments,  No.  58,  gave  only  98'12  and  98*02 
instead  of  100.  However,  the  error  may  be  rectified,  by  calculat- 
ing the  amount  of  strontium  sulphate  dissolved  in  the  filtrate  and 
the  wash-water,  basing  the  calculation  upon  the  known  degree  of 
solubility  of  strontium  sulphate  in  pure  and  acidified  water.  See 
Expt.  No.  59,  which,  with  this  correction,  gave  99'77  instead  of 
100.  The  necessity  for  making  the  correction  may  be  obviated  by 
washing  with  1  part  sulphuric  acid  mixed  with  20  parts  water  till 
all  substances  precipitable  by  alcohol  are  removed,  then  with  alco- 
hol till  all  the  sulphuric  acid  is  removed.  Strontium  sulphate  also 
carries  down  sulphates  of  other  strong  bases  in  small  quantities, 
and  this  point  must  not  be  overlooked  in  making  accurate  analy- 
ses. See  §  153. 

1}.  By  Evaporation. 

The  same  method  as  described  for  barium,  §  101,  1,  5. 

2.  Determination  as  Strontium  Carbonate. 

a.  In  Solutions. 

The  same  method  as  described  §  101,  2,  a.  For  the  proper- 
ties of  the  precipitate,  see  §  72.  The  method  gives  very  accurate 
results,  as  strontium  carbonate  is  nearly  absolutely  insoluble  in 
water  containing  ammonia  and  ammonium  carbonate.  A  direct 
experiment,  No.  60,  gave  99*82  instead  of  100.  Presence  of 
ammonium  salts  exercises  here  a  less  adverse  influence  than  the 
precipitation  of  barium  carbonate. 

b.  In  Salts  of  Organic  Acids. 

The  same  method  as  described  §  101,  2,  b.  The  remarks  made? 
there,  respecting  the  accuracy  of  the  results,  apply  equally  here. 


268  DETERMINATION.  [§  103v 

§103. 

3.  CALCIUM. 

a.  Solution. 

See  §  101,  a. — Solution  of  barium.  Calcium  fluoride  is,  by 
means  of  sulphuric  acid,  converted  into  calcium  sulphate,  and  the 
latter  again,  if  necessary,  decomposed  by  boiling  or  fusing  with 
an  alkali  carbonate  (§  132).  [Calcium  sulphate  dissolves  readily 
in  moderately  dilute  hydrochloric  acid.  It  is  much  less  soluble  in 
strong  hydrochloric  acid.] 

b.  Determination. 

Calcium  is  weighed  either  as  calcium  sulphate,  as  calcium 
carbonate,  or  calcium  oxide  (§  73).  It  may  be  converted  into 
sulphate  by  evaporation,  as  well  as  by  precipitation ;  and  into 
carbonate  or  oxide  by  precipitation  as  oxalate  or  carbonate,  or  by 
ignition.  Calcium,  in  the  form  of  oxide,  hydroxide,  or  carbon- 
ate, may  be  determined  also  by  the  volumetric  (alkalimetric) 
method.  Comp.  §  223.  The  volumetric  estimation  may  be  also 
effected  by  precipitating  the  calcium  as  oxalate,  either  by  a  direct 
or  an  indirect  method. 

We  may  convert  into 

1.  CALCIUM  SULPHATE. 

a.  By  Precipitation. 

All  calcium  salts  of  acids  soluble  in  alcohol,  provided  no  other 
substance  insoluble  in  alcohol  be  present. 

b.  By  Evaporation. 

All  calcium  salts  of  volatile  acids,  provided  no  non-volatile  body 
be  present. 

2.   CALCIUM  CARBON  ATP:,  OR  CALCIUM  OXIDE. 
a.   By  Precipitation  with  Ammonium  Carbonate. 
All  calcium  salts  soluble  in  water. 

b.  By  Precipitation  with  Ammonium  Oxalate. 

All  calcium  salts  soluble  in  water  or  in  hydrochloric  acid  with- 
out exception. 

c.  By  Ignition. 

Calcium  salts  of  organic  acids. 

Of  these  several  methods,  2,  b  (precipitation  with  ammonium 


§  103.]  CALCIUM.  269 

oxalate)  is  the  one  most  frequently  resorted  to.  This,  and  the 
method  1,  £>,  give  the  most  accurate  results.  The  method,  1,  #,  is 
usually  resorted  to  only  to  effect  the  separation  of  calcium  from 
other  basic  radicals ;  2,  a,  generally  only  to  effect  the  separation 
of  calcium  together  with  the  other  alkali-earth  metals  from  the 
alkalies.  As  many  bodies  (alkali  citrates,  and  metaphosphates) 
interfere  with  the  precipitation  of  calcium  by  the  precipitants 
given,  these,  if  present,  must  be  first  removed. 

3.  The  volumetric  methods  of  estimation,  which  are 
particularly  to  be  recommended  when  a  large  number  of  deter- 
minations are  to  be  made,  will  be  described  after  the  gravimetric 
methods. 

1.  Determination  of  Calcium  Sulphate. 

a.   13 y  Precipitation. 

Mix  the  solution  of  the  calcium  salt  in  a  beaker,  with  dilute  sul- 
phuric acid  in  excess,  and  add  three  or  four  times  the  volume  of 
alcohol ;  let  the  mixture  stand  twelve  hours,  filter,  and  thoroughly 
wash  the  precipitate  with  alcohol,  dry,  and  ignite  moderately 
(§  53).  For  the  properties  of  the  precipitate,  see  §  73.  The  re- 
sults are  very  accurate.  A  direct  experiment,  No.  61,  gave 
99-64  instead  of  100. 

J).   By  Evaporation. 

The  same  method  as  described  under  barium,  §  101,  1,  5. 

2.  Determination   as    Calcium    Carbonate   or    Calcium 
Oxide. 

a.  By  Precipitation  with  Ammonium  Carbonate. 

The  same  method  as  described  §  101,  2,  a.  The  precipitate 
can  be  most  conveniently  weighed  as  calcium  carbonate.  It  must 
be  exposed  only  to  a  very  gentle  red  heat,  but  this  must  be  con- 
tinued for  some  time.  For  the  properties  of  the  precipitate,  see 
§73. 

This  method  gives  very  accurate  results,  the  loss  of  substance 
incurred  being  hardly  worth  mentioning. 

If  the  solution  contains  ammonium  chloride  or  similar  ammo- 
nium salts  in  considerable  proportion,  the  loss  of  substance  in- 
curred is  far  greater.  The  same  is  the  case  if  the  precipitate  is 
washed  with  pure  instead  of  ammoniacal  water.  A  direct  experi- 


270  DETERMINATION.  [§  103. 

ment,  No.  62,  in  which  pure  water  was  used,  gave  99'17  instead 
of  100  parts  of  lime. 

If  it  is  feared  that  some  calcium  oxide  has  formed,  from  the 
heat  being  too  high,  the  residue  is  moistened  with  a  little  water, 
a  small  piece  of  ammonium  carbonate  added,  the  whole  slowly 
evaporated,  and  heated  again  to  gentle  redness,  i.e.,  till  the  bot- 
tom of  the  crucible  is  just  dull-red.  If  a  gas  blowpipe  is  at 
hand,  the  calcium  carbonate  may  be  converted  into  oxide  by  pro- 
longed, strong  ignition,  and  then  weighed  as  such.  Comp.  I  *. 

b.   B-y  Precipitation  with  Ammonium  Oxalate. 
a.    The  Calcium  Salt  is  soluble  in  Water. 

To  the  hot  solution  in  a  beaker  add  ammonium  oxalate  in 
moderate  excess,  and  then  ammonia  sufficient  to  impart  an  ammo- 
niacal  smell  to  the  fluid ;  cover  the  glass,  and  let  it  stand  in  a 
warm  place  until  the  precipitate  has  completely  subsided,  which 
will  require  twelve  hours,  at  least.  Pour  the  clear  fluid  gently 
and  cautiously,  so  as  to  leave  the  precipitate  undisturbed,  on  a 
filter ;  wash  the  precipitate  two  or  three  times  by  decantation  with 
hot  water;  lastly,  transfer  the  precipitate  also  to  the  filter,  by 
rinsing  with  hot  water,  taking  care,  before  the  addition  of  a  fresh 
portion,  to  wait  until  the  fluid  has  completely  passed  through  the 
filter,  f  Small  particles  of  the  precipitate,  adhering  firmly  to  the 
glass,  are  removed  with  a  feather.  If  this  fails  to  effect  their 
complete  removal,  they  should  be  dissolved  in  a  few  drops  of 
highly  dilute  hydrochloric  acid,  ammonia  added  to  the  solution, 
and  the  oxalate  obtained  added  to  the  first  precipitate.  Devia- 
tions from  the  rules  laid  down  here  will  generally  give  rise  to  the 
passing  of  a  turbid  fluid  through  the  filter.  After  having  washed 
the  precipitate,  dry  it  on  the  filter  in  the  funnel,  and  transfer  the 
dry  precipitate  to  a  platinum  crucible,  taking  care  to  remove  it  as 

*  In  converting  precipitated  calcium  carbonate  into  calcium  oxide,  FRITZ- 
SCHE  (Zeitschr.f.  analyl.  Chem.,  in,  179),  and  A.  COSSA  (ib.t  vm,  141)  obtained 
somewhat  too  little  (99  47  instead  of  100).  This,  I  think,  may  be  because  the 
calcium'  carbonate  employed  by  FRITZSCIIE  (which  he  dried  at  160°),  was  not 
anhydrous,  and  which  he  himself  hints. 

t  In  order  to  make  the  calcium  oxalate  settle  more  rapidly  and  filter  it  off 
clearly,  MUCK  recommends  adding  1  c.  c.  of  ammonia-alum  solution  containing 
O'OOl  grm.  alumina.  An  excess  of  ammonia  is  to  be  avoided,  and  0*001  grm. 
must  be  deducted  from  the  weight  of  the  calcium  oxide.  (Zeitschr.  /.  analyt. 
Chem.,  ix,  451.) 


§  103.]  CALCIUM.  271 

completely  as  possible  from  the  filter ;  burn  the  filter  on  a  piece 
of  platinum  wire,  letting  the  ash  drop  into  the  hollow  of  the  lid 
(§  53) ;  put  the  latter,  now  inverted,  on  the  crucible,  so  that  the 
filter  ash  may  not  mix  with  the  precipitate ;  heat  at  first  very 
gently,  then  more  strongly,  until  the  bottom  of  the  crucible  is 
heated  to  very  faint  redness.  Keep  it  at  that  temperature  from 
ten  to  fifteen  minutes,  removing  the  lid  from  time  to  time.  I 
am  accustomed  during  this  operation  to  move  the  lamp  back- 
wards and  forwards  under  the  crucible  with  the  hand,  since,  if 
you  allow  it  to  stand,  the  heat  may  very  easily  get  too  high. 
Finally  allow  to  cool  in  the  desiccator  and  weigh.  After  weigh- 
ing, moisten  the  contents  of  the  crucible,  which  must  be  perfectly 
white,  or  barely  show  the  least  tinge  of  gray,  with  a  little  water, 
and  test  this  after  a  time  with  a  minute  slip  of  turmeric  paper. 
Should  the  paper  turn  brown — a  sign  that  the  heat  applied  was 
too  strong — rinse  oft'  the  fluid  adhering  to  the  paper. with  a  little 
water  into  the  crucible,  throw  in  a  small  lump  of  pure  ammo- 
nium carbonate,  evaporate  to  dryness  (best  in  the  water-bath), 
heat  to  very  faint  redness,  and  weigh  the  residue.  If  the  weight 
has  increased,  repeat  the  same  operation  until  the  weight  remains 
constant.  I  wish  to  particularly  point  out  that  by  closely  adher- 
ing to  the  rules  laid  down  above  regarding  the  method  of  ignition, 
the  tedious  evaporation  with  ammonium  carbonate  may  be  al- 
ways avoided.  For  the  properties  of  the  precipitate  and  residue 
see  §  73.  This  method  gives  nearly  absolutely  accurate  results. 
A  direct  experiment,  No.  63,  gave  99 -99  instead  of  100. 
Equally  accurate  results  were  recently  obtained  also  by  A.  Sou- 
CHAY  in  my  laboratory.* 

If  a  gas  blowpipe  is  at  hand,  or  any  other  arrangement  by 
means  of  which  a  platinum  crucible  may  be  raised  to  a  white 
heat,  the  calcium  oxalate  may  be  converted  into  CAUSTIC  LIME 
with  results  almost  equally  accurate;  and  I  believe  that  this 
method,  which  requires  less  patience  than  the  other,  is  more  certain 
to  yield  good  results  in  the  hands  of  many  persons.  The  calcium 
oxalate  and  the  filter  ash  are  transferred  to  a  moderate-sized 
platinum  crucible,  which  is  ignited  first  over  the  BUNSEN  flame, 
and  then  over  the  blowpipe.  The  crucible  is  then  weighed,  and 

*Zeitschr.f.  analyt.  Chern.,  x,  323. 


272  DETERMINATION.  [§  103. 

ignited  again  over  the  blowpipe.  The  second  ignition  over  the 
blowpipe  should  not  reduce  the  weight.  The  duration  of  the 
ignition  necessary  varies  from  5  to  15  or  more  minutes,  accord- 
ing to  intensity  of  heat  and  quantity  of  the  precipitate.  It  is  well 
to  weigh  the  empty  crucible  again  at  the  end  of  the  operation,  as 
platinum  sometimes  loses  weight  after  violent  and  prolonged  igni- 
tion. The  results  obtained  by  FKITZSCHE,  COSSA,"*  and  SOTJCHAY 
scarcely  differ  from  the  calculated  numbers.  For  properties  of 
calcium  oxide,  see  §  73. 

The  calcium  oxalate  may  also  be  converted  into  sulphate. 
SCHKOTTER  ignites  in  a  covered  platinum  crucible  with  pure 
ammonium  sulphate.  Or  you  may  ignite  in  a  covered  platinum 
dish  till  the  precipitate  is  for  the  most  part  converted  into  oxide, 
add  a  little  water,  then  hydrochloric  acid  to  effect  solution,  then 
pure  sulphuric  acid  in  excess,  evaporate  and  ignite  moderately. 
This  process  is  also  quite  accurate. 

Some  chemists  collect  the  calcium  oxalate  on  a  weighed  filter, 
dry  at  100°,  and  weigh.     The  composition  of  the  precipitate  so 
obtained  is  CaC2O<  -f-  HaO.      This  method,  however,  is  more 
troublesome,  as  well  as  less  accurate,  than  the  first. 
/?.    The  Salt  is  insoluble  in  Water. 

Dissolve  the  salt  in  dilute  hydrochloric  acid.  If  the  acid  of 
the  calcium  salt  is  of  a  nature  to  escape  in  this  operation  (e.g., 
carbonic  acid),  or  to  admit  of  its  separation  by  evaporation  (e.g., 
silicic  acid),  proceed,  after  the  removal  of  the  acid,  as  directed 
in  ex.  But  if  the  acid  cannot  thus  be  readily  got  rid  of  (e.g., 
phosphoric  acid),  proceed  as  follows :  Add  ammonia  until  a  pre- 
cipitate begins  to  form,  re-dissolve  this  with  a  drop  of  hydro- 
chloric acid,  add  ammonium  oxalate  in  excess,  and  finally  sodium 
acetate ;  allow  the  precipitate  to  subside,  and  proceed  for  the  re- 
mainder of  the  operation  as  directed  in  ct.  In  this  process  the 
free  hydrochloric  acid  present  reacts  on  the  sodium  acetate  and 
ammonium  oxalate,  fprming  sodium  and  ammonium  chlorides, 
with  liberation  of  a  corresponding  amount  of  oxalic  and  acetic 
acids  in  which  calcium  oxalate  is  nearly  insoluble.  In  this 
method  the  loss  is  very  slight.  The  method  yields  accurate  re- 
sults. A  direct  experiment,  No.  64,  gave  99*78  instead  of  100. 

*FRITZSCHE  (Zeitschr.  f.  analyt.  Chem.,  in,  179)  and  A.  COSSA  (lb.,  vm,  141). 


§  103.]  CALCIUM.  273 

c.   By  l(jii'dlon. 

The  same  method  as  described  §  101,  2,  5  (barium).  The 
residue  remaining  upon  evaporation  with  ammonium  carbonate 
(which  operation  it  is  advisable  to  perform  twice)  must  be  ignited 
very  gently.  The  remarks  made  in  §  101,  2,  J,  in  reference  to 
the  accuracy  of  the  results,  apply  equally  here.  By  way  of  con- 
trol, the  calcium  carbonate  may  be  converted  into  oxide  or  into 
calcium  sulphate  (see  &,  ar),  or  it  may  be  determined  alkalimetri- 
cally  (§  223). 

3.   Volumetric  Methods. 

a.  Regarding    the    alkalimetric   determination   of  calcium 
oxide  or  carbonate,   see  §  223.     By  properly  carrying  out  the 
process,  a  mixture  of  calcium  oxide  and  carbonate,  obtained  by 
moderately  igniting  calcium  oxalate  in  the  air,  affords  very  good 
results  (Expt.  No.  65). 

b.  Precipitation  as  calcium  oxalate,  and  direct  estimation  of 
the  oxalic  acid  in  the  precipitate.     The  oxalic  acid  in  the  well- 
washed,  but  not  dried,  calcium  oxalate,  is  determined  by  means 
of   potassium    permanganate    (§   137).      Results  are   very    good 
(Expt.  No.  65). 

c.  Precipitation  of  calcium  oxalate  and  indirect  estimation 
of  the  oxalic  acid  in  the  precipitate.      In  this  method  (KRAUT*) 
the  calcium  salt  must  be  soluble  in  water.     Add  to  the  solutio'n 
of  the  calcium  salt,  contained  in  a  measuring-flask,  an  exactly 
measured   quantity   of  decinormal   oxalic-acid   solution  (§  215), 
more  than  sufficient  to  precipitate   the  calcium,   add  ammonia 
until  the  liquid  is  alkaline,  heat  to  boiling,  cool,  fill  the  flask  up 
to  the  mark,  and  shake.     Then  filter  through  a  dry  filter,  meas- 
ure off  an  aliquot  part  of  the  filtrate  (at  least  one-half),  determine 
in  it  the  oxalic  acid  by  potassium  permanganate,  as  in  §  137,  and 
calculate  the  quantity  for  the  whole  of  the  filtrate ;    the  quantity 
of  oxalic  acid  which  lias  been  used  up  to  combine  with  the  cal- 
cium, gives  the  quantity  of  the  latter  present.     1  c.  c.  of  deci- 
normal oxalic-acid  solution  =  0*0028  grm.  lime.     The  method 
is  rapid,  and  gives  accurate  results.     If  the  quantity  of  calcium 
is  small  in  comparison  with  the  volume  of  the  liquid,  no  correc- 
tion will  be  necessary  for  the  space  the  calcium  oxalate  occupies 
in  the  measuring  flask. 

*Chem  Centrattl.  1856,  316.  ~ 


274  DETERMINATION.  [§  104. 

§104. 

4.  MAGNESIUM. 

a.  Solution. 

Many  magnesium  salts  are  soluble  in  water ;  those  which  are 
insoluble  in  that  menstruum  dissolve  in  hydrochloric  acid,  with  the 
exception  of  some  silicates  and  aluminates  (see  §§  105  and  140). 

b.  Determination. 

Magnesium  is  weighed  (§  74)  either  as  sulphate  or  as  pyro- 
phosphate,  or  as  magnesium  oxide.  In  the  form  of  oxide  or  car- 
bonate, it  may  be  determined  also  by  the  alkalimetric  method 
described  in  §  223. 

We  may  convert  into 

V 

1.  MAGNESIUM  SULPHATE. 

a.  Directly.  b.  Indirectly. 

All  magnesium  salts  of  vola-          All  magnesium  salts  soluble 

tile    acids,   provided    no   other    in  water,  and  also  those  which, 

non-volatile  substance   be  pres-    insoluble    in    that    menstruum, 

ent.  dissolve    in    hydrochloric    acid, 

with  separation  of  their  acid 
(provided  no  ammonium  salts 
be  present). 

2.  MAGNESIUM  PYROPHOSPHATE. 

All  magnesium  compounds  without  exception. 

3.  MAGNESIUM  OXIDE. 

a.  Magnesium  salts  of  organic  acids,  or  of  readily  volatile  inor- 
ganic oxygen  acids. 

b.  Magnesium  chloride,  and  magnesium  compounds  convertible 
into  that  salt. 

The  direct  determination  as  magnesium  sulphate  is  highly 
recommended  in  all  cases  where  it  is  applicable.  The  indirect  con- 
version into  the  sulphate  serves  only  in  the  case  of  certain  separa- 
tions, and  is  hardly  ever  had  recourse  to  where  it  can  possibly  be 
avoided.  The  determination  as  pyrophosphate  is  most  generally 
resorted  to ;  especially  also  in  the  separation  of  magnesium  from 
other  bases.  The  method  based  on  the  conversion  of  magnesium 
chloride  into  oxide  is  usually  resorted  to  only  to  effect  the  separa- 


§  104.]  MAGNESIUM.  275 

tion  of  magnesium  from  the  alkali  metals.    Magnesium  phosphates 
are  analyzed  as  §  135  directs. 

1.  Determination  as  Magnesium  Sulphate. 

Add  to  the  solution  excess  of  pure  dilute  sulphuric  acid,  evapo- 
rate to  dryness,  in  a  weighed  platinum  dish,  on  the  water-bath;, 
then  heat  at  first  cautiously,  afterwards,  with  the  cover  on  more 
strongly — here  it  is  advisable  to  place  the  lamp  so  that  the  flame 
may  play  obliquely  on  the  cover  from  above — until  the  excess  of 
sulphuric  acid  is  completely  expelled ;  lastly,  ignite  gently  over 
the  lamp  for  some  time;  allow  to  cool,  and  weigh.  Should  no 
fumes  of  hydrated  sulphuric  acid  escape  upon  the  application  of  a 
strongish.  heat,  this  may  be  looked  upon  as  a  sure  sign  that  the 
sulphuric  acid  has  not  been  added  in  sufficient  quantity,  in  which 
case,  after  allowing  to  cool,  a  fresh  portion  of  sulphuric  acid  is 
added.  The  method  yields  very  accurate  results.  Care  must  be 
taken  not  to  use  a  very  large  excess  of  sulphuric  acid.  The  resi- 
due must  be  exposed  to  a  moderate  red  heat  only,  and  weighed 
rapidly.  For  the  properties  of  the  residue,  see  §  74. 

2.  Determination  as  Magnesium  Pyrophosphate. 

The  solution  of  the  magnesium  salt  is  mixed,  in  a  beaker,  with 
ammonium  chloride,  and  ammonia  added  in  slight  excess.  Should 
a  precipitate  form  upon  the  addition  of  ammonia,  this  may  be  con- 
sidered a  sign  that  a  sufficient  amount  of  ammonium  chloride  has 
not  been  used  ;  a  fresh  amount  of  that  salt  must  consequently  be 
added,  sufficient  to  effect  the  re-solution  of  the  precipitate  formed. 
The  clear  fluid  is  then  mixed  with  a  solution  of  sodium  phosphate 
or  sodium  ammonium  phosphate*  in  excess,  and  the  mixture  stirred,, 
taking  care  to  avoid  touching  the  sides  of  the  beaker  with  the  stir- 
ring-rod ;  otherwise  particles  of  the  precipitate  are  apt  to  adhere 
so  firmly  to  the  rubbed  parts  of  the  beaker,  that  it  will  be  found 
difficult  to  remove  them  ;  the  beaker  is  then  covered,  and  allowed 
to  stand  at  rest  for  twelve  hours,  without  warming ;  after  that  time 
the  fluid  is  filtered,  and  the  precipitate  collected  on  the  filter,  the 
last  particles  of  it  being  rinsed  out  of  the  glass  with  a  portion  of 
the  filtrate,  with  the  aid  of  a  feather;  when  the  fluid  has  completely 
passed  through,  the  precipitate  is  washed  with  a  mixture  of  3  parts 
of  water,  and  1  part  of  solution  of  ammonia  of  O90  sp.  gr.,  the 

*  According  to  MOHR  (NaNH4H)PO4  is  preferable  to  (NaaH)PO4  as  a  pre- 
cipitant. (See  Zeitschr.f.  analyt.  Chem.,  xn,  36.) 


276  DETERMINATION.  [§  104. 

operation  being  continued  until  a  few  drops  of  the  fluid  passing 
through  the  filter  mixed  with  nitric  acid  and  a  drop  of  silver  nitrate 
show  only  a  very  slight  opalescence. 

The  precipitate  is  now  thoroughly  dried,  and  then  transferred 
to  a  platinum  crucible  (§  53) ;  the  latter,  with  the  lid  on,  is  exposed 
for  some  time  to  a  very  gentle  heat,  which  is  finally  increased  to 
intense  redness.  The  filter,  as  clean  as  practicable,  is  incinerated 
in  a  spiral  of  platinum  wire,  and  the  ash  transferred  to  the  cru- 
cible, which  is  then  once  more  exposed  to  a  red  heat,  allowed  to 
cool,  and  weighed.  If  the  magnesium  pyrophosphate  is  not  per- 
fectly white,  moisten  with  a  few  drops  of  nitric  acid,  and  ignite 
again,  applying  the  heat  at  first  carefully. 

For  the  properties  of  the  precipitate  and  residue,  see  §  74. 

This  method,  if  properly  executed,  yields  most  accurate  results. 
The  precipitate  must  be  washed  completely,  but  not  over-washed, 
and  the  washing  water  must  always  contain  the  requisite  quantity 
of  ammonia. 

3.  Determination  as  Magnesium  Oxide. 

a.  hi   Magnesium  Salts  of  Organic  or    Volatile  Inorganic 
Acids. 

The  magnesium  salt  is  gently  heated  in  a  covered  platinum 
crucible,  increasing  the  temperature  gradually,  until  no  more  fumes 
escape ;  the  lid  is  then  removed,  and  the  crucible  placed  in  an 
oblique  position,  with  the  lid  leaning  against  it.  A  red  heat  is 
now  applied,  until  the  residue  is  perfectly  white.  For  the  prop- 
erties of  the  residue,  see  §  74.  The  method  gives  the  more  accu- 
rate results  the  more  slowly  the  salt  is  heated  from  the  beginning. 
Some  loss  of  substance  is  usually  sustained,  owing  to  traces  of  the 
salt  being  carried  oif  with  the  empyreumatic  products.  Mag- 
nesium salts  of  readily  volatile  oxygen  acids  (carbonic  acid,  nitric 
acid),  may  be  transformed  into  magnesium  oxide  in  a  similar  way,  by 
simple  ignition.  Even  magnesium  sulphate  loses  the  whole  of  its 
sulphuric  acid  when  exposed,  in  a  platinum  crucible,  to  the  heat 
of  the  gas  blowpipe-flame  (SONNENSCHEIN).  As  regards  small  quan- 
tities of  magnesium  sulphate,  I  can  fully  confirm  this  statement. 

b.  Conversion  of  Magnesium  Chloride  into  Magnesium  Oxide. 
To  the  concentrated  solution,  in  a  porcelain  crucible,  add  a 

mixture  of  water  and  pure  mercuric  oxide  in  more  than  sufficient 
quantity  to  enable  the  oxygen  of  the  oxide  to  convert  all  the 
magnesium  chloride  present  into  magnesium  oxide,  evaporate  the 


§  105.]  ALUMINIUM.  277 

mixture  on  a  water-bath,  dry  thoroughly,  cover  the  crucible,  and 
heat  to  redness  until  all  the  mercuric  chloride  formed  and  also 
the  excess  of  mercuric  oxide  have  been  expelled.  (The  operator 
should  carefully  guard  against  inhaling  any  of  the  vapors 
evolved.)  The  residue,  magnesium  oxide,  may  be  weighed  in  the 
crucible,  or,  if  its  separation  from  alkalies  is  intended,  it  is  col- 
lected on  a  filter,  washed  with  hot  water,  dried  and  ignited  (§53). 
Regarding  other  methods  whereby  the  object  intended  may  be 
attained,  and  which  are  frequently  more  convenient  for  effecting 
separations,  see  §  153,  B,  4  (17  to  21). 

THIRD  GROUP  OF  BASIC  RADICALS. 

ALUMINIUM GHROMIUM (TITANIUM). 

§  105. 
1.   ALUMINIUM. 

a.  Solution. 

Aluminium  compounds  which  are  insoluble  in  water  dissolve, 
for  the  most  part,  in  hydrochloric  acid.  Native  crystallized  alu- 
minium oxide  (sapphire,  ruby,  corundum,  &c.),  and  many  native 
aluminium  compounds,  and  also  artificially  produced  aluminium 
oxide  after  intense  ignition,  require  fusing  with  sodium  carbon- 
ate, caustic  potassa,  or  barium  hydroxide,  as  a  preliminary  step 
to  their  solution  in  hydrochloric  acid.  Many  aluminium  com- 
pounds (e.g.,  common  clay)  which  resist  the  action  of  concen- 
trated hydrochloric  acid,  may  be  decomposed  by  protracted  heat- 
ing with  moderately  concentrated  sulphuric  acid,  or  by  fusion 
with  sodium  disulphate ;  potassium  disulphate  also  effects  the  de- 
composition, but  it  gives  rise  to  the  formation  of  a  double  salt  of 
potassium  and  aluminium  difficultly  soluble  in  water  or  acids,  and 
which  renders  further  analysis  more  difficult  (L.  SMITH  *). 

b.  Determination. 

Aluminium  is  almost  invariably  weighed  as  aluminium  oxide; 
occasionally  also  as  phosphate  (compare,  for  instance,  §  209,  7,  n). 
In  the  former  case  the  several  aluminium  salts  are  converted  into 
aluminium  oxide,  either  by  precipitation  as  aluminium  hydroxide, 
and  subsequent  ignition,  or  by  simple  ignition.  Precipitation  as 
basic  acetate  or  basic  formate  is  resorted  to  only  in  cases  of  sepa- 


*  Amer.  Jour,  of  Sc.  and  Arts,  XL,  248;  Zeitschr.f.  analyt.  Chem.,  iv,  412. 


278  DETERMINATION.  [§  105. 

ration.     For  the  indirect  (acidimetric)  estimation  of  aluminium 
in  alum,  etc.,  see  §  215. 
We  may  convert  into 

ALUMINIUM    OXIDE. 

a.  By  Precipitation.  b.  By  Heating  or  Ignition. 

All  aluminium  salts  soluble  a.   All  aluminium  salts  of 

in  water,  and  those  which,  in-  readily   volatile    oxygen    acids 

soluble  in  that  menstruum,  dis-  (e.g.-,  aluminium  nitrate), 

solve  in  hydrochloric  acid,  with  ft.  All  aluminium  salts  of 

separation  of  their  acid.  organic  acids. 

With  regard  to  the  method  #,  it  must  be  remembered  that 
the  solution  must  contain  no  organic  substances  which  would 
interfere  with  the  precipitation — e.g.,  tartaric  acid,  sugar,  &c. 
Should  such  be  present,  the  solution  must  be  mixed  with  sodiuni 
carbonate  and  potassium  nitrate,  evaporated  to  dryness  in  a 
platinum  dish,  the  residue  fused,  then  softened  with  water,  trans- 
ferred to  a  beaker,  digested  with  hydrochloric  acid,  and  the  solu- 
tion filtered,  and  then,  but  not  before,  precipitated. 

The  methods  5,  a  and  fi  are  applicable  only  in  cases  where 
no  other  fixed  substances  or  ammonium  chloride  are  present  (on 
igniting  the  latter  with  aluminium  oxide,  aluminium  chloride 
volatilizes).  The  methods  of  determining  aluminium  in  its  com- 
binations with  phosphoric,  boric,  silicic,  and  chromic  acids 
will  be  found  in  Part  II.  of  this  Section,  under  the  heads  of 
these  several  acids. 

Determination  as  Aluminium  Oxide. 

a.  By  Precipitation. 

Mix  the  moderately  dilute  hot  solution  of  the  aluminium  salt, 
in  a  beaker  or  dish,  witli  a  tolerable  quantity  of  ammonium  chlo- 
ride, if  that  salt  is  not  already  present ;  add  ammonia  slightly  in 
•excess,  boil  gently  till  the  fluid  gives  a  neutral  or  barely  alkaline 
reaction  (the  fluid  adhering  to  the  test  paper  must  be  washed 
back).  The  fluid  must  not  be  heated  too  long,  or  it  may  become 
acid  through  decomposition  of  ammonium  chloride,  and  some  of 
the  precipitate  may  redissolve;  and  this  must,  of  course,  be 
avoided.  Precipitation  is  best  effected  in  a  large  platinum  dish ; 
in  default  of  this  a  porcelain  one  will  answer,  but  glass  is  not  to 


§  105.]  ALUMINIUM.  279 

be  recommended  because  it  is  markedly  attacked  by  hot  ammo- 
niacal  liquids  (see  p.  88).  Allow  the  precipitate  to  settle;  then 
decant  the  clear  supernatant  fluid  on  to  a  filter,  taking  care  not 
to  disturb  the  precipitate;  pour  boiling  water  on  the  latter  in  the 
beaker,  stir,  let  the  precipitate  subside,  decant  again,  and  repeat 
this  operation  of  washing  by  decantation  a  second  and  a  third  time ; 
transfer  the  precipitate  now  to  the  filter,  and  finish  the  washing 
with  boiling  water.  Suction  is  particularly  useful  in  filtering  off 
aluminium  hydroxide  (§  47),  as  the  precipitate  may ,  without  further 
treatment,  be  at  once  ignited  as  detailed  on  p.  117.  If  suction 
is  not  employed,  the  ignition  of  the  moist  precipitate  is  a  rather 
critical  operation.  If  the  precipitate  is  to  be  dried  before  igni- 
tion, however,  the  drying  must  first  be  very  thorough,  after  'which 
ignite  (§  52),  and  weigh.  The  heat  applied  should  be  very  gentle 
at  first,  and  the  crucible  kept  well 'covered,  to  guard  against 
the  risk  of  loss  of  substance  from  spirting,  which  is  always  to 
be  apprehended  if  the  precipitate  is  not  thoroughly  dry.  In 
whichever  way  the  precipitate  is  ignited,  it  is  always  advisable 
to  expose  it  for  some  time  to  an  incipient  white  heat  by  means  of 
the  gas  blowpipe,  before  weighing,  in  order  to  be  sure  that  every 
trace  of  moisture  has  been  expelled.  A.  MITSCHERLICH.*  In  the 
case  of  aluminium  sulphate  the  foregoing  process  is  apt  to  leave 
some  sulphuric  acid  in  the  precipitate,  which,  of  course,  vitiates 
the  result.  To  insure  the  removal  of  this  sulphuric  acid,  the 
precipitate  should  be  exposed  for  5-10  min.  to  the  heat  of  the  gas 
blowpipe  flame.  If  there  are  difficulties  in  the  way,  preventing 
this  proceeding,  the  precipitate,  either  simply  washed  or  mod- 
erately ignited,  must  be  re-dissolved  in  hydrochloric  acid  (which 
requires  protracted  warming  with  strong  acid),  and  then  precipi- 
tated again  with  ammonia ;  or  the  sulphate  must  first  be  con- 
verted into  nitrate  by  decomposing  it  with  lead  nitrate,  added  in 
very  slight  excess,  the  excess  of  lead  removed  by  means  of  hydro- 
gen sulphide,  and  the  further  process  conducted  according  to 
the  directions  of  a  or  I.  The  precipitation  may  also  be  effected 
by  means  of  ammonium  carbonate  or  ammonium  sulphate,  in- 
stead of  ammonia ;  the  accuracy  of  the  results  is  not,  however, 
thereby  increased. 

*Zeitschr.f.  analyt.  Chem.,  i,  67. 


280  DETERMINATION.  [§  106. 

For  the  properties  of  aluminium  hydroxide  and  ignited  alu- 
minium oxide,  see  §  75.  The  operator  should  never  neglect  to 
test  the  aluminium  hydroxide  for  silicic  acid  (which  is  often 
present).  This  is  readily  done  by  heating  with  dilute  sulphuric 
acid,  or  fusing  with  potassium-  or  sodium  disulphate  (§  75).  The 
method,  if  properly  executed,  gives  very  accurate  results.  But 
if  a  considerable  excess  of  ammonia  is  used,  more  particularly 
in  the  absence  of  ammonium  salts,  and  the  liquid  is  filtered  with- 
out boiling  or  long  standing  in  a  warm  place  to  remove  the 
ammonia,  no  trifling  loss  may  be  incurred.  This  loss  is  the 
greater,  the  more  dilute  the  solution,  and  the  larger  the  excess  of 
ammonia.  The  precipitate  cannot  well  be  sufficiently  washed  on 
the  filter  on  account  of  its  gelatinous  nature ;  on  the  other  hand, 
if  it  be  entirely  washed  by  decantation,  a  very  large  quantity  of 
wash- water  must  be  used,  hence  it  is  advisable  to  combine  the 
two  methods,  as  directed.* 

b.  By  Ignition. 

a.  Aluminium  Salts  of  Volatile  Oxygen  Acids. 
Ignite  the  salt  (or  the  residue  of  the  evaporated  solution)  in  a 
platinum  crucible,  gently  at  first,  then  gradually  to  the  very  high- 
est degree  of  intensity,  until  the  weight  remains  constant.  For 
the  properties  of  the  residue,  see  §  75.  Its  purity  must  be  care- 
fully tested.  There  are  no  sources  of  error. 

ft.  Aluminium  Salts  of  Organic  Acids. 
The  same  method  as  described  §  104,  3,  a  (Magnesium). 

§  106.  • 

2.  CHROMIUM. 

a.  Solution. 

Many  chromic  salts  are  soluble  in  water.  Chromic  hydroxide, 
and  most  of  the  salts  insoluble  in  water,  dissolve  in  hydrochloric 
acid.  Ignition  renders  chromic  oxide  and  many  chromium  salts 
insoluble  in  acids ;  this  insoluble  modification  must  be  prepared  for 


*  [When  a  solution  of  aluminium  hydroxide  in  potassium  or  sodium  hydrox- 
ide is  boiled  with  excess  of  ammonium  chloride,  the  aluminium  separates  com- 
pletely as  a  hydrated  oxide  with  two  mol.  of  water,  which  may  be  washed  with- 
comparative  ease.  In  certain  cases,  as  where  aluminium  is  separated  from  ferric 
iron  by  boiling  their  hydroxides  with  soda,  this  fact  may  be  taken  advantage  of. 
LOWE,  Fres.  Zeitsckrift,  iv,  355.] 


§  106.]  CHROMIUM.  281 

solution  in  hydrochloric  acid,  by  fusing  with  3  or  4  parts  of  po- 
tassa  in  a  silver  crucible.  In  the  process  of  fusing  a  small  quan- 
tity of  potassium  chromate  is  formed  by  the  action  of  air ;  this, 
however,  can  be  decomposed  by  heating  with  hydrochloric  acid 
witli  formation  of  chromic  chloride.  Addition  of  alcohol  greatly 
promotes  the  reduction  to  chromic  chloride.  Instead  of  this  fus- 
ing with  potassa,  we  frequently  prefer  to  adopt  a  treatment 
whereby  the  chromium  is  at  once  oxidized  and  converted  into  an 
alkali  chromate  (see  2).  For  the  solution  of  chromic  iron,  see 
§  160. 

b.  Determination. 

Chromium  is  always,  when  directly  determined,  weighed  as 
chromic  oxide.  It  is  brought  into  this  form  either  by  precipitation 
as  hydroxide  and  ignition,  or  by  simple  ignition.  It  may,  how- 
ever, also  be  estimated  by  conversion  into  chromic  acid,  and  deter- 
mination as  such. 

We  may  convert  into 

1.  CHROMIC  OXIDE. 

a.  By  Precipitation.  I.  By  Ignition. 

All  chromic  salts  soluble  in  a.  All  chromic  salts  of  vola- 

water,  and  also  those  which,  in-  tile  oxygen  acids,  provided  no 
soluble  in  that  menstruum,  dis-  non-volatile  substances  be  pres- 
solve  in  hydrochloric  acid,  with  ent. 

separation  of  their  acid.     Pro-  ft.  Chromic  salts  of  organic 

vided   always   that   no    organic     acids, 
substances  (such  as  tartaric  acid, 
oxalic  acid,  &c.)  which  interfere 
with  the  precipitation  be  present. 

2.  CHROMIC  ACID,  or.  more  correctly  speaking,  ALKALI  CHROMATE. 

Chromic  oxide  and  all  chromic  salts. 

The  methods  of  analyzing  chromic  phosphates,  borates,  silicates, 
and  chromic  chromate,  will  be  found  in  Part  II.  of  this  Section, 
under  the  heads  of  the  several  acids  of  these  compounds. 

1.  Determination  as  Chromic  Oxide. 

a.  By  Precipitation. 

The  solution,  which  must  not  be  too  highly  concentrated,  is 
best  heated  to  100°  in  a  platinum  dish.  One  of  porcelain  may 
also  answer,  but  is  not  so  good,  but  glass  should  be  avoided, 


282  DETERMINATION.  [ 

otherwise  considerable  error  is  caused  by  contamination  of  the 
precipitate  with  silica,  and  the  results,  therefore,  will  be  too  high 
(A.  SOUCHAY*).  If  porcelain  is  used,  this, error  is  slight.  Am- 
monia is  then  added  slightly  in  excess,  and  the  mixture  exposed 
to  a  temperature  approaching  boiling,  until  the  fluid  over  the  pre- 
cipitate is  perfectly  colorless,  presenting  no  longer  the  least  shade 
of  red ;  let  the  solid  particles  subside,  wash  three  times  by  decan- 
tation,  and  lastly  on  the  filter,  with  hot  water,  dry  thoroughly, 
and  ignite  (§  52).  The  heat  in  the  latter  process  must  be  in- 
creased gradually,  and  the  crucible  kept  covered,  otherwise  some 
loss  of  substance  is  likely  to  arise  from  spirting  upon  the  incan- 
descence of  the  chromic  oxide,  which  marks  the  passing  of  the  sol- 
uble into  the  insoluble  modification.  A  suction  filter  (§  47)  is 
very  convenient  for  washing  the  precipitate,  which  may  then  be 
transferred,  still  moist,  to  the  crucible  in  which  it  is  ignited  and 
weighed.  (See  p.  117.)  For  the  properties  of  the  precipitate 
and  residue,  see  §  76.  This  method,  if  properly  executed,  gives 
accurate  results.  Precipitation  may  also  be  effected  writh  ammo- 
nium sulphide,  instead  of  ammonia.  In  this  case  precipitation  is 
complete  in  the  cold ;  and  it  may  be  carried  out  in  glass  vessels. 

J.   By  Ignition. 

a.    Chfomic  Salts  of  Volatile  Oxygen  Acids, 
The  same  method  as  described,  §  105,  Z>,  a  (Aluminium). 

/?.    Chromic  Salts  of  Organic  Acids. 
The  same  method  as  described,  §  104,  3,  a  (Magnesium). 

2.   CONVERSION  OF  CHROMIUM  IN  CHROMIC  COMPOUNDS 
INTO  ALKALI  CHROMATE. 

(For  the  estimation  of  chromic  acid,  see  §  130.) 
The  following  methods  have  been  proposed  with  this  view : — 
a.  The  solution  of  the  chromic  salt  is  mixed  with  solution  of 
potassa  or  soda  in  excess,  until  the  chromic  hydroxide,  which  forms 
at  first,  is  redissolved.     Chlorine  gas  is  then  conducted  into  the 
cold  fluid  until  it  acquires  a  yellowish-red  tint ;  it  is  then  mixed 
with  potassa  or  soda  in  excess,  and  the  mixture  evaporated  to  dry- 
ness  ;  the  residue  is  ignited  in  a  platinum  crucible.     The  whole  of 

*  Zeitschr.  f.  analyt.  Chem.,  TV,  66. 


§106.]  CHROMIUM.  283 

the  potassium  (or  sodium)  chlorate  formed  is  decomposed  by  this 
process,  and  the  residue  consists,  therefore,  now  of  an  alkali  chro- 
mate  and  potassium  (or  sodium)  chloride. — (YoHL.) 

b.  Potassium  hydroxide  is  heated  in  a  silver  crucible  to  calm 
fusion ;  the  heat  is  then  somewhat  moderated,  and  the  perfectly 
dry  chromic  compound  projected  into  the  crucible.     When  the 
substance  is  thoroughly  moistened  with  the  potassa,  small  lumps  of 
fused  potassium  chlorate  are  added.    A  lively  effervescence  ensues, 
from  the  escape  of  oxygen ;  at  the  same  time  the  mass  acquires  a 
more  and  more  yellow  color,  and  finally  becomes  clear  and  trans- 
parent.    Loss  of  substance  must  be  carefully  guarded  against  (H. 
SCHWABZ). 

c.  Dissolve  chromic  hydroxide  in  solution  of  potassa  or  soda, 
add  lead  dioxide  in  sufficient  excess,  and  warm.     The  yellow  fluid 
produced  contains  all  the  chromium  as  lead  chromate  in  alkaline 
solution.     Filter  from  the  excess  of  lead  dioxide,  add  to  the  filtrate 
acetic  acid  to  acid  reaction,  and  determine  the  weight  of  the  pre- 
cipitated lead  chromate  (G.  CHANCEL*). 

d.  Mix   finely  comminuted    chromic   hydroxide    with    some 
potassium   chlorate  in  a  porcelain  dish,  add  nitric  acid  (sp.  gr. 
1*367),  cover  the  dish  with  a  funnel  of  somewhat  smaller  diam- 
eter, heat  on  the  wTater-bath,  and  add  from  time  to  time  a  frag- 
ment of  potassium  chlorate  until  all  the  chromic  hydroxide  is  dis- 
solved and  converted  into    potassium    chromate.     Even    with  a 
hydroxide  which  has  been  strongly  ignited,  the  operation  does  not 
last  longer  than  30  to  60  minutes.     The  chromic  acid  is  most  con- 

O 

veniently  determined  in  the  solution  by  precipitation  as  barium 
chromate  (STORES  f  ;  PEARSON  ;£). 

[e.  Render  the  solution  of  chromic  salt  nearly  neutral  by  a 
solution  of  sodium  carbonate,  add  sodium  acetate  in  excess,  heat 
and  add  chlorine  water,  or  pass  in  chlorine  gas,  keeping  the  solu- 
tion nearly  neutral  by  occasional  addition  of  sodium  carbonate. 
The  oxidation  proceeds  readily.  Boil  off  excess  of  chlorine,  when 
the  chromic  acid  may  be  precipitated  as  lead  chromate  or  barium 
chromate  (W.  GIBBS  §).] 

*  Comp.  rend.,  XLIII,  927.  -\Zeit8chr.f.  analyt.  Chem.,  ix,  71. 

\ Ibid.,  ix,  108.  §  [Am.  Journ.  8ci.  2  Ser.,  xxxix,  58.] 


284  DETERMINATION.  [§  107. 

§107. 

Supplement  to  the  Third  Group. 
TITANIUM. 

Titanium  is  always  weighed  as  titanic  oxide  (TiO2),  i.e.,  the 
oxide    or    anhydride    corresponding    to    titanic    acid    (Ti(OH)4). 
Titanic  acid  is  precipitated  with  an  alkali  or  by  boiling  its  dilute 
acid  solution.     In  precipitating  acid  solutions  of  titanic  acid  ammo- 
nia is  employed;  take  care  to  add  the  precipitating  agent  only 
in  slight  excess,  let  the  precipitate  formed,  which  resembles  alu- 
minium hydroxide,  deposit,  wash,  first  by  decantation,  then  com- 
pletely on  the  filter,  dry,  and  ignite  (§  52).     If  the  solution  con- 
tained sulphuric  acid,  put  some  ammonium  carbonate  into  the 
crucible,  after  the  first  ignition,  to  secure  the  removal  of  every 
remaining  trace  of  that  acid.    Lose  no  time  in  weighing  the  ignited 
titanic  oxide,  as  it  is  slightly  hygroscopic.     Occasionally  it  is  more 
convenient  to  precipitate  titanic  acid  from  its  acid  solutions  by 
nearly  neutralizing  with  ammonia,  adding  sodium  acetate  and  boil- 
ing.    The  precipitate  thus  obtained  is  easily  filtered  and  washed. 
If  we  have  titanic  acid  dissolved  in  sulphuric  acid,  as  for  instance 
occurs  when  we  fuse  it  with  potassium  disulphate  and  treat  the 
mass  with  cold  water,  we  may,  by  largely  diluting,  and  long  boil- 
ing, with  renewal  of  the  evaporating  water,  fully  precipitate  the 
titanic  acid.     If  much  free  acid  is  present  it  must  be  nearly  neu- 
tralized with  ammonia  before  boiling.      Boiling  is  best  effected  in 
a  platinum  dish.      After  filtration,  the  free  acid  in  the  filtrate  is 
still  further  neutralized,  and  the  liquid  boiled  again  for  sometime, 
to  see  that  no  titanic  acid  is  precipitated.    Testing  the  last  filtrate 
with  ammonia  affords  the  certainty  that  precipitation  is  complete. 
In  the  process  of  igniting  the  dried  precipitate,  some  ammonium 
carbonate  is  added.     From  dilute  hydrochloric-acid  solutions  of 
titanic  acid,  the  latter  separates  completely  only  upon  evaporating- 
the  fluid  to  dryness;   and  if  the  precipitate  in  that  case  were 
washed  Avith  pure  water,  the  filtrate  would  be  milky ;   acid  must, 
therefore,  be  added  to  the  water. 

Titanic  acid  precipitated  in  the  cold,  washed  with  cold  water, 
and  dried  without  elevation  of  temperature,  is  completely  soluble 
in  hydrochloric  acid ;  otherwise  it  dissolves  only  incompletely  in 
that  acid.  The  metaiitanic  acid  thrown  down  from  dilute  acid 


§  107.]  TITANIUM.  285 

solutions  by  boiling,  is  not  soluble  in  dilute  acids.  Titanic  oxide 
resulting  from  ignition  of  titanic  or  metatitanic  acid  does  not  dis- 
solve even  in  concentrated  hydrochloric  acid,  but  it  does  dissolve 
by  long  heating  with  tolerably  concentrated  sulphuric  acid.  The 
easiest  way  of  effecting  its  solution  is  to  fuse  it  for  some  time 
with  potassium  disulphate,  and  treat  the  fused  mass  with  a  large 
quantity  of  cold  water.  Upon  fusing  with  sodium  carbonate, 
sodium  titanate  is  formed,  which,  wrhen  treated  with  water,  leaves 
acid  sodium  titanate,  which  is  soluble  in  hydrochloric  acid.  Ti- 
tanic oxide  (TiO2)  consists  of  60 -07  per  cent,  of  titanium  and 
39*93  per  cent,  of  oxygen.  By  fusing  titanic  oxide  with  three 
times  its  quantity  of  potassium  hydrogen  fluoride,  potassium  titan 
ium  fluoride  is  formed,  which  readily  dissolves  in  very  dilute 
hydrochloric  acid  (of  sp.  gr.  1-015)  in  the  heat.  On  fusing,  a 
very  low  heat  must  be  applied  at  first,  till  the  excess  of  hydro- 
fluoric acid  has  escaped,  then  the  heat  is  quickly  raised  till  the 
mass  melts  and  the  titanic  oxide  is  just  dissolved  (MARIGNAC  *) . 
Or  heating  with  hydrofluoric  and  sulphuric  acids  practically  no 
titanium  fluoride  escapes,  but  by  heating  with  hydrofluoric  acid 
some  loss  does  occur  (RiLEY  f). 

Titanium  may  be  estimated  volumetrically  by  first  converting 
it  into  titanous  oxide,  Ti2O3,  and  then  oxidizing  this  to  titanic 
oxide  by  means  of  potassium  permanganate  (compare  §  112,  2) 
(PisANi ;{;).  Solutions  in  sulphuric  acid  are  to  be  avoided ;  but 
the  ordinary  solution  in  hydrochloric  acid,  or  the  solution  of 
titanium-potassium  fluoride  in  dilute  hydrochloric  acid,  is  used. 
The  reduction  is  effected  with  zinc  under  exclusion  of  air,  and 
with  or  without  the  application  of  heat.  In  case  of  the  hydro- 
chloric-acid solutions  it  is  accompanied  by  a  violet  color ;  in  solu- 
tions of  titanium-potassium  fluoride,  with  a  greenish  color.  After 
the  reduction  is  effected  the  zinc  is  removed,  and  solution  of 
potassium  permanganate  added  until  the  liquid  begins  to  remain 
red.  The  weak  point  in  this  method  lies  in  the  difficulty  of  ac- 
curately determining  the  moment  when  the  reduction  is  complete. 
MARIGNAC  §  has  fully  described  the  conditions  by  the  observance 
of  which  he  almost  invariably  obtained  good  results. 


*  Zeitschr.f.  analyt.  Chem.,  vn,  112.  jib.,  n,  71. 

J  lb.t  iv,  419.  $lb.,  vn,  113. 


286  DETERMINATION.  [§  108, 


FOURTH  GROUP  OF  BASIC  RADICALS. 

ZINC — MANGANESE NICKEL COBALT FERROUS  IRON FERRIC  IRON 

— (URANIUM  AND  URANYL). 

§108. 
1.   ZINC. 

a.  Solution. 

Many  of  the  zinc  salts  are  soluble  in  water.  Metallic  zinc, 
zinc  oxide,  and  the  salts  which  are  insoluble  in  water,  dissolve  in 
hydrochloric  acid.  For  effecting  the  solution  of  precipitated  zinc 
sulphide,  hydrochloric  acid  is  also  best.  To  dissolve  zinc  blende, 
however,  it  is  best  to  first  subject  the  finely  powdered  mineral  to- 
the  action  of  hot,  concentrated  hydrochloric  acid,  and  then  effect 
complete  solution  by  adding  some  nitric  acid,  potassium  chlorate, 
or  a  little  of  some  solution  of  bromine  in  hydrochloric  acid. 

b.  Determination. 

Zinc  is  weighed  either  as  oxide  or  as  sulphide  (§  77).  The 
conversion  of  zinc  salts  into  the  oxide  is  effected  either  by  precipi- 
tation as  basic  zinc  carbonate  or  sulphide,  or  by  direct  ignition. 
Besides  these  gravimetric  methods,  several  volumetric  methods  are 
in  use. 

We  may  convert  into 

1.  ZINC  OXIDE. 

a.  By  Precipitation  as  Zinc  b.  By  Precipitation  as  Zinc 
Carbonate.  Sulphide. 

All  zinc  salts  which  are  solu-  All  compounds  of  zinc  with- 

ble  in  water,  and  all  zinc  salts  of  out  exception, 
organic  volatile  acids ;  also  those 
salts  of  zinc  which,  insoluble  in 
water,  dissolve  in  hydrochloric 
acid,  with  separation  of  their 
acid. 

c.  By  direct  Ignition. 

Zinc  salts  of  volatile  inorganic  oxygen  acids. 


§  108.]  ZINC.  287 

2.  ZINC  SULPHIDE. 

All  compounds  of  zinc  without  exception. 

The  method  1,  c,  is  to  be  recommended  only,  as  regards  the 
more  frequently  occurring  compounds  of  zinc,  for  the  carbonate 
and  the  nitrate.  The  methods  1,  &,  or  2,  are  usually  only  resorted 
to  in  cases  where  1,  #,  is  inadmissible.  They  serve  more  especially 
to  separate  zinc  from  other  basic  radicals.  Zinc  salts  of  organic 
acids  cannot  be  converted  into  the  oxide  by  ignition,  since  this, 
process  would  cause  the  reduction  and  volatilization  of  a  small  por- 
tion 'of  the  metal.  If  the  acids  are  volatile,  the  zinc  may  be  deter- 
mined at  once,  according  to  method  1,  a:  if,  on  the  contrary,  the 
acids  are  non- volatile,  the  zinc  is  best  precipitated  as  sulphide.  For 
the  analysis  of  zinc  chromate,  phosphate,  borate,  and  silicate,  look 
to  the  several  acids.  The  volumetric  methods  are  chiefly  employed 
for  technical  purposes ;  see  Special  Part. 

1.  Determination  as  Zinc  Oxide. 

a.  By  Precipitation  as  Zinc  Carbonate, 

Heat  the  moderately  dilute  solution  nearly  to  boiling  in  a  capo- 
clous  vessel, — a  glass  vessel  is  poorly  adapted  for  this  purpose, 
porcelain  is  better,  and  platinum  best; — add,  drop  by  drop,  sodium 
carbonate  till  the  fluid  shows  a  strong  alkaline  reaction;  boil  a  few 
minutes;  allow  to  subside,  decant  through  a  filter,  and  boil  the 
precipitate  three  times  with  water,  decanting  each  time;  then 
transfer  the  precipitate  to  the  filter,  wash  completely  with  hot 
water,  dry,  and  ignite  as  directed  §  53,  taking  care  to  have  the  filter 
as  clean  as  practicable,  •  before  proceeding  to  incinerate  it.  In. 
order  to  prevent  the  reduction  of  the  zinc  oxide  and  volatilization 
of  zinc,  it  is  advisable  to  carefully  saturate  with  ammonium 
nitrate  the  filter  after  removing  as  much  of  the  precipitate 
from  the  latter  as  possible,  and  then  to  incinerate  it.  Should 
the  solution  contain  ammonium  salts,  the  ebullition  must  be  con- 
tinued until,  upon  a  fresh  addition  of  sodium  carbonate,  the  escap- 
ing vapor  no  longer  imparts  a  brown  tint  to  turmeric  paper.  If 
the  quantity  of  ammonium  salts  present  is  considerable,  the  fluid 
must  be  evaporated  boiling  to  dryness.  It  is,  therefore,  in  such 
cases  more  convenient  to  precipitate  the  zinc  as  sulphide  (see  b). 

The  presence  of  a  great  excess  of  acid  in  the  solution  of  zinc 
must  be  as  much  as  possible  guarded  against,  that  the  effervescence 
from  the  escaping  carbonic  acid  gas  may  not  be  too  impetuous.  The 


288  "  DETERMINATION.  [§  108. 

filtrate  must  always  be  tested  with  ammonium  sulphide  (with  addi- 
tion of  ammonium  chloride)  to  ascertain  whether  the  whole  of  the 
zinc  has  been  precipitated.  This  should  be  done  in  a  flask  filled  to 
the  neck  and  then  closed.  A  slight  precipitate  will  indeed  invari- 
ably form  upon  the  application  of  this  test;  but,  if  the  process  has 
been  properly  conducted,  this  is  so  insignificant  that  it  may  be  al- 
together disregarded,  being  limited  to  some  exceedingly  slight  and 
imponderable  flakes,  which  moreover  make  their  appearance  only 
after  many  hours'  standing.  If  the  precipitate  is  more  considerable, 
however,  it  must  be  treated  as  directed  in  &,  and  the  weight  of  the 
zinc  oxide  obtained  added  to  that  resulting  from  the  first  process. 
For  the  properties  of  the  precipitate  and  residue,  see  §  77.  This 
method  yields  pretty  accurate  results,  though  they  are  in  most 
cases  a  little  too  low,  as  the  precipitation  is  never  absolutely  com- 
plete, and  as  particles  of  the  precipitate  will  always  and  unavoid- 
ably adhere  to  the  filter,  which  exposes  them  to  the  chance  of 
reduction  and  volatilization  during  the  process  of  ignition.  On 
the  other  hand,  the  results  are  sometimes  too  high ;  this  is  owing 
to  defective  washing,  as  may  be  seen  from  the  alkaline  reaction 
which  the  residue  manifests  in  such  cases.  It  is  advisable  also  to 
ascertain  whether  the  residue  will  dissolve  in  hydrochloric  acid 
without  leaving  silica;  this  latter  precaution  is  indispensable  in 
cases  where  the  precipitation  has  been  effected  in  a  glass  vessel. 

[It  is  often  better,  especially  in  presence  of  ammonium  salts,  to 
heat  the  dry  zinc  salt  with  excess  of  sodium  carbonate  in  a  plati- 
num dish  cautiously  to  near  redness,  then  treat  with  hot  water  arid 
wash  as  directed.] 

b.  By  Precipitation  as  Zinc  Sulphide. 

Mix  the  solution,  contained  in  a  not  too  large  flask  and  suffi- 
ciently diluted,  with  ammonium  chloride,  then  add  ammonia,  till 
the  reaction  is  just  alkaline,  and  then  colorless  or  slightly  yellow 
ammonium  sulphide  in  moderate  excess.  If  the  flask  is  not  now 
quite  full  up  to  the  neck,  make  it  so  with  water,  cork,  allow  to 
stand  12  to  24  hours  in  a  warm  place,  wash  the  precipitate,  if  con- 
siderable, first  by  decantation,  then  on  the  filter  with  water  con- 
taining ammonium  sulphide  and  also  less  and  less  ammonium  chlo- 
ride (finally  none).  If  the  zinc  sulphide  is  to  be  weighed  as 
such,  it  is  best  to  replace  the  ammonium  chloride  by  ammonium 
nitrate.  In  decanting  do  not  pour  the  fluid  through  the  filter, 


§  109.]  ZINC.  289 

but  at  once  into  a  flask.  After  thrice  decanting,  filter  the 
fluid  that  was  poured  off,  and  then  transfer  the  precipitate  to  the 
filter,  finishing  the  washing  as  directed.  The  funnel  is  kept  cov- 
ered with  a  glass  plate.  If  the  zinc  is  not  to  be  determined  accord- 
ing to  2,  then  put  the  moist  filter  with  the  precipitate  in  a  beaker, 
and  pour  over  it  moderately  dilute  hydrochloric  acid  slightly  in 
excess.  Put  the  glass  now  in  a  warm  place,  until  the  solution 
smells  no  longer  of  hydrogen  sulphide ;  dilute  the  fluid  with  a  little 
water,  filter,  wash  the  original  filter  with  hot  water,  and  proceed 
with  the  solution  of  zinc  chloride  obtained  as  directed  in  a. 

The  following  method  also  effects  a  practically  complete  pre- 
cipitation of  zinc  from  acid  solution.  Add  sodium  carbonate,  at 
last  drop  by  drop  till  a  lasting  precipitate  forms,  dissolve  the  latter 
by  a  drop  of  hydrochloric  acid,  pass  .hydrogen  sulphide  till  the 
precipitate  ceases  to  increase  perceptibly,  add  sodium  acetate,  and 
again  pass  the  gas.  After  washing  with  water  containing  hydro- 
gen sulphide  (which  when  the  zinc  sulphide  had  been  thrown 
down  by  hydrogen  sulphide  from  acetic  acid  solution,  is  easily 
done),  treat  as  above  directed. 

From  a  solution  of  zinc  acetate  the  metal  may  be  precipitated 
completely,  or  nearly  so,  with  hydrogen  sulphide  gas,  even  in  pres- 
ence of  an  excess  of  acetic  acid,  provided  always  no  other  free 
acid  be  present  (Expt.  No.  66).  The  precipitated  zinc  sulphide  is 
washed  with  water  impregnated  with  hydrogen  sulphide,  and,  for 
the  rest,  treated  exactly  like  the  zinc  sulphide  obtained  by  precipi- 
tation with  ammonium  sulphide. 

Small  quantities  of  zinc  sulphide  may  also  be  converted  directly 
into  the  oxide,  by  heating  in  an  open  platinum  crucible,  to  gentle 
redness  at  first,  then,  after  some  time,  to  most  intense  redness. 
For  the  properties  of  zinc  sulphide  see  §77,  c. 

c.  By  direct  Ignition. 

The  salt  is  exposed,  in  a  covered  platinum  crucible,  first  to  a 
gentle  heat,  finally  to  a  most  intense  heat,  until  the  weight  of  the 
residue  remains  constant.  The  action  of  reducing  gases  is  to  be 
avoided. 

2.  Determination  as  Zinc  Sulphide. 

The  precipitated  zinc  sulphide,  obtained  as  in  1,  J,  may  be 
ignited  in  hydrogen   and    weighed.        H.    ROSE,*    who    recom- 
* Pogg.  Anal.,  ex,  128. 


290 


DETERMINATION. 


[§  108. 


mended    the    process,    employs    the    apparatus    represented    by 
Fig.  83. 

a  contains  concentrated  sulphuric  acid,  £>,  calcium  chloride. 
The  porcelain  crucible  has  a  perforated  porcelain  or  platinum 
cover,  into  the  opening  of  which  fits  the  porcelain  or  platinum 
tube,  d.  The  latter  is  provided  with  an  annular  projection  which 
rests  on  the  cover,  the  tube  itself  extends  some  distance  into  the 
crucible.  When  the  zinc  sulphide  has  dried  in  the  filter,  it  is 
transferred  to  the  weighed  porcelain  crucible,  the  filter  ashes  added, 
powdered  sulphur  is  sprinkled  over  the  contents  of  the  crucible, 
the  cover  is  placed  on,  and  hydrogen  is  passed  in  a  moderate 
stream,  a  gentle  heat  is  applied  at  first,  which  is  afterwards  raised 
for  five  minutes  to  intense  redness ;  finally  the  crucible  is  allowred 
to  cool  with  continued  transmission  of  the  gas,  and  the  zinc  sul- 
phide is  weighed. 


Fig.  83. 

Instead  of  the  hydrogen  apparatus  shown,  which  may  not  be 
at  the  disposal  of  the  operator,  any  apparatus  that  allows  the  cur- 
rent of  gas  to  be  regulated  may  be  used.  An  evolution-appara- 
tus in  which  the  current  is  not  under  control,  is  not  suitable. 

Instead  of  the  porcelain  tube  and  perforated  cover,  a  com- 
mon tobacco-pipe  may  be  employed,  the  bowl  of  the  latter  being 


§  109.]  MANGANESE.  291 

inverted  over  and  fitting  exactly  within  a  porcelain  crucible. 
[Hydrogen  sulphide  may  be  advantageously  substituted  for 
hydrogen.] 

OESTEN'S  experiments,  which  were  adduced  by  ROSE  in  sup- 
port of  the  accuracy  of  this  method,  were  highly  satisfactory. 

Zinc  sulphate,  carbonate,  and  oxide  may  be  converted  into 
sulphide  in  the  manner  just  described.  They  must,  however,  be 
mixed  with  an  excess  of  powdered  sulphur,  otherwise  you  will 
lose  some  zinc  from  the  reducing  action  of  the  hydrogen  011  the 
zinc  oxide.  Zinc  sulphate  is  best  ignited  first  with  excess  of  air 
and  before  mixing  with  sulphur,  and  then  igniting  in  a  current  of 
hydrogen.  (II,  ROSE.) 

The  properties  of  the  hydrated  and  anhydrous  zinc  sulphide  are 
given  §  77 ;  the  results  are  accurate.  Loss  occurs  only  when  the 
ignition  is  performed  over  the  gas  blowpipe  (which  is  quite  unnec- 
essary), and  continued  longer  than  five  minutes.  Compare  §  77,  c. 

§  109. 

2.  MANGANESE. 

a.  Solution. 

Many  manganous  salts  are  soluble  in  water.  The  inanganous 
salts  which  are  insoluble  in  that  menstruum,  dissolve  in  hydrochloric 
acid,  which  dissolves  also  all  oxides  of  manganese.  The  solution 
of  the  higher  oxides  is  attended  with  evolution  of  chlorine — equiva- 
lent to  the  amount  of  oxygen  which  the  oxide  under  examination 
contains,  more  than  mauganous  oxide  (MnO) — and  the  fluid,  after 
application  of  heat,  is  found  to  contain  inanganous  chloride. 

b.  Determination. 

Manganese  is  weighed  either  as  protosesquioxide,  as  sulphide, 
or  as  ptjt'<>j)ltoxj)hate  (§  78).  Into  the  form  of  protosesquioxide  it 
is  converted  either  by  precipitation  as  manganous  carbonate,  or 
as  manganous  hydroxide,  sometimes  preceded  by  precipitation  as- 
manganous  sulphide,  or  as  manganese  dioxide  ;  or,  finally,  by  direct 
ignition.  [When  determined  as  pyrophosphate  it  is  precipitated 
as  ammonium  manganous  phosphate.] 

Manganese  may  be  determined  volumetrically  in  three  differ- 
ent ways,  one  being  applicable  to  any  manganous  solution,  pro- 
vided it  be  free  from  any  other  substance  which  exerts  a  reducing 
action  on  alkaline  solution  of  potassium  f erricy anide ;  the  second  is 


292  DETERMINATION.  [§  109* 

applicable  only  when  iron  is  absent ;   the  third  is  admissible  only 
when  we  have  manganese  in  the  condition  of  a  perfectly  definite 
higher  oxide,  and  free  from  other  bodies,  which  evolve  chlorine 
on  boiling  with  hydrochloric  acid. 
We  may  convert  into 

1.  MANGANESE  PROTOSESQUIOXIDE. 

a.  By  Precipitation  as  Man-        b.  By  Precipitation  as  Man- 
ganous Carbonate.  ganese  Hydroxide. 

All   soluble   manganous  salts         All  the  compounds  of  manga- 
of  inorganic  acids,  and  all  man-     nese,  with  the  exception  of  its 
ganous  salts  of  volatile  organic     salts  of  non-volatile  organic  acids, 
acids  ;  also  those  manganous  salts 
which,  insoluble   in  water,  dis- 
solve in  hydrochloric  acid  with 
separation  of  their  acid. 

c.  By  Precipitation  as  Man-        d.  By  Separation  as  Manga' 
ganese  Sulphide.  nese  Dioxide. 

All  compounds  of  manganese         All  compounds  of  manganese 
without  exception.  in  a  slightly  acid  solution,  espe- 

cially manganous  acetate  and  ni- 
trate. 

e.  By  direct  Ignition. 

All  manganese  oxides ;  man- 
ganous salts  of  readily  volatile 
acids,  and  organic  acids. 

2.  MANGANESE  SULPHIDE. 

All  compounds  of  manganese  without  exception. 

3.  MANGANESE  SULPHATE. 

All  manganese  oxides,  as  well  as  salts  of  volatile  acids,  pro- 
vided no  non-volatile  substance  be  present. 

4.  MANGANESE  PYROPHOSPHATE. 

All  the  oxides  of  manganese  soluble  in  water,  and  those  salts, 
insoluble  in  water,  the  acids  of  which  may  be  removed  by  solution. 

The  method  1,  <?,  is  simple  and  accurate,  but  seldom  admis- 
sible. The  method  1,  «,  is  the  most  usually  employed;  if  one's 
choice  is  free,  it  is  to  be  preferred  to  1,  1).  The  methods  1,  <?, 
and  2,  are  generally  used,  when  the  methods  1,  a,  or  J,  cannot  be 
adopted — say  on  account  of  the  presence  of  a  non-volatile  organic 
substance,  and  also  when  we  have  to  separate  manganese  fr-jin 


§109.]  MANGANESE.  293 

other  metals.  The  latter  object  may  be  attained  also  by  the 
method  1,  d.  The  method  No.  3  is  at  times  convenient,  but  gives 
only  approximate  results.  The  method  No.  4  has  been  much 
recommended  recently,  and  is  quite  rapid,  but  yields  satisfactory 
results  only  when  the  solubility  of  tiie  ammonium -manganese 
phosphate  is  taken  into  account.  Manganous  phosphate  and 
borate  are  treated  either  according  to  the  method  2,  or,  in  the 
first  case,  according  to  the  method  4.  In  silicates  the  manganese 
is  determined  after  the  separation  of  the  silicic  acid  (§  140),  ac- 
cording to  1 ,  a,  or  3 ;  for  the  analysis  of  manganous  chromate, 
see  §  130  (chromic  acid).  The  first  two  volumetric  methods  are 
especially  suited  for  technical  work,  in  which  the  highest  degree 
of  accuracy  is  not  required.  The  estimation  of  manganese  from 
the  quantity  of  chlorine  disengaged  upon  boiling  the  oxides  with 
hydrochloric  acid,  is  resorted  to,  more  particularly,  to  determine 
the  degrees  of  oxidation  of  manganese,  and  permits  also  the  esti- 
mation of  manganese  in  presence  of  other  metals  (see  Section  Y.). 

1 .   Determination  as  Protosesquioxide  of  Manganese, 
a.   By  Precipitation  as  Manganous  Carbonate. 

The  precipitation  and  washing  are  effected  in  a  platinum  or 
porcelain  dish  in  exactly  the  same  way  as  directed  §  108,  1,  a 
(determination  of  zinc  as  oxide,  by  precipitation  as  carbonate). 
As  the  filtrate  and  washings  (which  are  at  times  slightly  turbid) 
are  never  quite  free  from  manganese,  evaporate  both  in  a  plati- 
num or  porcelain  dish  to  dryness,  treat  the  residue  with  boiling 
water,  and  collect  the  undissolved  flocks  of  hydrated  protosesqui- 
oxide  in  a  separate,  small  filter  and  wash  with  hot  water.  The 
two  filters  are  then  dried,  and,  together  with  the  precipitate,  ig- 
nited as  directed  in  §  53.  Then  exhaust  the  resulting  protoses- 
quioxide  repeatedly  with  boiling  water,  pouring  off  the  water  into 
a  small  filter,  and  finally  incinerate  this  filter  on  a  platinum  wire 
or  crucible  lid.  Transfer  the  ashes  to  the  crucible  and,  with  the 
lid  removed,  maintain  a  strong  heat  until  the  weight  of  the  residue 
remains  constant.  Care  must  be  taken  to  prevent  reducing  gases 
finding  their  way  into  the  crucible.  For  the  properties  of  the 
precipitate  and  residue,  see  §  78.  This  method,  if  properly  exe- 
cuted, gives  accurate  results.  If  the  small  quantity  of  manganese 
in  the  filtrate  is  neglected,  or  if  the  residue  is  weighed  without 


294  DETERMINATION.  [§  109. 

first  exhausting  it  with  boiling  water,  the  two  errors  may  compen- 
sate each  other ;  nevertheless  only  approximate  results  may  be  ex- 
pected. One  important  point  is  to  continue  the  application  of  a 
sufficiently  intense  heat  long  enough  to  effect  the  object  in  view. 
It  is  necessary  also  to  ascertain  whether  the  residue  has  not  an 
alkaline  reaction,  and,  having  removed  it  from  the  platinum  cru- 
cible, whether  it  dissolves  in  hydrochloric  acid  without  leaving 
silica. 

b.  By  Precipitation  as  Manganous  Hydroxide. 

The  solution  should  not  be  too  concentrated,  and  it  is  best  to 
have  it  in  a  platinum  dish.  Porcelain  may  be  used,  but  not  glass. 
Precipitate  with  solution  of  pure  soda  or  potassa,  and  proceed  in 
all  other  respects  as  in  a. 

If  phosphoric  or  boric  acid  is  present,  the  fluid  must  be, 
kept  boiling  for  some  time  with  an  excess  of  alkali.  For  the 
properties  of  the  precipitate,  see  §  78.  For  the  accuracy  of  the 
method  and  testing  of  the  precipitate  see  a. 

c.  By  Precipitation  as  Manganese  Sulphide. 
See  2. 

d.  By  Separation  as  Manganese  Dioxide. 

Heat  the  solution  of  manganous  acetate  or  some  other  manga- 
nous  salt  containing  but  little  free  acid,  after  addition  of  a  sufficient 
quantity  of  sodium  acetate,  to  from  50°  to  60°,  and  transmit 
chlorine  gas  through  the  fluid,  or  add  bromine  (KAMMEKER,* 
WAAGE  f).  The  whole  of  the  manganese  present  falls  down  as 
dioxide  (SCHIEL,  RIVOT,  BETID  ANT,  and  DAGUIN).  "Wash  first 
by  decantation,  then  upon  the  filter;  dry,  transfer  the  precipitate 
to  a  flask,  add  the  filter  ash,  heat  with  hydrochloric  acid,  filter, 
and  precipitate  as  directed  in  a.  If  the  sodium  acetate  is  defi- 
cient, and  especially  if  hydrochloric  acid  is  present,  it  may 
happen  that  the  precipitation  of  the  manganese  by  chlorine  or 
bromine  is  not  quite  complete ;  it  is  therefore  well,  after  filtering 
off  the  dioxide,  to  treat  the  filtrate  with  more  sodium  acetate,  and 
again  pass  chlorine  or  add  bromine.  If  the  filtrate  is  pink  from 
the  presence  of  permanganate,  add  a  little  alcohol,  and  heat,  in 
order  to  precipitate  the  small  remainder  of  the  manganese.  It  is 

*Ber.  d.  deutsch.  Chem.  Oesell,  iv,  218. 
\Zeitschr.f.  analyt.  Chem.,  x,  206. 


§  109.]  MANGANESE.  295 

impossible  to  convert  the  precipitate  of  dioxide  into  protosesqui- 
oxide  directly  by  ignition,  as  the  residue  contains  a  large  quantity 
of  alkali  which  cannot  be  completely  removed  by  washing.  The 
separation  of  manganese  as  dioxide,  by  evaporating  its  solution  in 
nitric  acid  to  dryness,  and  heating  the  residue,  finally  to  155°,  is 
given  in  Section  Y. 

e.    By  direct  Ignition. 

The  manganese  compound  under  examination  is  introduced 
into  a  platinum  crucible,  which  is  kept  closely  covered  at  first, 
and  exposed  to  a  gentle  heat ;  after  a  time  the  lid  is  taken  off, 
and  replaced"  loosely  on  the  crucible,  and  the  heat  is  increased  to 
the  highest  degree  of  intensity,  with  careful  exclusion  of  reducing 
gases ;  the  process  is  continued  until  the  weight  of  the  residue 
remains  constant.  The  conversion  of  the  higher  oxides  of  man- 
ganese into  protosesquioxide  of  manganese  requires  more  pro- 
tracted and  intense  heating  than  the  conversion  of  manganous 
oxide.  In  fact,  it  can  hardly  be  effected  without  the  use  of  a  gas 
blowpipe.  (It  is,  hence,  best  to  convert  them  into  sulphide  by 
adding  sulphur  and  igniting  them  in  a  current  of  hydrogen ;  see 
2.)  In  the  case  of  manganous  salts  of  organic  acids,  care  must 
always  be  taken  to  ascertain  whether  the  whole  of  the  carbon  has 
been  consumed ;  and  should  the  contrary  turn  out  to  be  the  case 
the  residue  must  either  be  dissolved  in  hydrochloric  acid,  and  the 
solution  precipitated  as  directed  in  #,  or  3,  or  it  must  be  re- 
peatedly evaporated  with  nitric  acid,  until  the  whole  of  the  carbon 
is  oxidized.  The  method,  if  properly  executed,  gives  accurate 
results.  On  the  other  hand,  if  the  directions  are  not  carefully 
attended  to,  one  must  not  be  surprised  at  considerable  differences. 
In  the  ignition  of  manganous  salts  of  organic  acids,  minute  par- 
ticles of  the  salt  are  generally  carried  away  with  the  em- 
pyreumatic  products  evolved  in  the  process,  which,  of  course, 
tends  to  reduce  the  weight  a  little. 

2.   Determination  as  Manganous  Sulphide. 

The  precipitation  as  sulphide  may  be  effected  in  two  ways, 
the  second  being  preferred  when  the  operation  is  to  be  completed 
with  comparative  rapidity. 

a.  The  solution  contained  in  a  comparatively  small  flask  and  not 
too  dilute  is  first  mixed  with  ammonium  chloride  (if  an  ammonium 


296  DETERMINATION.  [§  109, 

salt  is  not  already  present  in  sufficient  quantity),  then — if  the  fluid 
is  acid — with  ammonia,  till  it  reacts  neutral  or  very  slightly  alka- 
line; now  add  yellow  ammonium  sulphide,  in  moderate  excess;  if 
the  flask  is  not  already  quite  full  up  to  the  neck,  add  water  till 
it  is,  cork,  stand  it  in  a  warm  place  for  at  least  twenty-four  hours,, 
wash  the  precipitate  if  at  all  considerable,  first  by  decantation, 
then  on  the  filter,  using  water  containing  ammonium  sulphide, 
and  also  gradually-diminished  quantities  of  ammonium  chloride 
(finally  none).  In  decanting,  pour  the  fluid  in  a  flask,  not  on  the 
filter.  After  decanting  three  times,  filter  the  fluids  that  have 
been  poured  off,  transfer  the  precipitate  to  the  filter,  and  finish 
the  washing  as  above  directed,  without  interruption.  Keep  the 
funnel  covered  with  a  glass  plate. 

1).  Neutralize  the  boiling  liquid  with  ammonia,  add  ammonium 
sulphide  in  not  too  small  amount,  boil  for  about  ten  minutes,  then' 
allow  to  cool  somewhat,  add  ammonium  sulphide  again,  and  filter 
the  liquid  (which  must  smell  of  ammonium  sulphide)  through  a 
double  filter.  If  the  liquid  is  turbid,  return  it  to  the  filter  until 
it  filters  clear  (R.  FINKENEK*).  The  manganese  sulphide  fre- 
quently separates  on  boiling  the  liquid  in  an  anhydrous  state,  and 
with  a  green  color,  particularly  if  the  liquid  contains  but  little 
ammonium  salts,  while  much  free  ammonia  is  present.  The  pre- 
cipitate so  obtained  is  also  washed  with  water  to  which  some 
ammonium  sulphide  has  been  added. 

The  washed  manganese  sulphide  was  formerly  dissolved  in 
hydrochloric  acid,  and  then  precipitated  as  in  1  a.  The  object  is, 
however,  much  more  rapidly  and  conveniently  attained  by  drying 
it  and  igniting  it  strongly  (till  it  becomes  black)  with  the  filter- 
ash  and  a  little  sulphur  in  a  current  of  hydrogen,  and  then  weigh- 
ing the  anhydrous  manganese  sulphide  (H.  ROSE  f) ;  compare  the 
analogous  process  for  zinc,  §  108,  2.  For  the  properties  of  the 
precipitate  and  residue,  as  well  as  the  conditions  which  promote 
or  hinder  the  precipitation  of  manganese,  see  §  78,  e. 

The  results  obtained  by  OESTEN,  and  cited  by  ROSE,  are  per- 
fectly satisfactory.  Equally  satisfactory  results  were  obtained  by 
me  also. 


*  Handbuck  der  analyt.  Chem.,  von  H.  ROSE,  6.  Aufl.,  von  FINKENER,  p.  925. 
\Pogg.  Anal.,  ex,  122. 


§  109  ]  MANGANESE.  297 

Tliis  method  is  shorter  and  more  convenient  than  dissolving 
the  moist  sulphide  in  hydrochloric  acid,  and  precipitating  with 
sodium  carbonate. 

In  the  filtrate  from  the  sulphide  there  will  be  found  only  the 
most  insignificant  traces  of  manganese,  if  the  process  has  been 
properly  conducted. 

Tartaric  acid  retards  the  precipitation,  but  does  not  render  it 
less  complete;  citric  acid,  however,  prevents  precipitation,  or  at 
least  makes  it  incomplete. 

Manganous  sulphate  and  all  the  oxides  of  manganese  may  be 
converted  into  sulphide  by  ignition  with  sulphur  in  a  current  of 
hydrogen. 

3.   Determination  as  Manganous  Sulphate. 

Proceed  as  with  magnesia,  §  104,  1.  Care  must  be  taken  to 
avoid  too  great  an  excess  of  sulphuric  acid,  and  not  to  expose  to- 
mere  than  a  low  red  heat.  For  the  properties  of  the  residue  see 
§  78.  Accurate  results  are  obtained  only  by  chance,  as  on  heat- 
ing gently  the  weight  obtained  is  generally  too  high,  while  on 
heating  strongly  the  weight  is  too  low,  because  of  the  volatiliza- 
tion of  sulphuric  acid  (H.  HOSE  *).  To  obtain  accurate  results, 
therefore,  the  manganous  sulphate  should  be  converted  into  sul- 
phide according  to  2. 

4.     Determination     as     Manganous     Pyrophosphate. 

(W.  GIBBS  f). 

To  the  solution  of  the  manganous  salt,  which  may  contain 
ammonium  or  alkali  salt,  sodium  phosphate  is  added  in  large  ex- 
cess above  what  is  needful  to  convert  the  manganese  into  phos- 
phate. A  platinum  dish  is  best  for  the  purpose ;  a  porcelain  one 
will  answer,  but  glass  must  not  be  used.  The  white  precipitate 
which  is  formed  unless  considerable  free  acid  is  already  present  is 
then  redissolved  in  sulphuric  or  chlorhydric  acid,  the  liquid  is 
heated  to  boiling,  best  in  a  platinum  dish,  and  ammonia  added  in 
excess.  The  boiling  is  continued  10 — 15  minutes,  whereby  the 
white,  semi-gelatinous  precipitate  first  formed  is  converted  into 
rose-colored,  pearly  scales.  If  one  is  obliged  to  precipitate  in  a 
glass  beaker,  the  precipitate  may  be  converted  into  the  crystalline 

*  Pogg.  Annal.,  ex,  125. 

f  SILLIMAN,  Amer.  Journ.  (n)  XLIV,  216;  Zeitschr.f.  analyt.  CTiem.,  vn,  101.. 


298  DETERMINATION.  [§  109. 

form  more  safely  by  heating  on  the  water-bath  1  or  2  hours,  as  it 
is  likely  to  be  thrown  out  of  the  beaker  by  boiling.  The  whole  is 
kept  hot  for  an  hour  longer,  then  filtered  and  washed  with  water 
containing  a  little  ammonia.  The  precipitate  of  ammonium  man- 
ganous  phosphate  is  dried,  separated  from  the  filter,  and  con- 
verted by  ignition  into  pyrophosphate.  See  §  78  (  GIBBS,* 
HENRY  f).] 

It  is  advantageous  to  use  the  Bunsen  filtering  apparatus  for 
washing  the  precipitate,  on  account  of  its  slight  solubility  in  water, 
and  to  wash  with  cold,  and  not  boiling,  water,  according  to  GIBBS. 
(See  §  78,  (/.)  For  the  same  reason  when  great  accuracy  is  re- 
quired it  is  recommended  to  evaporate  the  filtrate  to  dryness,  re- 
dissolve  with  water  and  hydrochloric  acid,  make  alkaline  with 
ammonia,  and  boil  to  precipitate  and  recover  the  small  amount 
of  manganese — usually  from  2  to  4  milligrammes — which  may 
have  passed  into  the. filtrate. 

For  washing  the  precipitate  I  prefer  to  use  cold  water  because 
I  have  been  unable  to  confirm  Gibbs'  statement  that  the  precipitate 
is  insoluble  in  boiling  water.  J 

5.    Volumetric  Methods  of  Estimating  Manganese. 

a.   Determination    by   the   Reduction  of  Ferri- 
cyanide  of  Potassium  (E.  LENSSEN§). 

The  method  is  grounded  on  the  fact  that  if  a  solution  of  a 
manganous  salt,  containing  1  equivalent  of  Fe2O9  to  every  equiva- 
lent of  MnO,  is  acted  on  by  excess  of  alkaline  solution  of  potas- 
sium ferricyanide  at  a  boiling  temperature,  all  the  manganese  is 
precipitated  as  dioxide,  while  a  corresponding  quantity  of  potas- 
sium ferrocyanide  is  formed.  By  determining  the  latter,  the 
amount  of  manganese  present  is  obtained. 

*  Am.  Jour.  Sci.,  2d  Ser.,  XLIV,  p.  216.  f  lb.,  XLVII,  p.  130. 

^  According  to  estimations  made  by  me,  1  part  of  Mn(NH4)PO4  -f-  2H2O  is 
soluble  in  32092  parts  of  cold,  and  in  20122  parts  of  boiling  water;  and  in 
17755  parts  of  water  containing  ammonium  chloride  (1:  70).  The  double  salt 
forms  pearly,  pale-pink  scales,  becoming  somewhat  darker  in  color  on  the 
filter;  if  it  becomes  a  deep,  dark-red,  however,  it  is  evidence  that  the  salt  has 
nol  become  completely  converted  into  an  ammonium  salt.  In  this  case  the  pre- 
cipitate must  be  redissolved  in  hydrochloric  acid,  and  reprecipitated  after  add- 
ing sodium  phosphate,  etc.  The  manganous  pyrophosphate,  Mn2PaO7,  left  on 
igniting  the  ammonium-manganese  phosphate  is  white. 

%Journ.f.  prakt.  Chem.,  LXXX,  408. 


§  109.]  MANGANESE.  299 

K6FeaCy12+2KaO  +  MnSO4=2K4FeCy.  +  KfSO4  +  MnOf. 

Accordingly  1  at.  manganese  gives  rise  to  2  mol.  potassium 
ferrocyanide.  Of  course  all  other  reducing  substances  must  be 
absent,  and  the  manganese  must  be  present  entirely  in  the  form  of 
a  manganous  salt.  If  the  solution  contains  no  ferric  salt,  the  pre- 
cipitate is  a  combination  of  much  dioxide,  with  little  manganous 
oxide,  not  always  in  the  same  proportions.  In  performing  the 
process,  mix  first  with  the  acid  solution  of  the  manganous  salt  so 
much  ferric  chloride  that  you  may  be  sure  of  having  at  least  1 
mol.  FeaCl6  to  1  atom  Mn,  and  add  the  mixture  gradually  to  a 
boiling  solution  of  potassium  ferricyanide,  previously  rendered 
strongly  alkaline  with  potassa  or  soda.  After  boiling  together  a 
short  time  the  brownish-black  precipitate  becomes  granular  and 
less  bulky.  Allow  to  cool  completely,  filter  off  and  wash  the  pre- 
cipitate, acidify  the  filtrate  with  hydrochloric  acid,  and  estimate 
the  potassium  ferrocyanide  with  permanganate,  according  to  §  147, 
II.,  g.  If  the  liquid  is  filtered  hot,  the  results  are  too  high,  as 
the  filter  in  this  case  has  a  reducing  action.  The  method  may  be 
shortened,  as  follows:  After  boiling,  transfer  the  solution,  together 
with  the  precipitate,  to  a  measuring  flask,  allow  to  cool,  fill  up  to 
the  mark  with  water,  shake,  and  allow  to  settle.  Filter  through  a 
dry  filter,  take  out  a  certain  quantity  with  a  pipette,  and  determine 
the  ferrocyanide  in  this.  A  slight  source  of  error  is  here  intro- 
duced by  disregarding  the  volume  of  the  precipitate.  The  results 
adduced  by  LENSSEN  are  very  satisfactory.  I  have  myself  repeat- 
edly tested  this  method,  and  I  have  to  remark  as  follows : — 

a.  It  potassium  ferricyanide  is  long  boiled  with  pure  potassa,  a 
small  quantity  of  ferrocyanide  is  invariably  produced. 

b.  The  potassa  must  be  quite  free  from  organic  substances,  and 
should  therefore,  if  there  is  any  doubt  on  this  point,  be  fused  in  a 
.silver  dish  before  use,  otherwise  the  error  alluded  to  in  a  may  be 
considerably  increased. 

v.  The  complete  washing  of  the  voluminous  precipitate  is 
attended  with  so  much  difficulty  and  loss  of  time  as  to  render  the 
method  more  troublesome  than  a  gravimetric  analysis. 

d.  The  abridged  method,  on  the  other  hand,  may  be  of  great 
service  in  certain  cases,  especially  when  a  series  of  manganese 
determinations  have  to  be  made,  the  manganese  not  being  in  too 


300  DETERMINATION.  [§  109, 

minute  quantities,  and  the  highest  degree  of  accuracy  not  being 
required.  In  my  laboratory,  by  employing  a  slight  excess  of  ferric 
salt,  97  9 — 100-12 — 98-21 — 98-99,  and  100-4  were  obtained, 
instead  of  100.  The  inaccuracy  increases  on  using  a  large  excess 
of  the  iron.* 

b.  Estimation  by  Precipitating  the  Manganese  with  Potas- 
sium Permanganate  (A.  GUYARD-J-). 

This  method  depends  upon  the  following  reaction  :  The  action 
of  a  solution  of  potassium  permanganate,  at  a  temperature  of  80°, 
upon  a  diluted  neutral  or  very  weakly  acid  solution  of  a  man- 
ganous  salt,  results  in  the  precipitation  of  all  the  manganese,  i.e.,. 
both  that  in  the  original  solution  and  in  the  precipitant  is  thrown 
down  as  MnOa.H2O.  The  end  of  the  reaction  is  indicated  by  the 
liquid  becoming  pink.  By  standardizing  the  permanganate  solu- 
tion against  iron  or  manganese,  the  volume  used  will  give  the 
manganese  present. 

Dissolve  1  or  2  grammes  of  the  substance  to  be  tested  in 
nitrohydrochloric  acid,  boil  for  some  time  to  convert  all  the  man- 
ganese into  chloride,  nearly  neutralize  with  an  alkali,  dilute  with 
a  large  volume  of  boiling  water  (1  to  2  litres),  heat  to  80°,  and 
maintain  at  that  temperature  while  the  titrated  solution  of  potas- 
sium permanganate  is  being  added  gradually.  A  brown  floccu- 
lent  precipitate  immediately  forms.  This  is  allowed  to  settle  from 
time  to  time,  the  operation  being  concluded  when  the  liquid  ex- 
hibits a  distinct  red  color. 

The  critical  examination  of  this  method,  conducted  in  my 
laboratory  by  R.  HABICH,^;  gave  the  following  results:  a.  In 
neutral  solutions  the  results  are  accurate.  1).  A  very  small  quan- 
tity of  free  sulphuric  acid  noticeably  increases  the  quantity  of 
permanganate  solution  required,  and  the  results  are  less  accurate, 
but  are  still  sufficiently  so  for  technical  purposes,  c.  A  quite  large 
quantity  of  sulphuric  acid  prevents  the  reaction  altogether. 
d.  Hydrochloric  acid  acts  like  sulphuric  acid,  but  even  more 
powerfully  (its  influence  may  be  neutralized,  however,  according 
to  WlNXLER.S  by  adding  finely  divided  mercuric  oxide),  e.  In 
the  presence  of  iron  or  chromium,  the  method  is  useless. 

*Zeitschr.  f.  analyt.  Chem.,  in,  209. 

f  Chem.  News,  1863,  p.  292  ;  Zeitschr.  f.  analyt.  Chem.,  in,  373. 

$  Zeitschr.  f.  analyt.  Chem.,  iir,  474.  §  Ibid.,  m,  423. 


[§  110.  NICKEL. 

f.  Nickel,  cobalt,  zinc,   aluminium,   or  calcium  does  not  inter- 
fere with  the  reaction,  the  other  conditions  being  satisfactory. 

c.   Volumetric  Determination  ~by  loiling  the  higher  oxides 
with  hydrochloric  acid,  and  estimating  the  chlorine  evolved. 
The  methods  here  employed  will  be  found  all  together  in  the 
Special  Part  under  "  A^aluation  of  Manganese  Ores." 

§  no. 

3.  NICKEL. 

a.  Solution. 

Many  nickelous  salts  are  soluble  in  water.  Those  which  are 
insoluble,  as  also  nickelous  oxide,  in  its  common  modification, 
dissolve,  without  exception,  in  hydrochloric  acid.  The  peculiar 
modification  of  nickelous  oxide,  discovered  by  GENTH,  which  crys- 
tallizes in  octahedra,  does  not  dissolve  in  acids,  but  is  rendered 
soluble  by  fusion  with  potassium  disulphate.  Metallic  nickel  dis- 
solves slowly,  with  evolution  of  hydrogen  gas,  when  warmed  with 
dilute  hydrochloric  or  sulphuric  acid ;  in  nitric  acid,  it  dissolves 
with  great  readiness.  Nickel  sulphide  is  but  sparingly  soluble  in 
hydrochloric  acid,  but  it  dissolves  readily  in  nitrohydrochloric  acid. 
Niekelic  oxide  (Ni2O3)  dissolves  in  hydrochloric  acid,  upon  the 
application  of  heat,  to  nickelous  chloride,  with  evolution  of 
chlorine. 

I.   Determination. 

Nickel  is  best  weighed  as  metal;  it  may  be  weighed  also  as 
nickelous  oxide,  or  anhydrous  sulphate  (§  79).  The  compounds 
of  nickel  are  converted  into  nickelous  oxide,  usually  by  precipi- 
tation as  nickelous  hydroxide,  preceded,  in  some  instances,  by 
precipitation  as  nickel  sulphide,  or  by  ignition.  Nickel  may 
also  be  determined  volumetrically. 

We  may  convert  into 

1.   NICKELOUS  OXIDE. 

a.     By    Precipitation     as  b.     J3y    Precipitation     as 

Nickelous  Hydroxide.  Nickel  Sulphide. 

All  nickel  salts  of  inorganic  All    compounds    of    nickel 

acids  which  are  soluble  in  water,  without  exception, 
and  all  its  salts  of  volatile  or- 
ganic acids ;  likewise  all  salts  of 
nickel  which,  insoluble  in  water, 
dissolve  in  the  stronger  acids, 
with  separation  of  their  acid. 


302  DETERMINATION  [§  HO. 

c.  By  Ignition. 

Nickel  salts  of  readily  volatile  oxygen  acids,  or  of  such  oxygen 
acids  as  are  decomposed  at  a  high  temperature  (carbonic  acid, 
nitric  acid). 

2.  METALLIC  NICKEL  :  Nickelous  oxide  (and  the  compounds 
mentioned  under  1,  a,  5,  and  c),  also  nickel  chloride,  bromide, 
and  iodide. 

3.  NICKEL  SULPHATE:   Nickel  salts,   the  acids  of  which  are 
entirely  expelled  by  heating  and  evaporating  with  sulphuric  acid. 

The  methods  of  preparing  metallic  nickel  by  simply  igniting 
the  nickelous  oxide,  or  igniting  nickel  compounds  in  a  current  of 
hvdrogen,  are  very  accurate,  but  are  applicable  only  in  certain 
cases.  The  method  1,  #,  is  most  frequently  employed,  at  times 
in  connection  with  2.  In  the  presence  of  sugar  or  other  non- 
volatile organic  substance,  it  cannot  be  used.  In  this  case  weJ 
must  either  ignite  arid  thereby  destroy  the  organic  matter  before 
precipitating,  or  we  must  resort  to  the  method  1,  5,  which  other- 
wise is  hardly  used  except  in  separations.  By  whatever  method 
nickelous  oxide  is  obtained,  it  is  best  to  convert  it  into  metallic 
nickel  (by  method  2)  before  weighing.  The  conversion  into 
nickel  sulphate  (method  3)  is  quickly  executed,  but  it  requires 
the  greatest  care  to  obtain  trustworthy  results.  Nickel  salts  of 
chromic,  phosphoric,  boric,  and  silicic  acids  are  analyzed  accord- 
ing to  the  methods  given  under  the  several  acids.  The  volu- 
metric methods  are  seldom  used  with  advantage,  and  in  point  of 
simplicity  and  accuracy  leave  much  to  be  desired. 

1.   Determination  as  Nickelous  Oxide. 

a.  By  Precipitation  as  Nickelous  Hydroxide. 

Mix  the  solution  with  pure  solution  of  potassa  or  soda  in  excess, 
heat  for  some  time  nearly  to  ebullition,  decant  3  or  4  times,  boiling 
up  each  time,  filter,  wash  the  precipitate  thoroughly  with  hot 
water,  dry  and  ignite  strongly  (avoiding  contact  with  reducing 
gases  if  the  oxide  is  to  be  weighed)  (RUSSELL*)  (§  53).  The  pre- 
cipitation is  best  effected  in  a  platinum  dish;  in  presence  of 
nitrohydrochloric  acid,  or,  if  the  operator  does  not  possess  a  suf- 
ficiently capacious  dish  of  the  metal,  in  a  porcelain  dish ;  glass 
vessels,  however,  should  be  avoided.  Presence  of  ammoniacal 

*  Journ.  Chem.  Soc.,  xvi,  58. 


§  110.]  NICKEL.  303 

salts,  or  of  free  ammonia  does  not  interfere  with  the  precipita- 
tion. For  the  properties  of  the  precipitate  and  residue,  see  §  79. 
The  method,  if  properly  conducted,  gives  very  accurate  results. 
Instead  of  weighing  the  oxide  it  may  be  reduced  to  metal  accord- 
ing to  §  110,  2.  The  thorough  washing  of  the  precipitate  is  a 
most  essential  point.  It  is  necessary  also  to  ascertain  whether  the 
weighed  metal  (or  oxide)  has  not  an  alkaline  reaction,  and  whether 
it  dissolves  completely  in  nitric  acid  (or  hydrochloric  in  case  oxide 
is  weighed). 

I.   By  Precipitation  as  Nickel  Sulphide. 

This  process  requirest  the  greatest  care  and  attention.  It 
is  best  to  proceed  according  to  one  of  the  three  following 
methods:*  a.  Neutralize  with  ammonia  if  necessary  the  moder- 
ately dilute,  cold  solution  (the  reaction  should  be  rather  slightly 
acid  than  alkaline)  contained  in  not  too  large  a  flask,  add 
ammonium  chloride  if  this  or  some  similarly  acting  ammo- 
nium salt,  like  the  acetate,  is  not  already  present  in  suf- 
ficient quantity,  and  then  carefully  add  colorless  or  pale-yellow 
ammonium  sulphide  thoroughly  saturated  with  hydrogen  sul- 
phide, so  long  as  a  precipitate  still  forms.  A  large  excess  of 
the  reagent  must  be  avoided.  After  mixing  fill  the  flask  with 
water  up  to  the  neck,  cork  it,  and  allow  it  to  stand  for  twenty- 
four  hours  without  warming,  but  in  a  moderately  warm  place. 
The  precipitate  will  then  have  settled,  and  the  supernatant  liquid 
will  be  colorless  or  slightly  yellow.  Decant,  filter,  and  wash  a& 
directed  in  §  109,  1,0.  The  filtrate  and  wash- water  must  be  color- 
less or  only  slightly  yellow.  Dry  the  precipitate  in  the  funnel, 
and  transfer  as  completely  as  possible  to  a  beaker.  Incinerate  the 
filter  in  a  coil  of  platinum  wire  or  on  a  crucible-lid,  and  add  the 
ash  to  the  dry  precipitate.  Now  digest  the  mixture  with  concen- 
trated nitrohydrochloric  acid  at  a  gentle  heat  until  all  the  nickel  sul- 
phide is  dissolved,  and  the  residual  sulphur  appears  of  a  pure  yel- 
low color.  Add  hydrochloric  acid,  evaporate  to  drive  off  the 
nitric  acid,  dilute,  filter,  and  precipitate  as  directed  in  a.  The 
properties  of  the  precipitate  are  given  in  §  79.  If  carefully  exe- 
cuted the  method  gives  accurate  results.  If  the  solution,  when 
precipitating,  contains  free  ammonia,  or  no  ammonium  salt,  the 

*  Regarding  another  method  of  precipitation  (with  sodium  thiosulphate),  see 
page  191. 


304  DETERMINATION.  [§  110. 

liquid  filtered  off  from  the  nickel  sulphide  is  always  colored  more 
or  less  brown,  and  contains  nickel  sulphide  (§  79,  0),  which  must 
be  regained  by  acidulating  with  acetic  acid  and  boiling.  If  the 
precipitate  is  not  washed  as  directed,  some  nickel  is  very,  likely 
to  pass  through  with  the  wash-water.  If  the  filter  were  not  first 
incinerated,  but,  together  writh  the  precipitate,  treated  at  once 
with  nitrohydrochloric  acid,  the  nickel  could  not  be  completely 
precipitated  by  potassa  or  soda  from  the  solution  of  nickel  sul- 
phide because  of  the  organic  substances  which  the  solution  would 
contain. 

ft.  Add  ammonium  bicarbonate  to  the  slightly  acidulated 
solution  of  nickel  so  that  the  free  acid  may  be  neutralized,  and 
the  solution  contain  a  slight  excess  of  ammonium  bicarbonate  to- 
gether with  free  carbonic  acid,  and  then  pass  in  hydrogen  sul- 
phide. As  soon  as  the  nickel  is  precipitated,  which  is  very  soon 
effected,  filter  and  treat  the  precipitate  as  in  a. 

y.  To  the  nickel  solution  first  add  ammonia  to  alkaline  re- 
action, then  add  a  fairly  large  quantity  of  sodium  (or  ammonium) 
acetate,  ammonium  sulphide  in  good  excess,  then  acetic  acid  to 
decidedly  acid  reaction,  and  finally  heat  to  boiling.  The  precipi- 
tate formed  settles  well  and  is  treated  as  in  a.  Neutralize  the 
filtrate  with  ammonia,  and  test  it  by  adding  ammonium  sulphide ; 
if  a  black  color  develops  acidulate  with  acetic  acid  and  heat  in 
order  to  precipitate  the  last  portion  of  nickel  as  sulphide. 

It  is  not  advisable  to  weigh  the  nickel  sulphide  obtained  on 
ignition  with  sulphur  in  hydrogen. 

G.  By  direct  Ignition. 

The  same  method  as  described  in  §  109,  1,  e.     (Manganese.) 

2.  Determination  as  metallic  Nickel. 

Ignite  the  oxide  or  chloride  to  be  reduced  in  a  porcelain  cru- 
cible in  a  slow  stream  of  hydrogen  (compare  §  108,  2)  at  first 
gently,  then  more  strongly  till  the  weight  is  constant.  For 
properties  of  the  residue,  see  §  79,  c.  If  on  dissolving  the  metal 
in  nitric  acid  any  silica  remains,  this  must  be  weighed  and  de- 
ducted. 

3.  Determination  as  Nickel  Sulphate. 

The  nickel  solution  should  be  free  from  other  non-volatile 
.salts.  Evaporate  with  a  slight  excess  of  pure  sulphuric  acid  in  a 
platinum  dish  to  dryness  and  heat  for  15  or  20  minutes  rnoder- 


111-]  COBALT.  305 

ately,  so  as  just  to  drive  off  the  excess  of  sulphuric  acid  without 
blackening  the  yellow  sulphate  at  the  edges.  It  is  difficult  to  be 
sure  of  hitting  the  exact  point,  hence  we  can  place  no  dependence 
on  this  method  nor  on  that  of  GIBBS,  which  consists  in  dissolving 
the  sulphide  in  nitric  acid  and  evaporating  the  solution  with  sul- 
phuric acid.  For  the  properties  of  the  residue,  see  §  79,  d. 

4.  Determination  of  Nickel  Volumetrically. 
KUNZEL*  precipitates  with  sodium  sulphide,  using  sodium 
nitroprussiate  or  an  ammoniacal  silver  solution  as  an  indicator  of 
excess  of  reagent.  WICKE  f  and  FLEISCHER  f  precipitate  by  boil- 
ing with  sodium  hypochlorite  and  caustic  soda,  and  determine  the 
quantity  of  oxide  by  its  oxidizing  action  on  arsenous  acid  or  fer- 
rous oxide.  FR.  MOHR  §  determines  the  action  on  potassium 
iodide.  GIBBS  [  precipitates  with  oxalic  acid  and  alcohol,  and 
determines  the  oxalic  acid  in  the  precipitate  with  potart  urn  per- 
manganate. 

§111. 

4.   COBALT. 

a.  Solution. 

Cobalt  and  its  compounds  behave  with  solvents  like  the  corre- 
sponding compounds  of  nickel.  The  protosesquioxide  of  cobalt 
obtained  by  SCHWARZENBERG  in  microscopic  octahedra  does  not 
dissolve  in  boiling  hydrochloric  acid,  or  nitric  acid,  or  nitrohydro- 
chloric  acid;  but  it  dissolves  in  concentrated  sulphuric  acid,  and 
in  fusing  potassium  disulphate. 

5.  Determination. 

Cobalt  is  determined  in  the  metallic  state  (§  80)  or  as  sulphate, 
being  usually  first  precipitated  as  cobaltous  hydroxide,  sulphide,  or 
tripotassium-cobaltic  nitrate.  Cobalt  may  also  be  determined 
volumetrically. 

We  may  convert  into 
1 .   METALLIC  COBALT  : 

a.  By  direct  reduction.  All  salts  of  cobalt,  which  can  be 
immediately  reduced  by  hydrogen  (chloride,  nitrate,  carbonate, 
etc.). 

*  Zeitschr.  /.  analyt.  Chem. ,  n,  373.         f  Ibid. ,  iv,  424.         J  Ibid.,  x,  219. 
%Lehrbuch  der  Titrirmetkode ,  3d  edit.,  p.  303. 
I  Zeitschr.  f.  analyt.  Chem.,  vn,  259. 


306  DETERMINATION.  [§  111. 

h.  By  precipitation  as  cobaltous  hydroxide.  All  salts  of  in- 
organic acids  soluble  in  water,  and  insoluble  salts  of  such  acids  as 
may  be  removed  by  solution.  All  salts  of  volatile  organic  acids. 

c.  By  precipitation  as  sulphide.  All  compounds  of  cobalt 
without  exception. 

d.  By  precipitation    as   tripotassium-cobaltic  nitrite.     All 
compounds  of  cobalt  soluble  in  water  or  dilute  acetic  acid. 

2.  COBALT  SULPHATE  : 

a.  By  simple  evaporation  and  ignition. — The  oxygen    com- 
pounds of  cobalt  and  all  cobaltous  salts  of  acids  which  may  be 
completely  expelled  by  evaporation  and  ignition  with  sulphuric 
acid. 

b.  By  precipitation  as   sulphide. — All    compounds  of   cobalt 
without  exception. 

The  method  1,  a,  is  preferable  to  all  others  when  it  can  be 
applied ;  it  is  quick  and  gives  exact  results.  The  method  1,  J, 
gives  better  results  than  it  used  to  be  credited  with.  The  direct 
conversion  of  suitable  cobalt  compounds  into  sulphate  is  also  quite 
satisfactory.  The  precipitations  as  sulphide  and  as  tripotassium 
cobaltic  nitrate  are  rarely  used  except  in  separations.  The  volu- 
metric methods  are  better  adapted  for  technical  than  for  scientific 
purposes. 

1.  Determination  as  metallic  Cobalt. 

a.  By  direct  reduction. 

Evaporate  the  solution  of  cobaltous  chloride,  or  nitrate  (which 
must  be  free  from  sulphuric  acid  and  alkali),  in  a  weighed  crucible, 
to  dryness,  cover  the  crucible  with  a  lid  having  a  small  aperture  in 
the  middle,  conduct  through  this  a  moderate  current  of  pure  dry 
hydrogen,  and  then  apply  a  gentle  heat,  which  is  to  be  increased 
gradually  to  intense  redness.  When  the  reduction  is  considered 
complete,  allow,  to  cool  in  the  current  of  hydrogen,  and  weigh ; 
ignite  again  in  the  same  way  and  repeat  the  process  until  the 
weight  remains  constant.  The  results  are  accurate.  For  the 
properties  of  cobalt,  see  §  80. 

As  regards  the  apparatus  to  be  employed,  see  §  108,  2. 

~b.  By  precipitation  as  cobaltous  hydroxide. 

The  best  material  for  the  precipitating  vessel  is  platinum, 
porcelain  may  also  be  used,  but  not  glass.  First  remove  any  large 
excess  of  acid  which  may  be  present  by  evaporation.  Heat  nearly 


§  111.]  COBALT.  307 

to  boiling,  add  pure  potassa  in  slight  excess,  and  continue  heating 
till  the  precipitate  is  brownish-black.  Pour  the  supernatant  fluid 
through  a  filter,  wash  the  precipitate  by  decantation  with  boiling 
rater  repeatedly,  transfer  it  to  the  filter,  and  continue  the  washing 
with  boiling  water  till  the  washings  are  free  from  any  trace  of 
dissolved  substance.  Dry,  ignite  in  a  porcelain  crucible  (§  52)  till 
the  filter  is  thoroughly  burnt,  reduce  in  a  current  of  hydrogen, 
wash  the  metal  several  times  with  boiling  water,  dry,  ignite  again 
in  hydrogen  and  weigh.  Test  the  weighed  cobalt  by  dissolving  in 
nitric  acid.  If  any  silica  remains,  this  must  be  weighed  and  de- 
ducted. Mix  the  solution  with  ammonium  chloride  and  ammo- 
nium carbonate,  if  a  small  precipitate  (alumina  or  perhaps  a 
trace  of  ferric  hydroxide)  forms,  ignite  and  weigh  this  too  and 
deduct  it.  The  results  are  excellent ;  the  amount  of  alkali  which 
remains  with  the  metal  when  the  work  is  done  properly  being  ex- 
ceedingly minute.  Compare  §  80,  a. 

c.  By  precipitation  as  sulphide. 

Put  the  solution  in  a  flask,  add  ammonium  chloride,  then 
ammonia  just  in  excess,  then  ammonium  sulphide  as  long  as  a 
precipitate  is  produced,  fill  up  to  the  neck  with  water,  cork  and 
allow  to  stand  12  or  24  hours  in  a  warm  place.  Decant,  filter,  and 
wash  as  directed  §  109,  2.  Finally,  dry  and  proceed  as  directed 
§  110,  5,  a,  to  redissolve  the  cobalt  sulphide.  Determine  the 
cobalt  according  to  Z>.  There  are  no  sources  of  error  in  the  pre- 
cipitation with  ammonium  sulphide.  For  the  properties  of  cobalt 
sulphide,  see  §  80.  It  cannot  be  brought  into  a  weighable  form 
by  ignition  in  hydrogen,  as  the  residue  is  a  variable  mixture  of 
different  sulphides  (II.  ROSE).  Cobalt  may  also  be  thrown  down 
as  sulphide  by  the  other  methods  given  under  Nickel.  The 
thorough  precipitation  of  cobalt  is  much  easier  than  that  of  nickel. 

d.  By  precipitation  as  tripotassium  cobaltic  nitrate. 

To  the  moderately  concentrated  solution  of  the  cobalt  salt  add 
potassa  in  excess,  then  acetic  acid  till  the  precipitate  is  just  redis- 
solved,  then  a  concentrated  solution  of  potassium  nitrite  previously 
just  acidified  with  acetic  acid,  and  allow  to  stand  24  hours  at  a 
gentle  heat.  Filter,  wash  with  solution  of  potassium  acetate  (1  in 
10)  containing  some  potassium  nitrite,  till  all  foreign  substances  are 
removed,  dry,  dissolve  with  the  filter  ash  in  hydrochloric  acid, 
filter  and  determine  the  cobalt  according  to  1,  l>.  This  method 


308  DETERMINATION.  [§  111. 

was  introduced  by  A.  STKOMEYEK  ; *  the  present  modification,  first 
suggested  by  H.  ROSE,  and  improved  by  FR.  GATJHE,  is  the  surest 
to  yield  good  results  (GAUHEf).  For  the  properties  of  the  pre- 
cipitates, see  §  80,  e. 

2.  Determination  as  sulphate. 

a.  By  direct  conversion. 

To  the  solution  of  cobaltous  sulphate  add  a  little  more  sul- 
phuric acid  than  will  suffice  to  form  cobaltous  sulphate  with  all 
the  cobalt  present  if  a  volatile  acid  is  present.  Evaporate,  using 
a  platinum  dish  or  platinum  crucible,  at  all  events,  to  finish  the 
operation.  Heat  the  residue  cautiously  over  the  lamp,  gradually 
increasing  the  temperature  to  dull  redness,  and  maintain  at  this 
point  for  15  minutes.  Should  the  edges  blacken,  moisten  with 
dilute  sulphuric  acid,  dry,  and  ignite  again  with  greater  caution, 
Properties  of  the  precipitate,  §  80.  Results  quite  satisfactory. J 

b.  With  previous  precipitation  as  sulphide. 

Precipitate  the  cobalt  as  sulphide  according  to  1,  <?,  dissolve  it 
as  directed,  evaporate  with  excess  of  sulphuric  acid  in  a  porcelain 
dish  to  dryness,  take  up  the  residue  with  water,  transfer  the  solu- 
tion to  a  weighed  platinum  dish  and  proceed  according  to  2,  a. 

3.  Volumetric  Methods  of  Estimating  Cobalt. 

1.   According  to  CL.  WINKLEK.§ 

Principle:  On  adding  finely  divided  mercuric  oxide,  sus- 
pended in  water,  to  an  aqueous  solution  of  cobaltous  chloride 
(§  60,  4),  no  decomposition  takes  place,  and  no  cobaltous  hydrox- 
ide is  precipitated.  On  adding  a  solution  of  potassium  now,  hy- 
drated  manganese  dioxide  and  cobaltic  hydroxide  are  precipitated. 

(6CoCl2  +  5HgO  +  11H20  +  2KMn04  =  3Co2[OH]6 
+  2MnO2[H2O]  +  5IIgCl2+  2KC1). 

This  equation  does  not  exactly  express  the  change  that  occurs, 
because  with  the  cobaltic  hydroxide  there  is  always  precipitated 
a  certain  proportion  of  cobaltous  hydroxide  (or  perhaps  a  com- 
pound intermediate  between  Co2O3  and  CoO) ;  hence  the  perman- 

*  Annal.  d.  Chem.  u.  Pharm.,  xcvi,  218.         \Zeitschr.f.  analyl.  Chem.,  iv,  60. 
\  Compare  GAUHE,  Zeitschr.  f.  analyt.  Chem.,  iv,  55. 
§Zeitschr.f.  analyt.  Chem.,  in,  265;  in,  420;  vn,  48. 


§111.]  COBALT.  309 

ganate  solution  (§  112,  2)  used  must  be  standardized,  not  against 
iron  or  oxalic  acid,  but  against  cobaltous  chloride,  in  order  to 
determine  cobalt  volumetrically. 

Execution.  —Dissolve  O'l  or  0*2  grm.  of  pure  metallic  cobalt  * 
in  warm  hydrochloric  acid,  transfer  to  a  stoppered  300-c.  c.  flask, 
dilute  to  200  c.  c.,  add  an  excess  of  mercuric  oxide  suspended  in 
water,  and  into  the  cold  liquid  run  in  from  the  burette  the  potas- 
sium-permanganate solution  (5  to  6  grm.  of  the  pure  crystallized 
salt  to  the  litre)  in  small  portions  and  under  constant  agitation 
until  permanent  redness  of  the  liquid,  in  which  the  brown  pre- 
cipitate remains  suspended.  It  is  diffcult  at  first  to  see  the  color  of 
the  liquid,  but  it  is  readily  observed  towards  the  end,  because  the 
precipitate  settles  the  better  the  nearer  the  end  of  the  reaction. 
The  addition  of  more  mercuric  oxide  facilitates  the  deposition. 
The  operator  must  not  be  misled  by  the  gradual  disappearance  of 
the  color  which  occurs  on  long  standing.  The  c.  c.  of  perman- 
ganate solution  used  up  correspond  to  the  cobalt  weighed  off. 
In  applying  the  process  to  the  estimation  of  unknown  quantities 
of  cobalt,  proceed  similarly,  taking  pains  to  duplicate  as  nearly  as 
possible  the  conditions  as  regards  the  quantities  of  cobalt  and  the 
mercuric  oxide  added,  as  well  as  the  dilution. 

If  the  cobalt  solution  contains  sulphuric,  phosphoric,  or  arsenic 
acid,  or  oxygen  acids  of  nitrogen  or  chlorine,  or  organic  acids,  the 
method  as  above  detailed  is  useless ;  ferric  chloride,  however,  if 
present,  has  no  injurious  action,  because  the  mercuric  oxide  im- 
mediately precipitates  all  the  iron  as  ferric  hydroxide. 

The  injurious  influence  of  sulphuric  acid  may  always  be  neu- 
tralized by  adding  a  slight  excess  of  barium  chloride;  that  of 
moderate  quantities  of  phosphoric  or  arsenic  acid  may  be  destroyed 
by  first  adding  a  sufficient  quantity  of  ferric  chloride,  and  only 
then  adding  the  mercuric  oxide.  If  care  be  taken  to  add  1  part 
of  iron  for  every  part  of  arsenic  or  phosphoric  acid,  the  acid  will 


*  According  to  WINKLER  pure  metallic  cobalt  may  be  obtained  as  follows: 
Place  a  porcelain  crucible,  one-tliird  filled  with  purpureo-cobaltic  chloride  re- 
peatedly recrystallized  and  free  from  nickel,  within  a  large  platinum  crucible, 
the  lid  of  which  is  perforated  and  provided  with  a  gas-conducting  tube,  and  ig- 
nite in  a  current  of  hydrogen,  gently  at  first;  then,  when  most  of  the  ammonium 
chloride  is  expelled,  increase  the  heat,  and  finally  raise  to  the  highest  point,  and 
until  no  trace  of  hydrochloric  acid  is  given  off;  then  allow  to  cool  in  a  current 
of  hydiogen. 


310  DETERMINATION.  [§  112. 

be  completely  precipitated  in  the  form  of  basic  salts ;  nor  need 
these  salts  or  the  barium  sulphate  formed  be  filtered  off  before 
proceeding  to  titrate. 

If  the  cobalt-chloride  solution  contains  manganese,  the  method 
is  useless.  Small  quantities  of  nickel  do  no  harm,  but  large  are 
injurious.  Compare  §  160  (Separation  of  Cobalt  and  Nickel). 
The  results  do  not  satisfy  the  highest  requirements  so  far  as 
accuracy  is  concerned,  but  are  perfectly  satisfactory  for  technical 
purposes. 

2.  In  regard  to  other  methods  of  volumetrically  estimating 
cobalt,  see  Nickel.  All  the  methods  there  given  are  applicable  to 
cobalt.  FLEISCHER'S  method  will  also  be  detailed  under  Separa- 
tion of  Cobalt  and  Nickel  (§  160). 

§112. 

5.   FERROUS  IRON. 

a.   Solution. 

Many  ferrous  compounds  are  soluble  in  water.  Those  which 
are  insoluble  in  water  dissolve  almost  without  exception  in  hydro- 
chloric acid ;  the  solutions,  if  not  prepared  with  perfect  exclusion 
of  air,  and  with  solvents  absolutely  free  from  air,  contain  invari- 
ably more  or  less  ferric  chloride.  In  cases  where  it  is  wished  to 
avoid  the  chance  of  oxidation,  the  solution  of  the  ferrous  com- 
pound is  effected  in  a  small  flask,  through  which  a  slow  current  of 
carbonic-acid  gas  is  passed,  the  transmission  of  the  gas  being  con- 
tinued until  the  solution  is  cold.  Many  native  ferrous  compounds 
cannot  be  thus  dissolved.  They  are,  indeed,  rendered  soluble  by 
fusing  with  sodium  carbonate,  but  in  this  process  ferric  oxide  is 
formed.  It  is  therefore  advisable  to  heat  such  substances  (in  the 
finest  powder)  with  a  mixture  of  3  parts  concentrated  sulphuric 
acid  and  1  part  water  in  a  strong  sealed  tube  of  Bohemian  glass 
for  2  hours  at  about  210°,  or — in  the  case  of  silicates — to  \varm 
them  with  a  mixture  of  2  parts  hydrochloric  acid  and  1  part  strong 
hydrofluoric  acid  in  a  covered  platinum  dish  (A.  MITSCHEBLICH  *). 
It  is  advisable  to  cover  the  water-bath  on  which  the  platinum  dish  is 
heated  with  a  ring  of  plaster  of  Paris  about  O'l  metre  high,  and 

*  Journ.  f.  prakt.  CJiem.,  LXXXI,  116. 


§  112.]  FERROUS   IRON.  311 

to  place  on  this  a  plaster  plate  having  a  section  cut  out  at  one 
side  through  which  carbon  dioxide  is  conducted,  so  that  solution 
may  take  place  in  an  atmosphere  that  will  not  oxidize  the  iron.* 
Metallic  iron  dissolves  in  hydrochloric  acid,  and  in  dilute  sul- 
phuric acid,  as  ferrous  chloride  or  sulphate  respectively,  with  evo- 
lution of  hydrogen ;  in  warm  nitric  acid  it  dissolves  as  ferric 
nitrate,  and  in  nitrohydrochloric  acid  as  ferric  chloride. 

I .   Detenu  ination . 

Ferrous  iron  may  be  estimated  1,  by  dissolving,  converting 
into  ferric  iron,  and  determining  the  latter  gravimetrically  or 
volumetrically  ;  2,  by  precipitating  as  sulphide  and  weighing  it  as 
such,  or  determining  it  after  conversion  into  a  ferric  salt;  3,  by  a 
direct  volumetric  method  ;  and  4,  by  treating  with  gold  trichloride 
and  weighing  the  reduced  gold. 

The  methods  1  and  2  are,  of  course,  only  applicable  when  no 
ferric  compound  is  present ;  the  method  2  is  scarcely  ever  used 
except  for  separations.  The  methods  included  under  3  are  adapted 
to  most  cases,  and,  in  absence  of  other  reducing  substances,  are 
especially  worthy  of  recommendation.  The  method  A  will  be 
briefly  treated  of  in  the  supplement  to  §§  112  and  113. 

As  the  determination  of  iron  as  ferric  oxide  belongs  to  §  113, 
and  as  the  process  for  precipitating  ferrous  iron  as  sulphide  is  the 
same  as  that  for  precipitating  ferric  iron  in  this  form,  nothing 
remains  for  us  here  but  to  describe  the  methods  of  converting 
ferrous  into  ferric  salts  and  the  processes  included  under  3. 

1.  Methods  of  converting  Ferrous  into  Ferric  Iron. 

a.  Methods,  applicable  in  all  cases. 

Heat  the  solution  of  the  ferrous  salt  with  hydrochloric  acid  and 
add  small  portions  of  potassium  chlorate,  till  the  fluid,  even  after 
warming  for  some  time,  still  smells  strongly  of  chlorine.  Our 
object  may  be  also  attained  by  passing  chlorine  gas  or — in  the  case 
of  small  quantities — by  addition  of  chlorine  water,  or  very  con- 
veniently by  adding  solution  of  bromine  in  hydrochloric  acid.  If 
the  solution  is  required  to  be  free  from  excess  of  chlorine  or 
bromine,  it  is  finally  heated,  till  all  odor  of  chlorine  or  bromine 
lias  disappeared. 

*  Somewhat  more  complicated  apparatus  for  attaining  the  object  have  been 
ilcMTibcd  by  COOKE  (Zeitschr.  /.  analyl.  Ghent.,  vn,  98)  and  WILBUR  and 
WHITTLESEY  (ibid.,  x,  98). 


312  DETERMINATION  [§  112, 

J.  Methods  which  are  only  suitable  when  the  iron  is  to  "be  siibs& 
quently  precipitated  by  ammonia,  as  ferric  hydroxide. 

Mix  the  solution  of  the  ferrous  salt  in  a  flask  with  a  little 
hydrochloric  acid,  if  it  does  not  already  contain  any ;  add  some 
nitric  acid,  and  heat  the  mixture  for  some  time  to  incipient  ebulli- 
tion. The  color  of  the  fluid  will  .show  whether  the  nitric  acid  has 
been  added  in  sufficient  quantity.  Though  an  excess  of  nitric  acid 
does  no  harm,  still  it  is  better  to  avoid  adding  too  much  on  account 
of  the  subsequent  precipitation.  In  concentrated  solutions,  the 
addition  of  nitric  acid  produces  a  dark-brown  color,  which  disap- 
pears upon  heating.  This  color  is  owing  to  the  nitrogen  dioxide 
(N2O2)  formed  dissolving  in  the  portion  of  the  solution  which  still 
contains  ferrous  salt. 

c.  Methods  which  can  be  employed  only  when  the  ferric  iron  is. 
to  l)e  determined  volumetrically. 

Add  to  the  hydrochloric  solution  small  quantities  of  artificially 
prepared  iron-free  manganese  dioxide,  till  the  solution  is  of  a  dark 
olive-green  color  from  the  formation  of  manganic  chloride ;  boil 
till  this  coloration  and  the  odor  of  chlorine  have  disappeared  (Fit. 
MOHR)  ;  or  you  may  add  pure  potassium  permanganate  (in  crystals 
or  concentrated  solution)  till  the  fluid  is  just  red  and  then  boil,  till 
the  red  color  and  chlorine-odor  have  vanished.  These  methods 
present  the  advantage  of  permitting  complete  conversion  of  ferrous 
into  ferric  salts  without  the  use  of  any  considerable  excess  of  the 
oxidizing  agent. 

2.   Volumetric  Determination. 

a.  MARGUERITE'S  Method. 

If  we  add  to  a  solution  of  ferrous  salt,  containing  an  excess  of 
sulphuric  acid,  potassium  permanganate,  the  former  is  converted 
into  a  ferric  salt  by  the  oxidizing  action  of  the  latter  (10FeSO4  + 
8H2S04  +  K2Mn208=  5Fe,(SO4),  +  KQSO4  +  2MnSO4  +  8H2O). 
Now  if  we  possess  a  solution  of  potassium  permanganate,  and  know 
how  much  iron  100  c.c.  of  it  can  convert  from  the  ferrous  to  the  ferric 
condition,  we  can,  with  this,  readily  determine  an  unknown  quan- 
tity of  iron ;  we  have  simply,  for  this  purpose,  to  dissolve  the  iron 
in  acid,  in  the  form  of  a  ferrous  salt,  to  oxidize  the  solution  accu- 
rately, and  note  how  many  c.c.  of  the  solution  of  potassium  per- 
manganate have  been  used  to  accomplish  that  object. 


§112.]  FERKOUS   IKON.  313 

It  must  be  remarked  here  that  the  reaction  takes  place  accord- 
ing to  the  above  equation  only  if  the  free  acid  present  is  sulphuric 
acid,  whereas  in  the  presence  of  hydrochloric  acid  (see  y),  certain 
changes  occur  (LOWENTHAL  and  LENSSEN*).  The  changes  may, 
however,  be  to  some  extent  compensated  by  operating  in  a  certain 
manner,  but  the  results  cannot  be  considered  as  reliable  (see  y). 

a.    Titration  of  the  Solution  of  Potassium  Permanga- 
nate. 

Dissolve  5  grm.  (roughly  weighed)  of  pure  crystallized  potas- 
sium permanganate  in  distilled  water  by  the  aid  of  heat,  dilute  to 
1  litre,  and  preserve  in  glass- stoppered  bottle.  Action  of  direct 
sunlight  on  the  solution  should  be  avoided.  The  solution  if  care- 
fully kept  does  not  alter,  but  still  it  is  well  to  titrate  it  afresh 
occasionally,  while  care  must  be  taken  to  avoid  contamination  with 
organic  matter  (as  on  opening  the  bottle). 

aa.    Titration  ~by  Metallic  Iron. 

Weigh  off  accurately  about  1  grm.  thin  soft  iron  wire  previ- 
ously cleaned  with  emery  paper,  transfer  to  a  J-litre  measuring 
flask  containing  100  c.  c.  dilute  sulphuric  acid  (1  to  5),  add  about 
1  grm.  sodium  bicarbonate,  to  produce  carbonic  acid  and  expel 
the  air,  and  then  close  the  flask  with  an  india-rubber  stopper  pro- 
vided with  an  evolution  tube,  as  shown  in  Fig.  84 ;  c  contains  20 
or  30  c.  c.  water.  Heat  the  flask  at  first  gently,  finally  to  gentle 
boiling  till  the  iron  is  dissolved.  The  clip  b  is  open,  and  the 
hydrogen  escapes  through  the  water  in  c.  Meanwhile  boil  about 
300  c.  c.  distilled  water,  to  drive  out  all  the  air  it  contains,  and 
then  allow  it  to  cool.  As  soon  as  the  iron  is  entirely  dissolved, 
remove  the  lamp  and  close  the  evolution  tube  with  the  clip.  When 
the  iron  solution  has  cooled  a  little  loosen  the  clip  and  allow  the 
water  in  c  to  recede,  pour  the  boiled  water  into  c  and  allow  this 
also  to  recede  till  the  solution  nearly  reaches  the  mark.  Take  out 
the  evolution  tube  and  close  the  flask  with  an  unperf orated  stopper, 
allow  to  cool  to  the  temperature  of  the  room,  fill  with  water  to  the 
mark,  shake,  and  allow  to  stand,  so  that  the  particles  of  carbon 
usually  present  may  deposit.  Now  take  out  with  a  pipette  50  c.  c. 
of  the  clear  and  nearly  colorless  fluid  (containing  -J-  of  the  iron 

*Zeitschr.f.  analyt.  Chem.,  i,  329. 


314  DETERMINATION.  [§  112. 

weighed  off),  transfer  to  a  400  c.  c.  beaker,  and  dilute  till  the 
beaker  is  half  full.  Place  the  beaker  on  a  sheet  of  white  paper, 
or  better,  on  a  sheet  of  glass,  with  white  paper  underneath. 


Fig.  84. 

Fill  a  GAY-LUSSAC'S  or  GEISSLER'S  burette  of  30  c.  c.  capacity, 
divided  into  O'l  c.  c.  (see  Figs.  23  and  24),  up  to  zero  with  solu- 
tion of  potassium  permanganate,  of  which  take  care  to  have  ready 
a  sufficient  quantity  perfectly  clear  and  uniformly  mixed. 

Now  add  the  permanganate  to  the  ferrous  solution,  stirring  the 
latter  all  the  while  with  a  glass  rod.  At  first  the  red  drops  dis- 
appear very  rapidly,  then  more  slowly.  The  fluid,  which  at  first 
was  nearly  colorless,  gradually  acquires  a  yellowish  tint.  From 
the  instant  the  red  drops  begin  to  disappear  more  slowly,  add  the 
permanganate  with  more  caution  and  in  single  drops,  until  the  last 
drop  imparts  to  the  fluid  a  faint  but  unmistakable  reddish  color, 
which  remains  on  stirring.  A  little  practice  will  enable  you 
readily  to  hit  the  right  point.  As  soon  as  the  fluid  in  the  burette 
has  sufficiently  collected  read  off  again  and  mark  the  number  of 
c.  c.  used.  The  reading  off  must  be  performed  with  the  greatest 
exactness  (see  §  22) ;  the  whole  error  should  not  be  more  than 
0-1  c.  c. 


§112.]  FEKROUS   IRON.  315 

The  amount  of  permanganate  solution  used  should  be  about  20 
c.  c.  Repeat  the  experiment  with  another  50  c.  c.  of  the  iron  solu- 
tion. The  difference  between  the  permanganate  used  in  the  two 
cases  should  not  be  more  than  0-1  c.  c.  ;  if  it  is,  make  one  more 
experiment,  arid  when  the  results  are  sufficiently  near  take  the  mean. 
Now  calculate  what  quantity  of  iron  is  represented  by  100  c.  c.  of 
the  permanganate.  To  this  end  first  divide  the  iron  weighed  off 
by  5,  and  then  multiply  by  0*996,  since  soft  iron  wire  contains  on 
the  average  0*4  per  cent,  carbon,  &c. ;  this  gives  the  quantity  of 
pure  iron  contained  in  50  c.  c.  of  the  solution.  Suppose  we  took 
1-05  grm.  iron  wire  and  used  a  mean  of  21'3  c.  c.  permanganate, 
-uyuL  =  0-210,  0-210  X  0-996  =  0-20916,  and  then  by  rule 
of  three 

21-3:  0-20916  ::  100  :  #;  x  =  0-98197; 

therefore  100  c.  c.  permanganate  =  0*98197  pure  iron. 

If  there  is  a  deficiency  of  free  acid  in  the  solution  of  iron,  the 
fluid  acquires  a  brown  jcolor,  turns  turbid,  and  deposits  a  brown 
precipitate  (manganese  dioxide  and  ferric  hydroxide).  The  same 
may  happen  also  if  the  solution  of  potassium  permanganate  is 
added  too  quickly,  or  if  the  proper  stirring  of  the  iron  solution  is 
omitted  or  interrupted.  Experiments  attended  with  abnormal 
manifestations  of  the  kind  had  always  better  be  rejected.  That 
the  fluid  reddened  by  the  last  drop  of  solution  of  potassium 
permanganate  added,  loses  its  color  again  after  a  time,  need  create 
no  surprise  or  uneasiness;  this  decolorization  is,  in  fact,  quite 
inevitable,  as  a  dilute  solution  of  free  permanganic  acid  cannot 
keep  long  undecomposed. 

lib.  Titration  ~by  Ammonium  Ferrous  Sulphate. 

Weigh  off,  with  the  greatest  accuracy,  about  1*4  grm.  of  the 
pure  salt  prepared  according  to  the  directions  given  in  §  65,  4, 
dissolve  in  about  200  c.c.  distilled  water,  previously  mixed  with 
about  20  c.c.  dilute  sulphuric  acid,  and  proceed  as  in  aa. 

By  dividing  the  amount  of  salt  weighed  off  by  7*0014  (or  where 
great  accuracy  is  not  required  by  7)  we  obtain  the  quantity  of  iron 
corresponding. 

If  the  salt  is  not  pure,  for  instance  should  it  contain  basic 
radicals  isomorphous  with  ferrous  iron  (manganese,  magnesium, 


316  DETERMINATION.  [§  112. 

etc.);  or  if  it  contains  ferric  iron,  or  is  moist,  the  result  will  of 
course  be  too  high. 

cc.   Tit  ration  by  Oxalic  Acid. 

If  solution  of  potassium  permanganate  is  added  to  a  warm 
solution  of  oxalic  acid,  mixed  with  sulphuric  acid,  the  liberated 
permanganic  acid  oxidizes  the  oxalic  acid  to  carbon  dioxide  and 
water  [5H2C2O4  +  2KMnO4  +  3H,SO4  =  K2SO4  +  2MnSO4  + 
10CO,  +  8H2O].  For  the  oxidation  of  1  mol.  oxalic  acid  (H2C2O4) 
and  2  at.  iron  (in  the  ferrous  state)  equal  quantities  of  permanganic 
acid  are  accordingly  required;  therefore,  126*04:8  parts  (1  mol.) 
of  crystallized  oxalic  acid  correspond,  in  reference  to  the  oxidiz- 
ing action  of  permanganic  acid,  to  111-8  parts  (2  at.)  of  iron. 

A  solution  of  oxalic  acid  is  altered  by  the  action  of  light ;  it  is, 
therefore,  well  only  to  dissolve  as  much  as  will  be  required  for 
immediate  use.  Dissolve  1  to  1*2  grm.  pure  acid  prepared  by 
§  65,  1,  to  250  c.c. ;  50  c.c.  of  this  solution  are  introduced  into 
a  beaker,  diluted  with  about  100  c.c.  water,  from  6  to  8  c.c.  cone, 
sulphuric  acid  added,  and  the  fluid  heated  to  about  60°.  The  beaker 
is  then  placed  on  a  sheet  of  white  paper,  and  permanganate  added 
from  the  burette,  with  stirring.  The  red  drops  do  not  disappear 
at  first  very  rapidly,  but  when  once  the  reaction  has  fairly  set  inr 
they  continue  for  some  time  to  vanish  instantaneously.  As  soon 
as  the  red  drops  begin  to  disappear  more  slowly,  the  solution  of 
potassium  permanganate  must  be  added  with  great  caution;  if 
proper  care  is  taken  in  this  respect,  it  is  easy  to  complete  the 
reaction  with  a  single  drop  of  permanganate ;  this  completion  of 
the  reaction  is  indicated  with  beautiful  distinctness  in  the  colorless 
fluid.  To  find  the  iron  corresponding  to  the  permanganate  used, 
multiply  the  amount  of  crystallized  oxalic  acid  in  the  50  c.c.  by  8 
and  divide  by  9. 

If  the  oxalic  acid  was  not  perfectly  dry,  or  hot  quite  pure,  the 
result  of  the  experiment  will,  of  course,  lead  to  fixing  the  strength 
of  the  solution  of  potassium  permanganate  too  high.  Instead  of 
pure  oxalic  acid,  SAINT-GILLES  has  proposed  to  use  crystallized 
oxalate  of  ammonium  (NII4)2C2O4  +  H3O).  This  can  easily  be  pre- 
pared in  the  pure  state,  keeps  well,  and  can  be  weighed  with 
accuracy.  142*16  parts  of  the  crystallized  salt  correspond  to- 
Ill- 8  parts  iron. 


§  112.]  FERROUS    IKON.  317 

Of  the  foregoing  three  methods  of  standardizing  solution  of 
potassium  permanganate,  the  first  is  the  one  originally  proposed  by 
MARGUERITE.  Ammonium  ferrous  sulphate  was  first  proposed  by 
FR.  MOIIR,  and  oxalic  acid  by  HEMPEL,  as  agents  suitable  for  the 
purpose.  With  absolutely  pure  and  thoroughly  dry  reagents,  and 
proper  attention,  all  three  methods  give  correct  results. 

For  myself,  I  prefer  the  first  method,  as  the  most  direct  and 
positive,  the  only  doubtful  point  about  it  being  the  question 
whether  the  assumption  that  the  iron  wire  contains  99'6  per  cent, 
of  chemically  pure  iron  is  quite  correct ;  this,  however,  is  of  very 
trining  importance,  as  the  error  could  not  exceed  0*1  or  0*2  per 
cent.*  The  other  two  methods  are,  as  may  readily  be  seen,  some- 
what more  convenient,  but  they  are  not  so  trustworthy  unless  you 
can  insure  the  purity  and  dryness  of  the  preparations. 

For  the  analysis  of  very  dilute  solutions  of  iron,  e.g.,  chalybeate 
water,  in  which  the  amount  of  iron  may  be  very  approximately 
determined  with  great  expedition,  by  direct  oxidization  with  per- 
manganate, a  very  dilute  standard  solution  must  be  prepared. 
Such  a  solution  may  be  made  by  diluting  the  previous  solution 
with  9  parts  of  water  or  by  dissolving  0-5  grin,  crystals  of  potassium 
permanganate  in  1  litre  of  water.  It  is  to  be  directly  standardized 
with  correspondingly  small  quantities  of  iron,  ferrous  salt,  or  oxalic 
acid. 

In  experiments  of  this  kind,  the  fact  that  a  certain  quantity  of 
permanganate  is  required  to  impart  a  distinct  color  to  pure  acidi- 
fied water  (which  is  of  no  consequence  in  operations  where  the 
concentrated  solution  is  used)  must  be  taken  into  consideration  ;  for 
where  the  solution  used  is  so  highly  dilute,  it  takes  indeed  a  measur- 
able quantity  of  it  to  impart  the  desired  reddish  tint  to  the  amount 
of  water  employed.  In  such  cases,  the  volume  of  the  solution  of 
iron  used  for  standardizing  the  permanganate  and  the  volume  of 
the  weak  ferruginous  solution  subjected  to  analysis  should  be  the 
same,  and  either  the  two  solutions  should  contain  about  the  same 
quantity  of  iron,  or  by  means  of  a  special  experiment,  it  is  ascer- 
tained how  many  0*1  c.  c.  of  the  permanganate  are  required  to 
impart  the  desired  pale-red  color  to  the  same  volume  of  acidified 
water.  In  the  latter  case,  these  O'l  c.  c.  will  be  deducted  from 
the  amount  of  permanganate  used  in  the  regular  experiments. 

*  If  you  often  make  iron  determinations,  you  may  of  course  procure  a 
quantity  of  wire  and  determine  the  amount  of  the  foreign  matter  in  it. 


318  DETEKMINATION.  [ 

In  estimating  iron  in  mineral  waters  it  is  of  course  taken  for 
granted  that  the  water  contains  no  other  substances,  such  as  hy- 
drogen sulphide,  organic  matter,  nitrites,  etc.,  that  will  reduce 
the  permanganate. 


Fig.  85. 

ft.  Performance  of  the  Analytical  Process. 
This  has  been  fully  indicated  in  a.  The  compound  to  be  ex- 
amined is  dissolved,  preferably  with  application  of  a  current  of 
carbon  dioxide*  (see  Fig.  85),  in  dilute  sulphuric  acid,  allowed 
to  cool  in  the  current  of  carbon  dioxide,  and  suitably  diluted  (if 
practicable,  the  solution  of  a  substance  containing  about  0*2  grm. 
iron  should  be  diluted  to  about  200  c.  c.)  ;  if  free  acid  is  not  pres- 
ent in  sufficient  quantity,  dilute  sulphuric  acid  is  added  till  about 
20  c.  c.  are  present  altogether,  and  then  standard  permanganate 
from  the  burette,  to  incipient  reddening  of  the  fluid.  The  vol- 
ume of  standard  solution  used  is  then  read  off.  The  strength  of 
the  solution  of  permanganate  being  known,  the  quantity  of  iron 
present  in  the  examined  fluid  is  found  by  a  very  simple  calculation. 
Suppose  100  c.  c.  of  solution  of  potassium  permanganate  to  corre- 
spond to  0*98  grm.  iron,  and  25  c.  c.  of  the  solution  to  have  been 
used  to  effect  the  oxidation  of  the  ferrous  compound  examined, 
then 

100  :  25:  :  0-98  :  a?;    a?  =  0-245. 

*  If  commercial  hydrochloric  acid  is  used  for  the  preparation  of  CO2  by 
action  on  marble,  it  must  be  free  from  sulphurous  acid,  an  impurity  which  it 
often  contains, 


§  H2.]  FERROUS   IRON.  319 

The  quantity  of  ferrous  iron  originally  present  amounted 
accordingly  to  0-245  grm. 

For  the  method  of  determining  the  total  amount  of  iron 
present  in  a  solution  containing  both  ferrous  and  ferric  salts,  I 
refer  to  §  113 ;  for  that  of  determining  the  amount  in  each  con- 
dition separately,  to  Section  Y. 

y.  Process  to   be  used  when  titrating  hydrochloric- acid 

solutions  of  Iron  with  Permanganate. 

In  titrating  hydrochloric-acid  solutions  of  iron  with  perman- 
ganate, it  is  essential  that  the  standardizing  of  the  reagent  and  the 
actual  analysis  be  performed  under  similar  conditions  as  regards 
dilution,  amount  of  acid,  and  temperature.  Besides  the  proper 
reaction  lOFeCl,  +  2KMnO4  +  16HC1  =  5Fe3Cl6  +  2KC1  + 
2MnCl2  +  8H,O,  the  collateral  reaction  2KMnO4  +  16HC1  = 
2KC1  +  2MnCl2  +  8II.O  +  10C1  also  takes  place,  in  consequence 
of  which  a  little  chlorine  is  liberated.  This  chlorine  does  not 
combine  with  the  ferrous  chloride  to  form  ferric  chloride  in  the 
case  of  considerable  dilution,  but  there  occurs  a  condition  of 
equilibrium  in  the  fluid  containing  ferrous  chloride,  chlorine,  and 
hydrochloric  acid,  which  is  destroyed  by  addition  of  a  further 
quantity  of  either  body  (LOWENTHAL  and  LENSSEN*).  But  since 
it  is  difficult  to  observe  the  above  conditions  of  obtaining  correct 
results,  the  determination  in  presence  of  hydrochloric  acid  is. 
always  less  trustworthy  than  it  is  in  sulphuric  acid  solutions. 

The  following  method  I  have,  however,  found  f  to  give  the 
best  results : — 

Standardize  the  permanganate  by  means  of  iron  dissolved  in 
dilute  sulphuric  acid,  make  the  iron  solution  to  be  tested  up  to  \ 
litre,  add  50  c.c.  to  a  large  quantity  of  water  acidified  with  sul- 
phuric acid  (about  1  litre),  titrate  with  permanganate,  then  again  add 
50  c.c.  of  the  iron  solution,  and  titrate  again,  &c.  &c.  The  num- 
bers obtained  at  the  third  and  fourth  time  are  taken.  These  are 
constant,  while  the  number  obtained  the  first  time,  and  sometimes 
also  the  second  time,  differs.  The  result  multiplied  by  5  gives 
exactly  the  quantity  of  permanganate  proportional  to  the  amount 
of  ferrous  iron  present. 

J.  PENNY'S  Method  (recommended  subsequently  by  SCHABUS). 
If  potassium  dichromate  is  added  to  a  solution  of  a  ferrous  salt 

*  Zeitschr.f.  analyt.  Chem.,  i,  329.  \  lb.,  i,  361. 


320  DETERMINATION.  [§  112. 

in  presence  of  a  strong  free  acid,  the  ferrous  salt  is  converted  into 
ferric  salt,  whilst  a  potassium-  arid  a  chromic  salt  of  the  free  acid 
is  formed  (6FeSO4  +  KaCr2O7  +  7II2SO4  =  3Fe2(SO4)3  +  K2SO4 
+  Cri(S04)1  +  YHiO). 

Now,  with  29 '442  grin,  potassium  dichromate  dissolved  to  2 
litres  of  fluid,  33 '54  grin,  iron  may  be  changed  from  a. ferrous  to  a 
ferric  salt  (294-42  being  the  mol.  weight  of  KaCraO, ,  and  335-4 
being  6  times  the  at.  weight  of  iron) ;  50  c.  c.  of  the  above  solution 
correspond  accordingly  to  0-8385  grm.  iron. 

Care  must  be  taken  to  use  perfectly  pure  potassium  dichro- 
mate ;  the  salt  is  heated  in  a  porcelain  crucible  until  it  is  just 
fused ;  it  is  then  allowed  to  cool  under  the  desiccator,  and  the  re- 
quired quantity  weighed  off  when  cold.  Besides  the  above  solu- 
tion, another  should  also  be  prepared  ten  times  more  dilute  and 
cor.taining  hence  1-4721  grm.  per  litre. 

It  is  always  advisable  to  test  the  correctness  of  the  standard 
solution  of  potassium  dichromate  by  oxidizing  with  it  a  known 
quantity  of  pure  iron  dissolved  to  a  ferrous  salt  (see  §  112,  2,  aa). 

The  ferrous  solution  is  sufficiently  diluted,  mixed  with  a  suf- 
ficient quantity  of  dilute  sulphuric  acid,  and  the  standard  solution 
of  potassium  dichromate  slowly  added  from  the  burette,  the  liquid 
being  stirred  all  the  while  with  a  thin  glass  rod.  The  fluid,  which 
is  at  first  nearly  colorless,  speedily  acquires  a  pale  green  tint,  which 
changes  gradually  to  a  darker  chrome-green.  A  very  small  drop 
of  the  mixture  is  now  from  time  to  time  taken  out  by  means  of 
the  stirring-rod,  and  brought  into  contact  with  a  drop  of  a  solution 
of  potassium  ferricyanide  (free  from  f errocyanide)  on  a  porcelain 
plate,  which  has  been  spotted  with  several  of  such  drops.  When 
the  blue  color  thereby  produced  begins  to  lose  the  intensity  which 
it  exhibited  on  the  first  trials,  and  to  assume  a  paler  tint,  the 
addition  of  the  solution  of  potassium  dichromate  must  be  more 
carefully  regulated  than  at  first,  and  towards  the  end  of  the  process 
a  fresh  essay  must  be  made,  and  with  larger  drops  than  at  first, 
after  each  new  addition  of  two  drops,  and  finally,  even  of  a  single 
drop ;  drops  must  also  be  left  for  some  time  in  contact  before  the 
observation  is  taken.  When  no  further  blue  coloration  ensues,  the 
oxidation  is  terminated.  From  the  remarkable  sensitiveness  of  the 
reaction,  the  exact  point  may  be  easily  hit  to  a  drop.  To  heighten 
the  accuracy  of  the  results,  the  dilute  (ten  times  weaker)  standard 
fluid  should,  just  at  the  end  of  the  process,  be  substituted  for  the 


§  113.]  FERRIC   IRON.  321 

concentrated  solution  of  potassium  diclir  ornate;  the  iron  solution 
may  besides  be  diluted  to  measure  250  c.  c.,  50  c.  c.  of  this  being 
used  for  making  the  approximate  determination,  then  another 
50  c.  c.  being  taken  for  the  determination  proper,  thus  minimizing 
the  loss  incidental  to  the  method. 

Thus  if  exactly  0*84  grm.  of  a  substance  has  been  dissolved, 
the  number  of  half  c.  c.  of  the  standard  solution  will  show  the  per 
cents.,  while  the  diluted  solution  will  show  the  O'l  per  cents,  of 
iron  present. 

For  the  manner  of  proceeding  in  presence  of  ferric  salts, 
I  refer  to  §  113.  If  there  is  a  deficiency  of  free  acid  in  the 
solution,  brown  chromic  chromate  may  form,  upon  which  the 
solution  of  ferrous  salt  exercises  no  longer  a  deoxidizing  action. 

Of  the  two  methods  the  first  affords  the  advantage  that  the 
end  of  the  operation  is  at  once  known  by  the  red  color  acquired 
by  the  liquid,  thus  requiring  no  special  testing;  the  advantage 
possessed  by  the  second  method  is  that  the  standard  solution  of 
potassium  chromate  is  easily  prepared  and  may  be  readily  pre- 
served unchanged.  Since  it  has  been  found  that  the  titration  of 
iron  in  hydrochloric-acid  solution  does  not  afford  entirely  satisfac- 
tory results  with  permanganate,  recourse  has  been  had  again  of 
late  to  titration  with  potassium  chromate,  which  has  been  rieg- 
lected  for  some  time.  Attention  may  also  be  called  to  the  fact 
that  where  the  analyst  is  free  to  choose  between  a  solution  in 
hydrochloric  or  sulphuric  acid,  when  making  a  volumetric  esti- 
mation,  preference  should  always  be  given  to  the  sulphuric-acid 
solution,  because  it  is  affected  far  less  by  atmospheric  oxygen 
than  the  former 


6.   FERRIC  IRON. 

a.  Solution. 

Many  ferric  compounds  are  soluble  in  water.  Ferric  oxide 
and  most  ferric  compounds  which  are  insoluble  in  water  dissolve 
in  hydrochloric  acid,  but  many  of  them  only  slowly  and  with 
difficulty;  compounds  of  this  nature  are  best  dissolved  in  con- 
centrated hydrochloric  acid,  in  a  flask,  with  the  aid  of  heat; 

*  Zeitschr.f.  analyt.  CJwm.,  ix,  512. 


322  DETERMINATION.  [§  113* 

which,  however,  should  not  be  'allowed  to  reach  the  boiling- 
point;  the  compound  must,  moreover,  be  finely  powdered,  and 
even  then  it  will  often  take  many  hours  to  effect  complete  solu- 
tion. Sometimes,  as  in  the  case  of  strongly  ignited  ferric  oxide, 
the  substance  is  dissolved  in  potassium  disulphate  at  the  fusion 
point,  or  in  a  mixture  of  8  parts  sulphuric  acid  and  3  parts  water. 
It  is  frequently  advisable,  also,  to  reduce  the  ferric  oxide  to  the 
metallic  form  by  prolonged  ignition  in  hydrogen,  and  then  to 
dissolve  the  metal.  Iron-containing  silicates  which  are  not  decom- 
posable by  hydrochloric  acid,  are  treated  according  to  §  140,  5. 

5.   Determination. 

The  iron  of  ferric  compounds  is  usually  weighed  as  ferric 
oxide,  but  sometimes  as  ferrous  sulphide  (§81).  It  may,  how- 
ever, be  estimated  also  indirectly,  as  well  as  by  volumetric  analysis, 
both  directly  and  after  reduction  to  ferrous  iron.  The  conver- 
sion of  compounds  of  iron  into  ferric  oxide  is  effected  either  by 
precipitation  as  ferric  hydroxide,  preceded  in  some  cases  by  pre- 
cipitation as  ferrous  sulphide,  or  as  basic  ferric  acetate,  succinate, 
or  formate,  or  by  ignition.  "While  the  volumetric  and  the  now 
seldom  used  indirect  methods  are  applicable  in  almost  all  cases, 
we  may  convert  into 

1.  FERRIC  OXIDE. 

a.  By  Precipitation  as  Ferric  Hydroxide. 

All  salts  of  inorganic  or  volatile  organic  acids  and  soluble  in 
water,  and  likewise  those  which,  insoluble  in  water,  dissolve  in 
hydrochloric  acid,  with  separation  of  their  acid. 

b.  By  Precipitation  as  Ferrous  Sulphide. 
All  compounds  of  iron  without  exception. 

c.  By  Ignition. 

All  ferric  salts  of  volatile  oxygen  acids. 

2.  FERROUS  SULPHIDE. 

All  compounds  of  iron  without  exception. 

The  method  1,  0,  is  the  most  expeditious  and  accurate,  and  is 
therefore  preferred  in  all  cases  where  its  application  is  admis- 
sible. The  method  1,  #,  is  the  most  generally  used.  The ' 
methods  1,  &,  and  2,  serve  principally  to  effect  the  separation  of 
the  iron  from  other  bases ;  they  are  resorted  to  also  in  certain  in- 
stances where  a  is  inapplicable,  especially  in  cases  where  sugar  or 


§  113.]  FERRIC   IRON.  323 

other  non- volatile  organic  substances  are  present;  and  also  to  de- 
termine iron  in  ferric  phosphates  and  borates.  For  the  manner 
of  determining  iron  in  ferric  chromate  and  silicate,  I  refer  to 
§§  130  and  140.  The  volumetric  methods  for  estimating  the 
iron  of  ferric  compounds  are  used  in  technical  work  almost  to 
the  exclusion  of  all  others,  and  are  very  frequently  employed  in 
scientific  analyses.  The  methods  of  precipitating  iron  in  the  form 
of  basic  salts  will  be  given  in  Section  Y. 

1 .   Determination  as  ferric  Oxide. 

a.   By  Precipitation  as  Ferric  Hydroxide. 

Mix  the  solution  in  a  porcelain  dish  (a  glass  beaker  does  not 
answer  so  well)  with  ammonia  in  excess,  heat  nearly  to  boiling, 
decant  repeatedly  on  to  a  filter,  wash  the  precipitate  carefully 
with  hot  water,  dry  thoroughly  (which  very  greatly  reduces  the 
bulk  of  the  precipitate),  and  ignite  in  the  manner  directed  in  §  53. 

For  the  properties  of  the  precipitate  and  residue,  see  §  81. 
The  method  is  free  from  sources  of  error.  The  precipitate,  under 
all  circumstances,  even  if  there  are  no  fixed  bodies  to  be  washed 
out,  must  be  most  care/idly  and  thoroughly  washed,  since,  should 
it  retain  any  traces  of  ammonium  chloride,  a  portion  of  the  iron 
would  volatilize  in  the  form  of  ferric  chloride.  It  is  also  highly 
advisable  to  dissolve  the  weighed  residue,  or  a  portion  of  it,  in 
strong  hydrochloric  acid,  or  to  fuse  it  writh  potassium  disulphate 
and  dissolve  the  melt  in  dilute  hydrochloric  acid  to  see  whether 
it  is  quite  free  from  silicic  acid  (if  any  is  present  it  will  remain 
nndissolved).  The  solution  is  most  readily  effected  in  hydro- 
chloric acid  if  the  oxide  is  previously  reduced  to  metallic  iron  by 
ignition  in  hydrogen. 

1).   By  Precipitation  as  Ferrous  Sulphide. 

The  solution,  in  not  too  large  a  flask,  is  mixed  with  ammonia 
till  all  the  free  acid  is  neutralized.  (In  the  absence  of  organic, 
non-volatile  substances,  this  leads  to  the  precipitation  of  a  little 
ferric  hydroxide,  which,  however,  is  of  no  consequence.)  Add 
ammonium  chloride,  if  not  already  present  in  sufficient  quantity, 
then  colorless  or  yellowish  ammonium  sulphide  in  moderate  ex- 
cess, an~d  lastly  water,  till  the  fluid  reaches  to  the  neck  of  the  flask. 
Cork  it  up  and  stand  in  a  warm  place  till  the  precipitate  has 
subsided  and  the  supernatant  fluid  has  a  clear,  yellowish  appear- 


324  DETERMINATION.  [§  113. 

ance  (without  a  tinge  of  green).  Then  wash  the  precipitate  if  at  all 
considerable,  first  by  decantation,  then  on  the  filter,  using  water 
containing  ammonium  sulphide  and  gradually  decreasing  quan- 
tities of  ammonium  chloride.  "When  decanting  pour  the  liquids 
into  a  flask,  and  not  into  a  filter,  and  when  the  washing  is  com- 
plete, then  filter  the  mixed  fluids,  bring  the  precipitate  on  to  the 
filter,  and  continue  washing  uninterruptedly,  while  the  funnel  is 
kept  covered  with  a  glass  plate.  Neglect  of  any  of  these  precau- 
tions will  occasion  some  loss  of  substance,  the  ferrous  sulphide 
gradually  combining  with  the  oxygen  of  the  air  and  passing  thus 
into  the  filtrate  as  ferrous  sulphate.  As  this  sulphate  is  reprecip- 
itated  by  the  ammonium  sulphide  present,  the  filtrate  assumes, 
in  such  cases,  a  greenish  color,  and  gradually  deposits  a  black 
precipitate,  the  separation  of  which  is  greatly  promoted  by  addi- 
tion of  ammonium  chloride. 

When  the  operation  of  washing  is  completed,  the  moist  pre- 
cipitate (if  it  is  not  dried  and  determined  according  to  2)  is  put, 
together  with  the  filter,  into  a  beaker,  some  water  added,  and  then 
hydrochloric  acid,  until  the  whole  is  redissolved.  Heat  is  now 
applied,  until  the  solution  smells  no  longer  of  hydrogen  sulphide ; 
the  fluid  is  then  filtered  into  a  flask,  the  residual  paper  carefully 
washed,  incinerated,  the  ash  treated  with  warm  strong  hydrochloric 
acid.  The  solution  thus  obtained  (if  yellowish)  is  added  to  the 
main  filtrate,  which  is  next  heated  with  nitric  acid  (see  §  112,  1) ; 
the  solution  (now  ferric)  is  finally  precipitated  with  ammonia,  as 
in  a. 

If  a  solution  of  potassium  ferric,  ammonium  ferric,  or  sodium 
ferric  tartrate  contains  a  considerable  excess  of  alkali  carbonate, 
the  precipitation  of  the  iron  as  sulphide  is  prevented  to  a  greater 
or  less  extent  (BLUMENAU).  In  such  cases  the  fluid  must  therefore 
be  nearly  neutralized  with  an  acid,  before  the  precipitation  with 
the  ammonium  sulphide  can  be  effected. 

c.  By  Ignition. 

•  Expose  the  compound,  in  a  covered  crucible,  to  a  gentle  heat 
at  first,  and  gradually  to  the  highest  degree  of  intensity ;  continue 
the  operation  until  the  weight  of  the  residuary  ferric  oxide  remains 
constant. 


.§  113.]  FERRIC   IRON.  325 

2.   Determination  as  Anhydrous  Ferrous  Sulphide. 

The  hydrated  ferrous  sulphide  obtained,  as  in  1,  &,  may  be  very 
conveniently  determined  by  conversion  into  the  anhydrous  sul- 
phide. The  process  is  the  same  as  for  zinc  (§  108,  2).  The  heat 
to  which  it  is  finally  exposed  in  the  current  of  hydrogen  must  he 
strong,  as  an  excess  of  sulphur  is  retained  with  some  obstinacy.  In 
fact,  it  is  advisable  after  weighing  to  re-ignite  in  hydrogen  and 
weigh  a  second  time.  It  is  of  no  importance  if  the  hydrated  sul- 
phide has  oxidized  on  drying. 

Ferrous  sulphate  and  ferric  hydroxide  can  be  transformed  into 
sulphide  in  the  same  manner,  after  having  been  dehydrated  by 
ignition  in  a  porcelain  crucible  (H.  ROSE*). 

The  results  obtained  by  OESTEN,  and  adduced  by  ROSE,  as  well 
as  those  obtained  in  my  own  laboratory,  are  exceedingly  satisfac- 
tory. (Expt.  No.  67.) 

3.     Volumetric  Determination. 

a.  Preceded  hy  Reduction  of  Ferric  to  Ferrous  Iron. 

The  methods  here  to  be  described  depend  upon  the  reduction 
of  ferric  to  ferrous  iron,  the  quantity  of  which  latter  is  then  esti- 
mated. We  have  hence  to  occupy  ourselves  simply  with  the  re- 
duction of  ferric  to  ferrous  solutions,  the  other  part  of  the  process 
having  been  fully  discussed  in  §  112  (Ferrous  Iron).  This  reduc- 
tion can  be  effected  by  many  substances  (zinc,  stannous  chloride, 
hydrogen  sulphide,  sulphurous  acid,  &c.),  but  only  those  can  be 
used  with  advantage  an  excess  of  which  may  be  added  with  im- 
punity. If  an  excess  must  be  very  carefully  avoided,  or,  being 
added,  must  be  carefully  removed,  the  method  becomes  trouble- 
some, and  a  ready  source  of  inaccuracy  is  introduced.  On  these 
grounds,  zinc,  although  somewhat  slow  in  action,  unquestionably 
deserves  to  be  preferred  to  all  other  reducers. 

Reduction  hy  Zinc. — Heat  the  hydrochloric-  or  sulphuric-acid 
solution,  which  must  contain  a  moderate  excess  of  acid,  but  be 
free  from  nitric  acid,f  in  a  small,  long-necked  flask  placed  in  a 
slanting  position  ;  drop  in  small  pieces  of  iron-free  zinc  (§  60)  and 

*  Pogg.  AnnaL,  ex,  126. 

f  If  nitric  acid  is  present,  there  forms,  under  the  action  of  zinc,  nitrous  acid, 
•which  reduces  potassium  permanganate.  This  would  hence  give  rise  to  erro- 
neous results  (TERREIL,  Zeitschr.  /.  analyt.  Chem.,  vi,  116). 


326  DETERMINATION.  [§  113. 

conduct  a  slow  current  of  carbon  dioxide  through  the  flask  (Fig. 
85).  Evolution  of  hydrogen  gas  begins  at  once,  and  the  color  of 
the  solution  becomes  paler  in  proportion  as  the  ferric  sulphate  (or 
chloride)  changes  to  ferrous  sulphate  (or  chloride).  Apply  a  mod- 
erate heat  to  promote  the  action,-  and  add  also,  if  necessary,  a 
little  more  zinc.  As  soon  as  the  hot  solution  is  completely  de- 
colorized (one  cannot  judge  of  the  perfect  reduction  of  a  cold  solu- 
tion so  well,  as  the  color  of  a  ferric  salt  is  deeper  when  hot) ,  and 
all  the  zinc  is  dissolved, *  allow  to  cool  completely  in  the  stream 
of  carbon  dioxide  (to  hasten  the  cooling  the  flask  may  be  im- 
mersed in  cold  water) ;  then  dilute  the  contents  with  water,  pour 
off  and  wash  carefully  into  a  beaker,  leaving  behind  (so  far  as 
possible)  any  flocks  of  lead  that  may  have  separated  from  the  zinc, 
wash  repeatedly  with  water,  and  in  the  case  of  a  sulphuric-acid 
solution  proceed  preferably  as  directed  in  §  112,  2,  /?;  in  the  case 
of  a  hydrochloric-acid  solution  proceed  according  to  p.  319,  y.  If 
the  solution  contain  metals  precipitable  by  zinc,  these  will  separate 
and  may  render  filtration  necessary.  In  this  case  the  filtrate 
must  be  again  heated  with  zinc  before  using  the  standard  solution. 
If  iron-free  zinc  cannot  be  procured,  the  percentage  of  iron  in 
the  metal  used  must  be  determined  and  weighed  portions  of  it 
employed  in  the  process  of  reduction ;  the  known  amount  of  iron 
contained  in  the  zinc  consumed  is  then  subtracted  from  the  total 
amount  of  iron  found. 

In  the  analysis  of  solid  ferric  compounds  it  is  advisable  to  add 
some  zinc  while  they  are  dissolving  in  hydrochloric  acid.  Solu- 
tion is  thereby  facilitated  (O.  L.  ERDMANN-J-).  Regarding  the  re- 
duction of  ferric  chloride  by  stannous  chloride  compare  with  J. 

[Reduction  by  Hydrogen  /Sulphide. — Pass  hydrogen  sulphide 
through  the  cold  ferric  solution  in  a  flask.  The  solution  should 
occupy  about  two-thirds  of  the  capacity  of  the  flask,  and  should 
not  contain  much  more  than  0'2  grm.  iron  per  100  c.  c.,  but  may 
be  more  dilute  when  but  little  iron  is  present.  Continue  the 
treatment  with  hydrogen  sulphide  at  least  10  minutes  after 
the  color  due  to  the  ferric  salt  has  disappeared,  or  until  the 

*  If  any  zinc  remains  undissolved,  the  results  may  be  too  low,  because  iron 
often  deposits  on  zinc  and  does  not  dissolve  until  the  zinc  itself  dissolves 
(A.  MITSCHERLICH,  Zeitschr.  f.  analyt.  Chem.,u,  72). 

•\Journ.f.prakt.  Chem.,  LXXVI,  176. 


§  113.]  FERRIC    IKON.  327 

solution  appears  to  be  well  saturated  with  that  gas.  Heat,  at  first 
cautiously,  to  boiling.  Escape  of  hydrogen  sulphide  at  this  period 
indicates  that  enough  of  that  reagent  has  been  applied.  Continue 
boiling  so  rapidly  that  air  cannot  enter  the  flask,  the  mouth  of 
which  may  be  partially  closed  by  a  loose  roll  of  filter  paper,  or  other 
means,  until  the  solution  is  reduced  to  one  half  its  first  volume. 
This  will  insure  the  removal  of  excess  of  hydrogen  sulphide.  (The 
escaping  vapor  will  cease  to  blacken  paper  dipped  in  an  alkaline 
lead  solution  somewhat  before  this  point  is  reached.)  During  the 
boiling,  let  the  flask  be  inclined  so  as  to  prevent  mechanical  loss*. 
When  the  boiling  is  discontinued  fill  the  flask  immediately  with 
cold  water  to  within  an  inch  of  the  mouth,  close  with  a  stopper, 
and  cool  in  a  stream  of  water.  Before  reducing  the  ferric  solu- 
tion by  either  of  the  above  processes,  it  is  desirable  to  remove 
hydrochloric  acid,  if  it  is  present,  so  that  the  iron  after  reduction 
can  be  satisfactorily  determined  by  KMnO4.  Chlorides  can  be 
decomposed  and  HC1  removed  by  evaporating  the  solution  with 
excess  of  sulphuric  acid  so  long  as  hydrochloric  acid  vapors  are 
given  off  at  a  temperature  slightly  exceeding  100°.  A  liberal 
excess  of  sulphuric  acid  is  advantageous.  After  cooling  add  water 
and  digest  till  the  ferric  sulphate  is  dissolved.  This  treatment  is 
simple  and  safe  when  nothing  is  present  which  is  thereby  converted 
into  a  compound  insoluble  in  dilute  sulphuric  acid  (silicic  acid, 
barium,  strontium,  much  calcium,  &c.).  Such  insoluble  com- 
pounds may  persistently  retain  iron.  When,  therefore,  by  evapo- 
ration with  sulphuric  acid  and  subsequent  treatment  with  water 
an  insoluble  residue  remains,  it  should  riot  be  thrown  away  before 
testing  it  to  ascertain  whether  it  retains  iron.] 

&.    Without  Previous  Reduction  to  Ferrous  Iron. 

These  methods  all  depend  on  adding  a  reducing  agent  to  the 
solution  till  the  sesquioxide  is  entirely  converted  into  protoxide, 
and  then  determining,  either  directly  or  indirectly,  the  reducing 
agent  used.  Many  methods  have  been  proposed,  but  I  have 
found  those  given  under  a  and  ft  the  best. 

a.   Reduction  ~by  stannous  chloride.* 

This  method,  if  properly  executed,  gives  very  good  results; 

*  The  reduction  of  ferric  chloride  by  stannous  chloride  has  already  been 
utilized  by  PENNY  and  WALLACE  in  another  manner,  but  I  believe  I  was  the 
first  to  give  the  method  a  practical  form  (Zeitschr.f.  analyt.  Chem.,  i,  26). 


328  DETERMINATION.  [§  113, 

and,  having  had  many  years'  experience  with  it,  I  can  strongly 
recommend  it.  There  are  required  for  it : 

a.  A  Ferric-Chloride  Solution  of  known  strength.  This  is 
prepared  by  dissolving  10*04  grm.  of  perfectly  clean,  thin,  soft 
iron  wire  (corresponding  to  10  grm.  pure  iron)  in  IIC1  in  a  long- 
necked  flask  placed  slantingly,  oxidizing  with  potassium  chlorate,, 
completely  driving  off  the  excess  of  chlorine  by  protracted,  gentle 
boiling,  and  finally  making  up  the  volume  to  1  litre. 

&.  A  clear  Solution  of  Stannous  Chloride.  Tin's  should  be  of 
such  a  strength  that  one  volume  may  reduce  about  two  volumes 
of  the  ferric- chloride  solution. 

c.  A  Solution  of  Iodine  in  Potassium  Iodide,  each  c.  c.  of 
which  contains  O'Ol  grm.  iodine.  The  iodine  content  need  not 
be  exactly  known.  The  operations  are  as  follows: 

1.  Measure  off  2  c.  c.  of  the  stannous-chloride   solution  and 
add  a  little  starch  solution,   5  c.  c.  water,  and  sufficient  of  the 
iodine  solution  to  permanently  color  the  liquid  blue.     Note  the 
quantity  used.       Every  c.   c.   of  stannous-chloride  solution  will 
require  about  5  c.  c.  of  iodine  solution.* 

2.  Measure  off  50  c.  c.  of  the  ferric-chloride  solution,  add  a 
little  hydrochloric  acid,  and  heat  the  liquid  in  a  small  flask,  pref- 
erably on  an  iron  plate,  to  boiling.      Then  run  in  the  stannous- 
chloride  solution  from  a  burette,  first  in  large  quantities,  then  in 
smaller,  with  suitable  intervals  between  the  additions,  while  the 
fluid  is  kept  gently  boiling  all  the  time.      The  yellow  color  be- 
comes lighter  in  proportion  as  the  reduction  progresses.    Towards 
the  end  the  stannous-chloride  solution  is  added  by  drops,  with 
sufficient  time  for  action.      The  precise  moment  when  reduction 
is  complete  may  be  readily  noted  by  the  yellow  liquid  becoming 
colorless.      Now  coolthe  contents  of  the  flask,  add  some  starch 
solution,  and  run  in  from  a  burette  solution  of  iodine  by  drops 
until  a  permanent  blue  color  supervenes.      The  quantity  of  iodine 
solution  used  gives,  according  to  the  relations  determined  in   1 , 
the  excess  of  the  stannous-chloride  solution  added. f     On  deduct- 

*  The  quantity  of  iodine  solution  here  used  varies  somewhat  according  as 
more  or  less  hydrochloric  acid  is  added  to  the  stannous-chloride  solution.  The 
differences  are,  however,  so  trifling  as  to  have  no  appreciable  influence  on  the 
result,  because  In  this  method  only  a  very  small  excess  of  stannous  chloride  has 
to  be  determined. 

f  If  the  stannous  chloride  has  been  added  very  cautiously  toward  the  last, 
no  determinable  excess  of  stannous  chloride  is  frequently  left  in  solution,  par- 


§  113.]  FERRIC    IRON.  329 

ing  this  excess  from  the   quantity  of   stannous-chloride  solution 
used  at  first,  the  corrected  quantity  which  is  necessary  to  reduce- 
0*5  grm.  of  iron  from  a  ferric  to  a  ferrous  condition  is  found. 

3.  The  stannous-chloride  solution  having  been  thus  standard- 
ized, it  may  be  used  for  estimating  unknown  quantities  of  iron 
by  dissolving  these  in  hydrochloric  acid,  converting  any  ferrous 
chloride  into  ferric  chloride  by  one  of  the  methods  detailed  in 
§  112,  1,  a  or  <?,  driving  off  every  trace  of  chlorine,  and  finally 
adding  the  stannous-chloride  solution  to  the  suitably  concentrated 
iron  solution  as  described  under  2,  until  the  liquid  becomes  color- 
less, the  excess  of  stannous  chloride  present  being  then  determined 
with  the  iodine  solution.  The  iron  content  may  then  be  calculated,, 
from  the  volume  of  stannous-chloride  solution  used,  by  the  rule- 
of -three.  Assuming  that  25  c.  c.  of  stannous-chloride  solution  cor- 
respond to  0-5  grm.  iron  (i.e.,  sufficient  to  reduce  0*5  grm.  iron 
from  a  ferric  to  a  ferrous  condition),  and  assuming  that  we  have 
used  20  c.  c.  of  the  solution  to  reduce  the  unknown  quantity  of 
ferric  salt,  then  the  latter  will  have  contained  0*4  grm.  of  iron, 

25:  0-5:  :  20:  a?;   a?  =  0-4  grm. 

This  method,  as  already  stated,  affords  excellent  results,*  but  only 
when  all  the  operations  are  carried  out  at  once,  in  order  that  the 
titer  of  the  stannous  chloride  may  not  be  altered  by  the  action  of 
the  air.  On  this  account,  also,  it  is  preferable  to  operate  with 
rather  concentrated  stannous-chloride  solutions,  and  consequently 
fairly  large  quantities  of  iron,  rather  than  with  dilute  solutions, 
on  which  the  air  would  act  more  strongly. 

To  prepare  the  stannous-chloride  solution,  heat  pure  powdered 
tin  (obtained  by  melting  tin  in  a  porcelain  dish  and  triturating 
with  a  pestle  until  cold J  with  hydrochloric  acid  (sp.  gr.  1-12)  until 
the  tin,  being  in  excess,  evolves  no  more  hydrogen.  Cool  the 
liquid,  pour  or  filter  it  off  from  the  undissolved  tin  and  add 
3  volumes  of  hydrochloric  acid  and  6  volumes  of  water.  The 
solution  is  best  preserved  in  an  apparatus  which  prevents  or  limits 
the  action  of  the  air  on  it. 

ticularly  when  operating  with  concentrated  iron  solutions.     In  other  cases, 
however,  a  small  excess  will  be  found.    In  order  to  render  the  method  perfectly 
reliable  hence  L  consider  it  absolutely  indispensable  to  test  for  an  excess  of 
stannous  chloride  in  the  manner  stated,  and  to  determine  it. 
*  These  are  given  in  the  Zeitschr.  f.  analyt.  Chem.,  i,  26. 


330 


DETERMINATION. 


[ 


Formerly  *  the  air  entering  the  vessel  containing  the  solution 
was  made  to  pass  through  tubes  filled  with  phosphorus  and  potas- 
sium pyrogallate  in  order  to  deprive  it  of  oxygen ;  the  apparatus 
shown  in  Fig.  86  is,  however,  now  preferred,  and  especially  in 
large  laboratories  where  relatively  much  of  the  solution  is  fre- 
quently used. 


Fig.  86. 

a  is  a  vessel  containing  the  stannous- chloride  solution  ;  f  is 
a  siphon,  which  is  filled  by  blowing  air  into  &,  after  which  the 
compression-cock  g  is  closed,  and  the  constant  carbonic-acid  ap- 
paratus G  connected  with  the  tube  &.  The  stopper  of  a  is  then 
loosened  and  the  air  in  the  vessel  replaced  by  carbonic  acid,  when 
the  stopper  is  again  tightly  inserted.  On  withdrawing  some  of 
the  solution  by  opening  <7,  an  equal  volume  of  it  is  replaced  by 
carbonic  acid  which  passes  from  o  to  #,  the  evolution  of  gas  ceas- 
ing as  soon  as  g  is  closed  again  and  the  acid  is  forced  out  of  the 
flask  d,  which  contains  marble  and  is  provided  with  a  small  open- 
ing at  the  bottom ;  the  flask  is  held  down  by  a  plaster  plate,  h. 
*  Zeitschr.  /.  anatyt.  Chem. ,  u,  58. 


§  113.]  FERRIC   IRON.  331 

/?.  Reduction  by  Potassium  Iodide ,  and  the  determination 
of  the  liberated  Iodine  by  Sodium  Thiosulphate.  * 

The  principle  of  the  method  is  as  follows  :  When  an  excess  of 
potassium  iodide  acts  upon  ferric  chloride  at  a  moderate  heat, 
there  are  formed  ferrous  chloride,  potassium  chloride,  and  free 
iodine,  the  last  being  dissolved  by  the  excess  of  potassium  iodide, 
and  remaining  in  solution  ;  (Fe2Cl6  +  2KI  =  2FeCl3  +  2KC1 
+  21).  On  determining  the  free  iodine  by  means  of  a  standard 
solution  of  sodium  thiosulphate  [2XaaS8O8  +  21  =  2XaI  + 
Na2S4O6  (sodium  tetrathionate)],  the  quantity  of  iron  present  may 
be  calculated,  since  1  eq.  of  iodine,  126-85,  liberated  corresponds 
to  1  eq.  of  iron,  55 '9,  present. 

The  method  requires:  1,  a  solution  of  sodium  thiosulphate 
(about  12  grm.  of  the  crystallized  salt  in  the  litre) ;  2,  potassium 
iodide,  which  must  be  free  from  iodate  (§  65,  b) ;  3,  a  ferric-chlo- 
ride solution  of  known  iron  strength,  and  free  from  ferrous 
chloride  and  free  chlorine,  see  §  113,  J,  <*,  the  solution  there 
described,  containing  O'l  grm.  iron  in  10  c.  c.,  being  well  adapted 
for  the  purpose ;  4,  several  flasks  with  well-fitting  ground -glass 
stoppers  and  of  100  to  150  c.  c.  capacity;  5,  thin,  freshly  pre- 
pared starch  paste. 

The  sodium-thiosulphate  solution  is  first  standardized  as  fol- 
lows :  10  c.  c.  of  ferric-chloride  solution  are  introduced  into  each 
of  two  flasks,  and  diluted  caustic-soda  solution  added  to  each  until 
a  few  flocks  of  ferric  hydroxide  begin  to  precipitate,  when  suffi- 

*  This  method  passed  through  many  phases  before  it  reached  the  state  of 
development  in  which  it  is  at  present  used.  After  DUFLOS,  and  later  on, 
STRENW,  had  employed  the  method  of  estimating  iron  by  reducing  ferric  chlo- 
ride with  hydriodic  acid,  C.  Moira  in  1858  ( Annal.  d.  Chem.  u.  Pharm.,  cv, 
53)  studied  the  influence  of  dilution  in  this  reaction.  In  1860  FR.  MOHR 
(Annal.  d.  Chem.  u.  Pharm.,  cxui,  257)  described  a  method  in  which  the  po- 
tassium iodide  was  not  used  in  excess,  but  played  the  part  more  of  an  indica- 
tor. The  method  did  not,  however,  answer  the  strict  requirements  demanded 
of  a  good  process.  In  the  same  year  C.  D.  BRAUN  (Journ.  f.  pr<>kt.  Chem., 
LXXXI,  423)  employed  the  method  in  estimating  iron  in  ferric  chloride  obtained 
by  oxidizing  ferrous  chloride  with  nitric  acid,  but  he  improved  the  process  by 
employing  an  excess  of  potassium  iodide,  facilitating  the  reaction  by  warming, 
and  estimating  the  liberated  iodine  on  cooling.  In  1863  FR.  MOHR  (Zeitschr.  f. 
analyt.  Chem.,  n,  243)  described  the  method,  adopting  BRAUN'S  improvements, 
but  standardizing  the  thiosulphate  solution  with  iodine  liberated  by  potassium 
bichromate.  In  1864  BRAUN  (Zeitschr.  analyt.  Chem.,  in,  452)  again  described 
his  method  in  detail. 


332  DETERMINATION.  [§  113, 

cient  hydrochloric  acid  (about  0'5  to  1  c.  c.)  of  I'l  specific  gravity 
is  added  to  each  to  render  the  solutions  again  clear.  By  this 
method  undue  acidity  of  the  liquids  is  avoided,  and  the  fluids  will 
no  longer  be  brownish- red,  but  will  have  a  dark-yellow  color. 
Three  grm.  of  potassium  iodide  are  now  introduced  into  each  of 
the  flasks,  the  stoppers  tightly  inserted  and  tied  down  with 
moistened  parchment  paper  or  with  wire  or  cord,  and  the  flasks 
warmed  to  50°  or  60°(  best  accomplished  by  suspending  the  flasks 
in  the  ascending  steam  of  a  water-bath).  The  reduction  of  the 
ferric  chloride  is  completed  in  from  15  to  20  minutes,  when  the 
solutions  will  have  a  brownish-red  color.  After  allowing  to  cool, 
run  in  from  the  burette  the  sodium-thiosulphate  solution  until 
the  liquid  has  a  wine-yellow  color,  then  add  from  O5  to  1  c.  c.  of 
thin  starch  paste,  and  continue  again  to  add  the  thiosulphate  solu-, 
tion  until  the  blue  color  of  the  starch  iodide  just  disappears.  The 
volume  used  up  corresponds  to  the  iodine  liberated  by  Ol  grm. 
iron,  and  hence  also  to  O'l  grm.  iron  present  as  ferric  chloride. 

The  Estimation  of  the  Iron  in  a  solution  of  unknown  strength 
is  accomplished  in  the  same  manner  as  in  standardizing  the 
sodium-thiosulphate  solution.  Care  must  be  taken  that  all  the 
iron  must  be  present  as  ferric  oxide  or  chloride,  and  that  the 
liquid  contains  no  other  substance  that  will  decompose  potassium 
iodide,  e.g. ,  free  chlorine  or  nitric  acid.  It  is  also  advisable  to 
employ  solutions  containing  as  nearly  as  possible  about  O'l  grm. 
iron,  and  that  not  too  little  nor  too  much  thiosulphate  be  used. 
The  free  acid  must  also  be  reduced  in  quantity,  as  detailed  above. 

If  it  was  found  that  18 -4  c.  c.  of  the  thiosulphate  solution  corre- 
sponded to  0*1  grm.  iron,  and  if  there  had  been  required  24*5  c.  c. 
of  the  thiosulphate  solution  to  combine  with  the  iodine  liberated 
by  the  unknown  quantity  of  iron  present,  this  last  would  then 
amount  to  0*13315  grm.,  since  18-4:  O'l  :  :  24-5:0-13315. 

The  method  gives  good  results,  and  is  much  to  be  recommended 
for  determining  small  quantities  of  iron. 

y.  Reduction  by  Sodium  TTiiosulpKate  in  the  presence  of  a 
Cupric  Salt,  after  OUDEMANS.* 

If  an  acid  solution  of  ferric  chloride  is  mixed  witli  a  little  cupric 


*  Sodium  tliiosalpbate  was  first  employed  by  SCIIERER  (Gel.  Ans.  der  K. 
Bayerischen  Akademie,  vom  31  Aug.  1859),  afterwards  by  KREMER  and  LAN- 
DOLT  (Zeitschr.f.  analyt.  Chem.,  I,  214).  The  method  of  OUDEMANS  is  to  be  found 


§  113.]  FERRIC   IRON.  333 

sulphate  and  some  potassium  sulphocyanate  and  then  sodium 
thiosulphate  is  added,  the  red  color  of  the  ferric  sulphocyanate 
gets  paler  and  paler,  and  finally  when  the  ferric  salts  are  reduced 
to  ferrous,  disappears  altogether.  Warming  is  unnecessary.  To 
hit  the  point  is  not  easy,  so  we  add  a  slight  excess  of  sodium  thio- 
sulphate and  then  titrate  back  with  standard  iodine.  The  reaction 
is  as  follows:  Fe2Cl6  +  21Sa2!S2O3  _  2FeCla  +  2NaCl  -f  !STa3S4O6; 
it  is  promoted  by  the  addition  of  a  small  quantity  of  cupric  sul- 
phate, which  is  alternately  reduced  by  the  thiosulphate  and  oxi- 
dized by  the  ferric  chloride.  If  a  small  quantity  of  cuprous  salt 
is  produced  by  the  excess  of  thiosulphate  this  does  not  matter,  as 
its  action  on  the  iodine  solution  is  the  same  in  extent  as  the  action 
of  the  thiosulphate  which  produced  it.  The  method  is  not 
accurate  unless  the  fluid  remains  clear;  neither  cuprous  sul- 
phocyanate nor  cuprous  iodide  nor  sulphur  must  be  thrown 
down.  Hence  care  must  be  taken  to  maintain  the  proper  amounts 
of  the  reagents  and  to  dilute  the  fluid  sufficiently. 

This  method  is  much  like  that  detailed  under  /?,  in  so  far  as  it 
is  most  convenient  to  standardize  the  thiosulphate  against  a  ferric- 
chloride  solution  of  known  strength,  and  then  to  use  it  on  solutions 
of  unknown  strength  to  determine  their  iron  content. 

"We  require — 1.  A  solution  of  sodium  thiosulphate  containing 
about  12  grm.  (of  the  crystallized  salt)  per  litre.  2.  A  solution  of 
ferric  chloride  of  known  strength,  prepared  by  dissolving  10 -04 
grm.  of  clean,  fine,  and  soft  iron  wire  (=10  grm.  pure  iron)  in 
hydrochloric  acid  in  a  slanting  long-necked  flask,  oxidizing  the 
solution  with  potassium  chlorate,  completely  removing  the  excess 
of  chlorine  by  protracted  gentle  boiling,  and  finally  diluting 
the  solution  to  1  litre.  3.  A  solution  of  cupric  sulphate,  1 
in  100.  4.  A  solution  of  potassium  sulphocyanate,  1  in  100. 
5.  A  solution  of  iodine  in  potassium  iodide,  containing  5  or 
6  grm.  iodine  in  the  litre  (compare  §  146,  3).  6.  Thin  starch 
paste. 

Measure  off  some  of  the  sodium  thiosulphate,  add  starch  paste 
(§  146,  3),  and  then  titrate  with  iodine  solution,  in  order  to  de- 


in  Zeitschr.  f.  analyt.  Chem.,  vi,  129;  it  was  criticised  and  rejected  in  MOHR'S 
Lehrb.  d.  Titrirmethode,  3  Aufl.  291.  OUDEMANS  replied  toMomi  in  Zeitschr.  f. 
•analyt.  Chem.,  ix,  342,  and  an  examination  of  the  method  by  C.  BALLING  ap- 
peared in  the  same  journal,  ix,  99. 


334  DETERMINATION.  [§  113. 

terraine  the  relation  between  the  two  solutions.  Now  transfer  10 
or  20  c.  c.  of  the  ferric  chloride  to  a  beaker,  add  2  c.  c.  concen- 
trated hydrochloric  acid,  100  or  150  c.  c.  water,  3  c.  c.  copper 
solution,  and  1  c.  c.  potassium-sulphocyanate  solution,  titrate  with 
sodium  thiosulphate  till  the  fluid  just  loses  its  color,  add  at  once 
some  starch  paste,  and  titrate  back  with  iodine  solution  till  the 
blue  color  appears.  Deduct  the  thiosulphate  equivalent  to  the 
iodine  solution  from  the  total  quantity  of  thiosulphate  used ;  the  re- 
mainder will  represent  the  amount  required  to  reduce  the  iron 
present.  In  the  analysis  the  conditions  should  be  similar  to  those 
in  the  standardizing  of  the  thiosulphate. 

This  method  is  very  rapid,  and  the  results,  though  not  so 
accurate  as  those  by  methods  a  and  /?,  are  quite  good  enough  for 
many  technical  purposes. 

Supplement  to  §§  112  and  113. 

Besides  the  methods  given  in  §§  112  and  113,  there  have 
been  many  others,  particularly  indirect  ones,  advocated.  Since 
these,  however,  possess  no  advantages  over  those  described  above, 
or  are  capable  of  only  limited  application,  I  will  confine  myself  to 
a  description  of  only  the  most  important. 

1.  FITCH'S  method*  :   Add  hydrochloric  acid  to  the  solution, 
which  must  contain  the  iron  as  a  ferric  salt,  and  be  free  from 
nitric  acid,  and  boil  in  contact  with  a  few  strips  of  metallic  copper 
until  the  solution  acquires  a  light-green  color.     Then  estimate  the 
iron  from  the  loss  in  weight  of  the  copper  (FeaCl6  -|-  Cu  =  2FeCl2 
-]-  2CuCl).      The  method  yields  good  results  only  when  the  most 
careful  attention  is  paid  in  excluding  the  air.    The  conditions  most 
favorable  to  success  have  been   studied  by  J.  LOWE  and  KONIG, 
and   are  detailed,  under  the    "Analysis  of  Iron   Ores,"  in  the 
Special  Part. 

2.  The  solution  containing  the  iron  as  a  ferric  salt,  and  free 
from  the  metals  of  the  fifth  and  sixth  groups,  as  well  as  other  sub- 
stances decomposable  by  hydrogen  sulphide,  is  precipitated  by  an 
excess  of  a  clear  solution  of  hydrogen  sulphide,  avoiding  all  heat. 
The  precipitated  sulphur  is  determined  after  a  few  days,  and  the 
quantity  of  iron  calculated  therefrom  according  to  the  equation 

*Jour.f.  prakt.  Ghem.,  xvn,  160. 


5  114.]  URANIUM   AND   URANYL.  335 

FeaO3  +  HaS  =  2FeO  +  HaO  +  S  (H.  EOSE).    Eesults  accurate. 
Compare  also  DELFFS.* 

3.  Add  an  excess  of  gold  and  sodium  chloride  to  the  solution 
containing  the  iron  as  a  ferrous  salt,  close  the  bottle,  and  deter- 
mine the  precipitated  gold :  6FeCla  +  2  AuCl,  =  3FeaCl.  +  2Au 
(H.  KOSE). 

Supplement  to  the  Fourth  Group. 

§  H4. 
7.  URANIUM  AND  URANYL. 

If  the  compound  in  which  the  uranium  is  to  be  determined 
contains  no  other  fixed  substances,  it  may  often  be  converted  into 
uranous  uranate  U(UO4)2 — (called  also  uranoso-uranic  oxide  UO,- 
2UO3) — by  simple  ignition.  If  sulphuric  acid  is  present,  small  por- 
tions of  ammonium  carbonate  must  be  thrown  into  the  crucible 
towards  the  end  of  the  operation. 

In  cases  where  the  application  of  this  method  is  inadmissible, 
the  solution  of  uranium  (which,  if  it  contains  uranous  salts,  must 
ti  rat  be  warmed  with  nitric  acid,  until  they  are  converted  into  uranyl 
suits)  is  nearly  boiled  in  a  platinum  or  porcelain  dish,  and  pre- 
cipitated with  ammonia  in  slight  excess.  The  yellow  precipitate 
formed,  which  consists  of  hydrated  ammonium  uranate,  is  filtered 
off  hot  and  washed  with  a  dilute  solution  of  ammonium  chloride,  to 
prevent  the  fluid  passing  milky  through  the  filter.  The  precipitate 
is  dried  and  ignited  (§  53).  To  make  quite  sure  of  obtaining  the 
uranous  uranate  in  the  pure  state,  the  crucible  is  ignited  for  some 
time  in  a  slanting  position  and  uncovered ;  the  lid  is  then  put  on, 
while  the  ignition  is  still  continuing ;  the  crucible  is  allowed  to 
cool  under  the  desiccator,  and  weighed  (RAMMELSBERG). 

If  the  solution  from  which  the  uranyl  is  to  be  precipitated  con- 
tains other  basic  radicals  (alkali-earth  metals,  or  even  alkali  metals), 
portions  of  these  will  precipitate  along  with  the  ammonium  uranate. 
For  the  measures  to  be  resorted  to  in  such  cases,  I  refer  to  Sec- 
tion Y. 

The  reduction  of  the  uranous  uranate  to  the  state  of  uranous 

*Chem.  CentralbL,  1856,  839. 


336  DETERMINATION. 

> 

oxide  (UO2)  is  an  excellent  means  of  ascertaining  its  purity  for  the 
purpose  of  control.  This  reduction  should  never  be  omitted,  since 
PELIGOT  has  found  the  uranous  uranate  to  be  variable  in  composi- 
tion. It  is  effected  by  ignition  in  a  current  of  hydrogen  gas,  in  the 
way  described  §  111,  1  (Cobalt).  In  the  case  of  large  quantities 
the  ignition  must  be  several  times  repeated,  and  the  residue  must 
be  occasionally  stirred  with  a  platinum  wire.  While  cooling 
increase  the  current  of  gas  to  prevent  reabsorption  of  oxygen.  By 
intense  heating  the  property  of  spontaneous  ignition  in  the  air  is 
destroyed.  If  after  evaporating  a  solution  of  uranyl  chloride,  the 
residue  is  to  be  ignited  in  hydrogen,  heat  gently  at  first  in  the  gas 
to  avoid  loss  by  volatilization.  The  separation  of  uranyl  from 
phosphoric  acid  is  effected  by  fusing  the  compound  with  potassium 
cyanide  and  sodium  carbonate.  Upon  extracting  the  fused  mass 
with  water,  the  phosphoric  acid  is  obtained  in  solution,  whilst  ura- 
nium is  left  as  uranous  oxide.  KNOP  and  ARENDT*  have  employed 
this  method. 

Taking  239 '6  as  the  atomic  weight  of  uranium,  uranous  uran- 
ate, U(UO4)a ,  contains  84-88  percent,  of  uranium  and  15-12  per 
cent,  of  oxygen.  UOa,  uranous  oxide,  contains  88*22  per  cent, 
uranium  and  11 '78  per  cent,  of  oxygen. 

According  to  BELOHOFBECK,t  uranium  may  be  also  determined 
volumetrically  by  reducing  the  solution  of  uranyl  acetate  or  sul- 
phate to  uranous  salts  with  zinc,  as  in  the  case  of  iron  (§  113,  3,  a). 
As  the  color  of  the  solution  is  no  safe  criterion  of  the  end  of  the 
reduction,  you  must  allow  the  action  of  the  zinc  to  continue  for  a 
considerable  time.  BELOHOUBECK  says,  a  quarter  of  an  hour  is 
sufficient  for  small  quantities,  half  an  hour  for  large  quantities. 
The  solution  of  the  uranous  salt  is  diluted,  mixed  with  dilute  sul- 
phuric acid,  and  then  titrated  with  permanganate  to  incipient  red- 
dening. The  permanganate  is  standardized  by  §  112,  2;  1  at. 
uranium  =  2  at.  iron. 

BELOHOUBECK  obtained  good  results  also  in  hydrochloric  solu- 
tions, but  experiments  made  in  this  laboratory  have  shown  that 
these  are  liable  to  the  error  pointed  out  in  the  case  of  iron  (Comp. 
p.  319,7),  at  least  in  the  presence  of  considerable  quantities  of 
hydrochloric  acid. 

*  Ctiem.  Centralblatt,  1856,  773.         f  Zeitschr.f.  analyt.  Chem.,  vi,  120. 


§  115.]  SILVER.  337 

Fifth  Group. 

SILVER LEAD MERCURY    IN    MERCUROTJS    COMPOUNDS MERCURY    IN 

MERCURIC     COMPOUNDS COPPER BISMUTH — CADMIUM (PALLA- 
DIUM). 

§115. 

1.  SILVER. 

a.  Solution. 

Metallic  silver,  and  those  of  its  compounds  which  are  insoluble 
in  water,  are  best  dissolved  in  nitric  acid  (if  soluble  in  that  acid). 
Dilute  nitric  acid  suffices  for  most  compounds ;  silver  sulphide, 
however,  requires  concentrated  acid.  The  solution  is  effected  best 
in  a  flask,  which  should  be  heated  if  necessary,  and  placed  in  a 
slanting  position  if  gas  is  evolved.  In  the  case  of  metallic  silver, 
or  silver  sulphide,  the  solution  is  heated  finally  to  gentle  boiling 
to  drive  off  nitrous  acid.  Silver  chloride,  bromide,  and  iodide  are 
insoluble  in  wrater  and  in  nitric  acid.  To  get  the  silver  contained 
in  chloride  and  bromide  in  solution,  proceed  as  follows : — Fuse  the 
salt  in  a  porcelain  crucible  (this  operation,  though  not  absolutely 
indispensable,  had  better  not  be  omitted),  pour  water  over  it,  put 
u  piece  of  clean  cadmium,  zinc,  or  iron  upon  it,  and  add  some 
dilute  sulphuric  acid.  Wash  the  reduced  spongy  silver,  first  with 
dilute  sulphuric  acid,  then  with  water,  and  finally  dissolve  it  in 
nitric  acid.  However,  as  we  shall  see  below,  the  quantitative 
analysis  of  these  salts  does  not  necessarily  involve  their  solution. 

1>.  Determination. 

Silver  may  be  weighed  as  chloride,  sulphide,  or  cyanide,  or  in 
the  metallic  state  (§  82).  It  is  also  frequently  determined  by  volu- 
metric analysis. 

We  may  convert  into 

1.  SILVER  CHLORIDE  :  All  compounds  of  silver  without  excep- 
tion. 

2.  SILVER  SULPHIDE  :  3.  SILVER  CYANIDE  :  All  compounds  so^u 
ble  in  water  or  nitric  acid. 

4.  METALLIC  SILVER  :  Silver  oxide  and  some  silver  salts  of  readily 
volatile  acids;  silver  salts  of  organic  acids;  silver  chloride,  bro- 
mide, iodide,  sulphide,  and  sulphate. 

The  method  -1  is  the  most  convenient,  especially  when  con- 
ducted in  the  dry  way,  and  is  preferred  to  the  others  in  all  cases 


338  DETERMINATION.  [§  115. 

where  its  application  is  admissible.  The  method  1  is  that  most 
generally  resorted  to.  2  and  3  serve  mostly  only  to  effect  the 
separation  of  silver  from  other  metals. 

In  assays  for  the  Mint,  silver  is  usually  determined  volumetric- 
ally  by  GAY-LUSSAC'S  method.  PISANI'S  volumetric  method  is 
especially  suited  to  the  determination  of  very  small  quantities  of 
silver.  H.  YOGEL'S  method  is  specially  useful  to  photographers. 
The  estimation  of  silver  by  cupellation  will  be  detailed  under 
"  Analysis  of  Galena,"  in  the  Special  Part. 

1.  Determination  of  Silver  as  Chloride. 

a.  In  the  Wet  Way. 

Mix  the  moderately  dilute  solution  in  a  beaker  with  nitric  acid,, 
heat  to  about  70°,  and  add  hydrochloric  acid  with  constant  stirring 
till  it  ceases  to  produce  a  precipitate.  A  large  excess  of  hydro- 
chloric acid  must  be  avoided,  as  the  precipitate  is  not  absolutely 
insoluble  therein.  While  protecting  the  contents  of  the  beaker 
from  the  action  of  direct  sunlight  continue  the  heat  till  the  precipi- 
tate has  fully  settled,  pour  off  the  clear  fluid  through  a  small  filter,, 
rinse  the  precipitate  on  to  the  latter  by  means  of  hot  water  mixed 
with  some  nitric  acid,  wash  with  hot  water  containing  nitric  acid,, 
then  with  pure  hot  water,  dry  thoroughly,  transfer  the  precipitate 
to  a  watch-glass  as  nearly  as  possible,  incinerate  the  filter  in  a 
weighed  porcelain  crucible,  treat  the  ash  (which  always  contains 
some  metallic  silver)  with  a  few  drops  of  nitric  acid  in  the  heat ; 
add  two  or  three  drops  of  hydrochloric  acid,  evaporate  cautiously 
to  dryness,  add  the  main  bulk  of  the  precipitate,  using  a  camel's- 
hair  brush  to  transfer  the  last  portions,  heat  cautiously  till  it  begins 
to  fuse  at  the  edge,  allow  to  cool,  and  weigh. 

To  remove  the  fused  mass  without  breaking  the  crucible,  lay 
a  small  piece  of  iron  or  zinc  upon  it,  and  then  add  very  dilute 
hydrochloric  or  sulphuric  acid.  The  chloride  will  be  reduced,  and 
the  silver  may  now  be  detached  from  the  crucible  with  the  greatest 
ease. 

For  the  properties  of  the  precipitate  see  §  82.  The  method 
gives  very  exact  results,  at  all  events  in  the  absence  of  any  con- 
siderable quantities  of  those  salts  in  which  silver  chloride  is  some^ 
what  soluble ;  compare  §  82.  To  avoid  error  in  this  respect,  it  is 
well  to  test  the  clear  filtrate  with  hydrogen  sulphide. 


§  115.] 


SILVER. 


339 


Ij.   In  the  Dry  Way. 

This  method  serves  more  exclusively  for  the  analysis  of  silver 
bromide  and  iodide,  although  it  can  be  applied  in  the  case  of  other 
compounds. 


Fig.  87. 

The  process  is  conducted  in  the  apparatus  illustrated  by  Fig.  87. 

a  is  a  flask  for  disengaging  chlorine ;  it  is  completely  filled  with 
pieces  of  pyrolusite  (native  manganese  dioxide)  of  the  size  of  hazel- 
nuts,  and  half  filled  with  strong  hydrochloric  acid ;  £  contains 
concentrated  sulphuric  acid ;  c  contains  calcium  chloride;  d  is  a 
bulb  containing  the  silver  iodide  or  bromide;  e  conducts  the 
chlorine  by  means  of  a  rubber  tube  into  the  open  air  or  into  a 
flask  containing  calcium  hydroxide.  The  operation  is  commenced 
by  introducing  the  compound  to  be  analyzed  into  the  bulb,  and 
applying  heat  to  the  latter  until  its  contents  are  fused ;  when  cold, 
the  tube  is  weighed  and  connected  with  the  apparatus.  Chlorine 
gas  is  then  evolved  from  a ;  when  the  evolution  of  the  gas  has 
proceeded  for  some  time,  the  contents  of  the  bulb  are  heated  to 
fusion,  and  kept  in  this  state  for  about  fifteen  minutes,  agitating 
now  and  then  the  fused  mass.  The  bulb-tube  is  then  removed 
from  the  apparatus,  allowed  to  cool,  and  held  in  a  slanting  position 
to  replace  the  chlorine  by  atmospheric  air;  it  is  subsequently 


340  DETERMINATION.  [§  115. 

weighed,  then  again  connected  with  the  apparatus,  and  the  former 
process  repeated,  keeping  the  contents  of  d  in  a  state  of  fusion  for 
a  few  minutes.  The  operation  may,  in  ordinary  cases,  be  con- 
sidered concluded  if  the  weight  of  the  tube  suffers  no  variation 
by  the  repetition  of  the  process.  If  the  highest  degree  of  accuracy 
is  to  be  attained,  heat  the  silver  chloride  again  to  fusion,  passing 
;at  the  same  time  a  slow  stream  of  pure,  dry  carbon  dioxide  through 
the  tube,  in  order  to  drive  ont  the  traces  of  chlorine  absorbed  by 
the  fused  chloride.  Allow  to  cool,  hold  obliquely  for  a  short  timer 
so  as  to  replace  the  carbon  dioxide  by  air,  and  finally  weigh. 

2.  Determination  as  Silver  Sulphide. 

Hydrogen  sulphide  precipitates  silver  completely  from  acid, 
neutral,  and  alkaline  solutions ;  ammonium  sulphide  precipitates  it 
from  neutral  and  alkaline  solutions.  The  precipitate  does  not 
settle  clearly  and  rapidly  except  a  free  acid  or  salt  be  present  (such 
as  nitric  acid  or  an  alkali  nitrate).  Recently  prepared  perfectly 
clear  solution  of  hydrogen  sulphide  may  be  employed  to  precipitate 
small  quantities  of  silver ;  to  precipitate  larger  quantities,  the  solu- 
tion of  the  salt  of  silver  (which  must  riot  be  too  acid)  is  moderately 
diluted,  and  washed  hydrogen  sulphide  gas  conducted  into  it. 
After  complete  precipitation  has  been  effected,  and  the  silver  sul- 
phide has  perfectly  subsided  (with  exclusion  of  air),  it  is  collected 
on  a  weighed  filter,  washed,  dried  at  100°,  and  weighed.  For  the 
properties  of  the  precipitate,  see  §  82.  This  method,  if  properly 
executed,  gives  accurate  results.  The  operator  must  take  care  to 
filter  quickly,  and  to  prevent  the  access  of  air  as  much  as  possible 
during  the  filtration,  since,  if  this  precaution  be  neglected,  sulphur 
is  likely  to  separate  from  the  hydrogen  sulphide  water,  which,  of 
course,  would  add  falsely  to  the  weight  of  the  silver  sulphide.  If 
the  presence  of  a  minute  quantity  of  sulphur  in  the  precipitate  is 
suspected,  treat  it  after  drying  with  pure  carbon  disulphide  on  the 
filter  repeatedly,  till  the  fluid  running  through  gives  no  residue  on 
evaporation  in  a  watch-glass ;  dry  and  weigh. 

The  sulphide  must,  however,  never  be  weighed  as  just  described, 
imless  the  analyst  is  satisfied  that  no  considerable  amount  of  sul- 
phur has  fallen  down  with  it,  as  would  occur  if  the  fluid  contained 
liyponitric  acid,  a  ferric  salt,  or  any  other  substance  which  decom- 
poses hydrogen  sulphide.  In  case  the  precipitate  does  contain 
much  admixed  sulphur,  the  simplest  process  is  to  convert  it  into 


§115.]  SILVER.  341 

metallic  silver  (H.  ROSE*).  For  this  purpose  it  is  transferred  to  a 
weighed  porcelain  crucible,  the  filter  ash  is  added,  and  the  whole 
is  heated  to  redness  in  a  stream  of  hydrogen,  the  apparatus 
described  in  §  108  being  employed.  Results  accurate. 

Should  the  apparatus  in  question  not  be  at  the  operator's  dis- 
posal, he  may,  after  complete  washing  of  the  precipitate,  carefully 
rinse  it  into  a  porcelain  dish  (without  injuring  the  weighed  filter), 
heat  it  once  or  twice  with  a  moderately  strong  solution  of  pure 
sodium  sulphite,  retransfer  the  precipitate  (now  freed  from  admixed 
sulphur)  to  the  old  filter,  wash  well,  dry  and  weigh  (J.  LowEf) ; 
or  he  may  treat  the  dried  precipitate,  together  with  the  filter-ash, 
with  moderately  dilute  chlorine-free  nitric  acid  at  a  gentle  heat, 
till  complete  decomposition  has  been  effected  (till  the  undissolved 
sulphur  has  a  clean  yellow  appearance),  filter,  wash  well,  and  pro- 
ceed according  to  1,  a. 

3.  Determination  as  Silver  Cyanide. 

Mix  the  neutral  solution  of  silver  with  potassium  cyanide,  until 
the  precipitate  of  silver  cyanide  which  forms  at  first  is  redissolved  ; 
add  nitric  acid  in  slight  excess,  and  apply  a  gentle  heat.  If  the 
solution  contains  free  acid,  this  must  be  first  neutralized  with  pot- 
ash or  sodium  carbonate.  After  some  time,  collect  the  precipitated 
silver  cyanide  on  a  weighed  filter,  wash,  dry  at  100°,  and  weigh. 
For  the  properties  of  the  precipitate,  see  §  82.  The  results  are 
accurate. 

4.  Determination  as  Metallic  Silver. 
a.  In  the  Dry  Way. 

Silver  oxide,  silver  carbonate,  &c.,  are  easily  reduced  by  simple 
ignition  in  a  porcelain  crucible.  In  the  reduction  of  salts  of 
organic  acids,  the  crucible  is  kept  covered  at  first,  and  a  moder- 
ate heat  applied ;  after  a  time  the  lid  is  removed,  and  the  heat 
increased,  until  the  whole  of  the  carbon  is  consumed.  For  the 
properties  .of  the  residue,  see  §  82.  The  results  are  absolutely 
accurate,  except  as  regards  silver  salts  of  organic  acids ;  in  the 
analysis  of  the  latter,  it  not  unfrequently  happens  that  the  reduced 
silver  contains  a  minute  portion  of  carbon,  which  increases  the 
weight  of  the  residue  to  a  trifling  extent. 

If  it  is  desired  to  transform  silver  chloride,  bromide,  or  sulphide 
*  Pogg.  AnnaL,  ex,  139.  \Journ.f.  prakt.  Chem.,  LXXVII,  73. 


342  DETERMINATION.  [§  115. 

into  metallic  silver,  for  the  purpose  of  analysis,  they  are  heated  in 
a  current  of  pure  hydrogen  to  redness,  till  the  weight  remains 
constant.  The  process  may  be  conducted  in  a  porcelain  crucible 
or  a  bulb-tube.  In  the  former  case,  the  apparatus  described  in 
§  108  is  used ;  in  the  latter  the  apparatus  represented  in  Fig.  87, 
with  the  substitution,  of  course,  of  hydrogen  for  chlorine  (§  64, 
14).  If  the  bulb-tube  is  used,  it  must,  after  cooling  and  before 
being  weighed,  be  held  in  an  inclined  position,  so  that  the  hydrogen 
may  be  replaced  by  air.  The  results  are  perfectly  accurate.  Silver 
iodide  cannot  be  reduced  in  this  way. 

I.    In  the  Wet  Way. 

If  the  silver  solution  is  that  of  a  nitrate,  as  is  usual,  add  a  little 
sulphuric  acid  and  evaporate  till  all  the  nitric  acid  is  expelled,  dis- 
solve the  silver  sulphate  in  hot  water,  transfer  it  to  a  weighed 
porcelain  crucible,  arid  immerse  in  the  solution  a  rod  of  cadmium. 
The  silver  is  rapidly  reduced,  and  the  precipitated  metal  may  be 
easily  removed  from  the  cadmium  and  collected  into  a  coherent 
mass.  Warm  the  latter  with  the  acid  liquid  until  no  more 
hydrogen  is  evolved,  wash  with  hot  water  by  decantation,  dry,  and 
ignite.  Results  accurate  (A.  CLASSEN*).  Cadmium  is  preferable 
to  zinc,  because  the  latter  usually  leaves  behind  a  little  lead  on 
solution  in  sulphuric  acid. 

5.    Volumetric  Methods. 

I.    GAY-LUSSAC'S. 

This,  the  most  exact  of  all  known  volumetric  processes,  was 
introduced  by  GAY-LUSSAC  as  a  substitute  for  the  assay  of  silver  by 
cupellation,  was  thoroughly  investigated  by  him,  and  will  be  found 
fully  described  in  his  work  on  the  subject.  This  method  has  been 
rendered  still  more  precise  by  the  researches  of  G.  J.  MULDER,  to 
whose  exhaustive  monograph  f  I  refer  the  special  student  of  this 
branch.  I  shall  here  confine  myself  to  giving  the  process  so  far 
as  to  suit  the  requirements  of  the  chemical  laboratory,  taking  only 
for  granted  that  the  analyst  has  the  ordinary  measuring  apparatus, 
<fec.,  at  his  disposal.  MULDER'S  results  will  be  made  use  of  to  the 
full  extent  possible  under  these  circumstances. 

a.  REQUISITES. 

a.  SOLUTION    OF    SODIUM    CHLORIDE.       Take    chemically  pure 

*  Zeitschr.  f.  analyt.  Ckem.,  v,  402.  The  method  given  by  MILLON  and 
COMMAILLE  (Zeitschr.  f.  analyt.  Cliem.,  n,  212),  in  which  silver  is  precipitated 
by  means  of  copper-ammonium  chloride,  is  not  to  be  recommended,  according 
to  the  investigations  of  STAS  (ibid.,  vi,  426),  as  well  as  those  conducted  by  me. 

f  Die  Silberprobirmethode  (see  note  p.  198). 


§  115.]  SILVER.  343 

sodium  chloride— either  artificially  prepared  or  pure  rock-salt- 
powder  it  roughly  and  ignite  moderately  (not  to  fusion*).  Now 
dissolve  5*4207  grm.  in  distilled  water  to  1  litre,  measured  at  16°. 
100  c.c.  of  this  solution  contains  a  quantity  of  sodium  chloride 
equivalent  to  1  grm.  of  silver,  Ag.  The  solution  is  kept  in  a 
.stoppered  bottle  and  shaken  before  use. 

/?.  DECIMAL  SOLUTION  OF  SODIUM  CHLORIDE.  Transfer  50  c.c. 
of  the  solution  described  in  a  to  a  500-c.c.  measuring  flask, 
fill  up  to  the  mark  with  distilled  water  and  shake.  Each  c.c. 
of  this  decimal  solution  corresponds  to  O'OOl  grm.  silver.  The 
measuring  must  be  performed  at  16°.  The  solution  is  preserved 
like  the  other. 

y.  DECIMAL  SILVER  SOLUTION.  Dissolve  0*5  grm.  chemically 
pure  silver  f  in  2  to  3  c.c.  pure  nitric  acid  of  1  *2  sp.  gr.,  and  dilute 
the  solution  with  water  exactly  to  500  c.c.  measured  at  16°.  Each 
c.c.  contains  0*  001  grm.  silver.  The  solution  is  kept  in  a  stoppered 
bottle  and  protected  against  the  influence  of  light. 

d.  TEST-BOTTLES.  These  should  be  of  colorless  glass,  holding 
easily  200  c.c.,  closed  w^ith  well-ground  glass-stoppers,  running  to 
a  point  below.  The  bottles  fit  into  cases  blackened  on  the  inside, 
and  reaching  up  to  their  necks.  In  order  to  protect  the  latter  also 
from  the  action  of  light,  a  black-cloth  cover  is  employed. 

b.  PRINCIPLE. 

Suppose  we  know  the  value  of  a  solution  of  sodium  chloride, 
•i.e.,  the  quantity  that  is  necessary  to  precipitate  a  given  amount  of 
•silver,  say  1  grm.,  we  are  in  the  position,  with  the  aid  of  this  solu-. 

*  On  fusion,  if  the  flame  can  in  the  least  way  act  upon  it,  it  takes  an  alkaline 
reaction,  since  under  the  influence  of  vapor  of  water  and  carbon  dioxide,  a  little 
hydrochloric  acid  is  formed  and  escapes,  while  a  corresponding  quantity  of 
sodium  carbonate  remains. 

f  For  the  preparation  of  pure  silver  STAS  recommends  the  following  method : 
Take  crude  silver  nitrate  containing  copper,  fuse  in  order  to  decompose  any 
platinum  nitrate  which  may  be  present,  dissolve  in  dilute  ammonia,  allow  to 
stand  48  hours,  filter  and  dilute  till  the  fluid  does  not  contain  more  than  2  per 
cent,  silver.  Add  ammonium  sulphite  in  excess.  To  ascertain  how  much  sul- 
phite will  be  required  make  a  small  preliminary  test;  as  soon  as  after  heating  the 
blue  oolution  loses  all  color,  you  may  be  sure  that  enough  of  the  sulphide  has 
been  added.  Warm  on  a  water-bath  to  60°  or  70°,  when  all  the  silver  will  be 
thrown  down  as  a  metallic  powder,  allow  to  cool  and  wash  by  decantation  with 
diluted  ammonia  till  the  washings  are  free  from  copper  and  sulphuric  acid.  Now 
•digest  the  metal  for  several  days  with  strong  ammonia,  wash,  dry,  and  fuse  with 
a  flux  of  borax  and  sodium  nitrate. 


344  DETEEMI^ATION.  [§  115. 

tion,  to  determine  an  unknown  amount  of  silver,  for  if  we  put  x 
for  the  unknown  amount  of  silver,  then 


c.c 


.  of  solution  used  for  1  grm.  :  c.c.  used  for  x : :  1  grm.  :  x. 


But  if  we  examine  whether  1  mol.  sodium  chloride  dissolved  in 
water  actually  precipitates  1  at.  of  silver  dissolved  in  nitric  acid 
exactly,  we  find  that  this  is  not  the  case.*  On  the  contrary,  the 
clear  supernatant  fluid  gives  a  small  precipitate  both  on  the  addition 
of  a  little  solution  of  sodium  chloride,  and  on  the  addition  of  a 
little  silver  solution,  as  MULDEK  has  most  accurately  determined. 
The  value  of  a  solution  of  sodium  chloride  in  the  sense  explained 
above  cannot,  therefore,  be  reckoned  from  the  amount  of  salt  it 
contains,  by  calculating  1  at.  silver  for  1  mol.  sodium  chloride,  but 
it  can  only  be  obtained  by  experiment.  MULDEK  has  shown  that 
the  temperature  and  the  degree  of  dilution  have  some  influence, 
and  also  that  this  fact  is  to  be  explained  011  the  ground  of  the  sol- 
vent power  of  the  sodium  nitrate  produced  on  the  silver  chloride. 
In  the  solution  thus  formed  we  have  to  imagine  NaNO3  and  NaCl 
with  AgNO3  in  a  certain  state  of  equilibrium,  which  on  the  addition 
of  either  Nad  or  AgNO3  is  destroyed,  silver  chloride  being  pre- 
cipitated. 

From  this  interesting  observation  it  follows,  that  if  to  a  silver- 
solution  we  add  at  first  concentrated  solution  of  sodium  chloride, 
then  decimal  solution  drop  by  drop,  till  the  exact  point  is  reached 
when  no  more  precipitate  appears,  now,  on  addition  of  decimal 
silver-solution,  a  small  precipitate  will  be  again  produced ;  and  if 
we  add  the  latter  drop  by  drop,  till  the  last  drop  occasions  no  tur- 
bidity, then  again  decimal  solution  of  sodium  chloride  will  give  a 
small  precipitate.  On  noticing  the  number  of  drops  of  both  deci- 
mal solutions  which  are  required  to  pass  from  one  limit  to  the 
other,  we  find  that  the  same  number  of  each  are  used.  Let  us- 
suppose  that  we  had  added  decimal  solution  of  sodium  chloride  till 
it  ceased  to  react,  and  had  then  used  20  dropsf  of  decimal  silver- 
solution,  till  this  ceased  to  produce  a  further  turbidity,  we  must 
now  again  add  20  drops  of  decimal  solution  of  sodium  chloride,  in 

*  If  sodium  bromide  or  potassium  bromide  is  used,  complete  precipitation 
would  ensue  on  addition  of  an  equivalent  quantity  of  silver  solution,  since  bro- 
mide of  silver  is  not  at  all  soluble  in  the  supernatant  fluid  (STAB,  Compt.  rend. 
LXVII,  1107). 

f  Twenty  drops  from  MULDER'S  dropping  apparatus  are  equal  to  1  c.  c. 


§  115.]  SILVER.  345 

order  to  reach  the  point  at  which  this  ceases  to  react.  Were  we  to 
add  only  10  instead  of  these  20  drops,  we  have  the  neutral  point, 
as  MULDER  calls  it,  i.e.,  the  point  at  which  both  silver  and  sodium 
chloride  produce  equal  precipitates. 

We  have,  therefore,  3  different  points  to  choose  from  for  our 
final  reaction :  «,  the  point  at  which  sodium  chloride  has  just 
ceased  to  precipitate  the  silver ;  5,  the  neutral  point ;  c,  the  point 
at  which  silver-solution  has  just  ceased  to  precipitate  sodium 
chloride.  Whichever  we  may  choose,  we  must  keep  to  it,  i.e.,  we 
must  not  use  a  different  point  in  standardizing  the  sodium  chloride 
solution  and  in  performing  an  analysis.  The  difference  obtained, 
by  using  first  a  and  then  ~b  is,  according  to  MULDER,  for  1  grm. 
silver,  at  16°,  about  0*5  mgrm.  silver;  by  employing  first  a  and 
then  c,  as  was  permitted  in  the  original  process  of  GAY-LUSSAC,  the 
difference  is  increased  to  1  mgrm. 

For  our  object,  it  appears  most  convenient  to  consider,  once  for 
all,  the  point  a  as  the  end,  and  never  to  finish  with  the  silver- 
solution.  If  the  point  has  been  overstepped  by  the  addition  of  too 
large  an  amount  of  decimal  solution  of  sodium  chloride,  2  or  3 
c.c.  of  decimal  silver-solution  should  be  added  all  at  once.  The 
end-point  is  then  found  by  carefully  adding  decimal  solution  of 
sodium  chloride  again,  and  the  quantity  of  silver  in  the  silver-solu- 
tion added  is  added  to  the  original  amount  of  silver  weighed  off. 

c.  PERFORMANCE  OF  THE  PROCESS. 

This  is  divided  into  two  operations — <*,  the  titration  of  the 
sodium-chloride  solution;  fi,  the  assay  of  the  silver  alloy  to  be 
examined. 

a.  TITRATION  OF  THE  SODIUM- CHLORIDE  SOLUTION. 

Weigh  off  exactly  from  1*001  to  1*003  grm.  chemically  pure 
silver,*  put  it  into  a  test-bottle,  add  5  c.c.  perfectly  pure  nitric 
acid,  of  1*2  sp.  gr.,  and  heat  the  bottle  in  an  inclined  position  in  a 
water-  or  sand-bath  till  complete  solution  is  effected.  Now  blow 
out  the  nitrous  fumes  from  the  upper  part  of  the  bottle,  and  after 
it  has  cooled  a  little,  place  it  in  a  stream  of  water,  the  temperature 
of  which  is  about  16°,  and  let  it  remain  there  till  its  contents  are 
cooled  to  this  degree,  wipe  it  dry,  and  place  it  in  its  case. 

Now  fill  the  100-c.c.  pipette  with  the  concentrated  solution  of 
sodium  chloride,  which  is  then  allowed  to  flow  into  the  test-bottle 


.*  Sec  note,  p.  34*3. 


346  DETEEMINATIOtf.  [§  115. 

containing  the  silver-solution*.  Insert  the  glass-stopper  firmly 
(after  moistening  it  with  water),  cover  the  neck  of  the  bottle  with 
the  cap  of  black  stuff  belonging  to  it,  and  shake  violently  without 
delay,  till  the  silver  chloride  settles,  leaving  the  fluid  perfectly 
clear.  Then  take  the  stopper  out,  rub  it  on  the  neck,  so  as  to 
remove  all  silver  chloride,  replace  it  firmly,  and  by  giving  the 
bottle  a  few  dexterous  turns,  rinse  the  chloride  down  from  the 
upper  part.  After  allowing  to  rest  a  little,  again  remove  the 
stopper,  and  add,  from  a  burette  divided  into  O'l  c.c.,  decimal 
sodium-chloride  solution,  allowing  the  drops  to  fall  against  the 
lower  part  of  the  neck,  the  bottle  being  held  in  an  inclined 
position.  If,  as  above  directed,  1-001  to  1*003  grm.  silver  have 
been  employed,  the  portions  of  sodium  chloride  solution  at  first 
added  may  be  -J  c.c.  After  each  addition,  raise  the  bottle  a  little 
out  of  its  case,  observe  the  amount  of  precipitate  produced,  shake 
till  the  fluid  has  become  clear  again,  and  proceed  as  above,  before 
adding  each  fresh  quantity  of  sodium-chloride  solution.  The 
smaller  the  precipitate  produced,  the  smaller  should  be  the  quan- 
tity of  sodium  chloride  next  added ;  towards  the  end  only  two 
drops  should  be  added  each  time ;  and  quite  at  the  end  read  off 
the  height  of  the  fluid  in  the  burette  before  each  further  addition. 
"When  the  last  two  drops  give  no  more  precipitate,  the  previous 
reading  is  the  correct  one. 

If  by  chance  the  point  has  been  overstepped,  and  the  time  has 
been  missed  for  the  proper  reading  off  of  the  burette,  add  2  to  3 
c.c.  of  the  decimal  silver  solution  (the  silver  in  which  is  to  be 
added  to  the  quantity  first  weighed),  and  try  again  to  hit  the  point 
exactly  by  careful  addition  of  decimal  sodium-chloride  solution. 

The  value  of  the  sodium-chloride  solution  is  now  known. 
Reckon  it  to  1  grm.  silver. 

Suppose  we  had  used  for  1-002  grm.  silver,  100  c.c.  of  concen- 
trated and  3  c.c.  of  decimal  sodium-chloride  solution ;  this  makes 
altogether  100-3  of  concentrated  ;  then 

1-002  :  1-000  ::  100-3  :  x 

x  =  100-0998 

We  may  without  scruple  put  100-1  for  this  number.     We  now 

*  The  pipette,  having  been  filled  above  the  mark,  should  be  fixed  in  a  support, 
before  the  excess  is  allowed  to  run  out,  otherwise  the  measurings  will  not  be  suffi- 
ciently accurate. 


§  115.]  SILVER.  347 

know  that  100*1  c.c.  of  the  concentrated  solution  of  sodium 
chloride,  measured  at  16°,  exactly  precipitates  1  grin,  of  silver. 
This  relationship  serves  as  the  foundation  of  the  calculation  in 
.actual  assaying,  and  must  be  re-examined  whenever  there  is  reason 
to  imagine  that  the  strength  of  the  sodium  chloride  solution  may 
have  altered. 

P.    THE    ACTUAL    ASSAY    OF   THE    SlLVEK-ALLOY. 

"Weigh  off  so  much  as  contains  about  1  grm.  of  silver,  or  better, 
a  few  mgrm.  more  ;*  dissolve  in  a  test-bottle  in  5  to  T  c.c.  nitric 
acid,  and  proceed  in  all  respects  exactly  as  in  a. 

Suppose  we  had  taken  1-116  grm.  of  the  alloy,  and  in  addition 
to  the  100  c.c.  of  concentrated  sodium-chloride  solution,  had  used 
5  c.c.  of  the  dilute  (=0-5  concentrated),  how  much  silver  would 
the  alloy  contain  ? 

Presuming  that  we  use  the  same  sodium  chloride  solution 
which  served  as  our  example  in  a,  100-1  c.c.  of  which  =  1  grm. 
.•silver,  then 

100-1  :  100-5  : :  1-000  :  so 

x  =  1-003996  (say  (1-004). 

We  may  also  arrive  at  the  same  result  in  the  following  manner  : — 

NaCl  Solution. 
For  the  precipitation  of  the  silver  in  the  alloy 

were  used 100-5  c.c. 

For  1  .grm.  silver  are  necessary 100*1  c.c. 

Difference 0*4  c.c. 

There  are,  therefore,  4  mgrm.  of  silver  present  more  than  a  grm., 
on  the  presumption  that  0*1  of  the  concentrated  sodium-chloride 
.solution  (=  1  c.c.  of  the  decimal  solution)  corresponds  to  1  mgrm. 

*  In  coins  containing  9  parts  of  silver  and  1  part  of  copper,  therefore  take 
about  1*115  or  1-120.  In  weighing  off  alloys  of  silver  and  copper,  which  do  not 
correspond  to  the  formula  Ag3Cu2  (standard  JL££^'L,)  =  we  must  remember  that 
they  are  never  homogeneous  in  the  mass  ;  thus,  for  instance,  the  pieces  of  metal, 
from  which  coins  are  stamped,  often  show  1*5  to  1  7  in  a  thousand  more  silver  in 
the  middle  than  at  the  edges.  In  assaying  alloys,  then,  portions  from  various 
parts  of  the  mass  must  be  taken,  in  order  to  get  a  correct  result.  The  inaccuracy, 
however,  proceeding  Irom  the  cause  above-mentioned,  can  only  be  completely 
overcome  by  fusing  the  alloy  and  taking  out  a  portion  from  the  well-stirred  mass 
for  the  assay. 


348  DETERMINATION".  [ 

silver.  This  supposition,  although  not  absolutely  correct,  may  be 
safely  made,  for  the  inexactness  it  involves  is  too  minute,  as  is- 
evident  from  the  previous  calculation. 

Before  we  can  execute  this  process  exactly,  we  must  know  the 
quantity  of  silver  the  alloy  contains  very  approximately.  In 
assaying  coins  of  known  value  this  is  the  case,  but  with  other  silver 
alloys  it  is  usually  not  so.  Under  the  latter  circumstances  an 
approximate  estimation  must  precede  the  regular  assay.  This  is 
performed  by  Aveighing  off  0*5  grm.  (or  in  the  case  of  alloys  that  are 
poor  in  silver,  1  grm.),  dissolving  in  3  to  6  c.c.  nitric  acid,  and 
adding  from  the  burette  sodium  chloride  solution, — first  in  larger,, 
then  in  smaller  quantities — till  the  last  drops  produce  no  further 
turbidity.  The  last  drops  are  not  reckoned  with  the  rest.  The 
operation  is  conducted,  as  regards  shaking,  &c.,  as  previously 
given.  Suppose  we  had  weighed  off  0'5  grm.  of  the  alloy,  and 
employed  25  c.c.  of  the  sodium-chloride  solution — taking  the 
above  supposed  value  of  the  latter — 

We  have  100-1  :  25  : :  1-000  :  x 

x  =  0-249T 

that  is,  the  silver  in  0*5  grm.  of  the  alloy ;  and  as  to  the  quantity  of 
alloy  we  have  to  weigh  off  for  the  assay  proper, 

We  have  0-2497  :  1-003  : :  -5  :  x 

x  =  2-008. 

This  quantity  will,  of  course,  require  more  nitric  acid  for  solution 
than  was  previously  used  (use  10  c.c.).  In  cases  where  the  highest 
degree  of  accuracy  is  not  required,  the  results  afforded  by  this 
rough  preliminary  estimation  will  be  accurate  enough,  if  the 
experiment  is  carefully  conducted,  since  they  give  the  quantity  of 
silver  present  to  within  ToVir  or  Trirr- 

With  alloys  which  contain  sulphur,  and  with  such  as  consist  of 
gold  and  silver,  and  contain  a  little  tin,  LEVOL*-  employs  concen- 
trated sulphuric  acid  (about  25  grm.)  as  solvent.  The  portion  of 
the  alloy  is  boiled  with  it  till  dissolved ;  after  cooling,  the  fluid  is 
treated  in  the  usual  manner.  As,,  however,  concentrated  sulphuric 
acid  fails  to  dissolve  all  the  silver  when  there  is  much  copper 
present,  MAscAzziNif  digests  the  weighed  portion  of  alloy  (which 

*  Annal.  de  Chim.  et  de  Phys.  (3),  XLIV,  34?  ;  Journ.f.  prakt.  Chem.,  LXVI, 
382.  t  Chem.  CentralbL,  1357,  300. 


§115.]  SILVER.  349 

may  contain  small  quantities  of  lead,  tin,  and  antimony,  besides 
gold)  first  with  the  least  possible  amount  of  nitric  acid,  as  long  as 
red  vapors  are  formed ;  he  then  adds  concentrated  sulphuric  acid, 
boils  till  the  gold  has  settled  well  together,  adds  water  after 
cooling,  and  then  titrates.  In  the  presence  of  mercury,  the 
chloride  of  that  metal  is  carried  down  with  the  silver,  rendering 

'  o 

the  method  inaccurate.  If  the  quantity  of  mercury  is  but  small, 
you  may  get  over  the  difficulty  by  adding  25  c.c.  ammonia  and 
20  c.c.  acetic  acid  (LEVOL).  The  ammonium  acetate  acts  by 
decomposing  the  mercuric  chloride,  and  thus  preventing  its 
precipitation  (DEBBAT*).  If  the  quantity  of  mercury  is  large  the 
addition  of  an  alkali  acetate  is  not  effective,  and  DEBRAY  recom- 
mends to  drive  off  the  mercury  by  igniting  for  four  hours  in  a 
small  crucible  of  gas  carbon  in  a  muffle.  The  presence  of  other 
volatile  metals,  such  as  zinc,  does  not  interfere  with  this  oper- 
ation. 

II.  PISANI'S  METHOD. f 

This  process  depends  on  the  following  reaction  :  a  solution  of 
iodide  of  starch  added  to  a  very  dilute  neutral  solution  of  silver 
nitrate,  forms  silver  iodide  and  silver  hypoiodite.  The  blue  color 
consequently  vanishes,  and  on  continued  addition  of  the  iodide  of 
starch,  the  fluid  does  not  become  permanently  blue  till  all  the  sik 
ver  nitrate  present  is  decomposed  in  the  above  manner.  The 
iodide  of  starch  solution  used  is  therefore  proportional  to  the  quan- 
tity of  silver  nitrate.  Hence,  if  the  value  of  the  iodide  of  starcli 
solution  be  determined,  by  allowing  it  to  act  on  a  certain  amount 
of  silver  solution  of  known  strength,  we  shall  be  able  to  estimate 
unknown  quantities  of  silver  with  the  greatest  ease,  provided  that 
the  silver  solution  is  free  from  all  other  substances  which  exert  a 
decomposing  action  on  the  iodide  of  starch.  Besides  the  ordinary 
reducing  agents,  the  following  salts  must  be  especially  mentioned 
as  possessing  this  power :  Mercurous  and  mercuric  salts,  stannous 
salts,  mafiganous,  ferrous,  and  antimonous  salts,  also  auric  chloride 
and  arsenites ;  lead  and  copper  salts,  on  the  other  hand,  do  not 
affect  iodide  of  starch. 

The  iodide  of  starch  is  prepared  as  follows :  make  an  intimate 

*  Compl.  rend  ,  i.xx,  849  ;  Zeitxchr.  f.  Clu'tn.,  1870,  349. 

f  Annal.  d.  Min.,  x,  83 ;  Jahrcsber.  von  Liebig  u.  Kopp,  1856,  749. 


350  DETERMINATION.  [§  115, 

mixture  in  a  mortar  of  2  grin,  iodine  and  15  grin,  starch  with  the 
addition  of  G  to  8  drops  of  water,  and  heat  the  slightly-moist  mix- 
ture in  a  closed  flask  in  a  water-Lath,  till  the  original  violet-blue 
color  has  passed  into  dark  grayish-blue — it  takes  about  an  hour. 
The  iodide  of  starch  thus  prepared  is  then  digested  with  water ;  it 
dissolves  completely  to  a  deep  bluish-black  fluid. 

The  value  of  this  fluid  is  determined  by  allowing  it  to  act  on 
10  c.c.  of  a  neutral  solution  of  silver  nitrate,  containing  1  grni.  of 
pure  silver  in  1  litre — the  silver  solution  is  mixed  with  a  little 
pure  precipitated  calcium  carbonate  before  adding  the  iodide  of 
starch.  The  strength  of  this  latter  is  right,  if  50  to  60  c.c.  are 
used  in  this  experiment.  On  adding  it,  at  flrst  the  blue  color  dis- 
appears rapidly,  and  the  fluid  becomes  yellowish  from  the  silver 
iodide.  The  end  of  the  operation  is  attained  as  soon  as  the  fluid  is 
bluish-green.  The  point  is  pretty  easy  to  hit,  and  an  error  of  0*0 
c.c.  is  of  no  importance,  as  it  only  corresponds  to  about  0*0001  grni. 
silver.  The  calcium  carbonate,  besides  neutralizing  the  free  acid, 
has  the  effect  of  rendering  the  final  change  of  the  color  more  dis- 
tinctly observable.  To  analyze  an  alloy  of  silver  and  copper,  dis- 
solve about  0*5  grm.  in  nitric  acid,  dilute  to  100  c.c.  to  lower  the 
color  of  the  copper,  saturate  5  c.c.  with  calcium  carbonate,  and  add 
iodide  of  starch  till  the  coloration  appears.  Or  you  may  deter- 
mine very  approximately  the  amount  of  silver  in  2  c.c.  of  the  solu- 
tion, then  precipitate  the  greater  part  (about  99£)  of  the  silver 
from  50  c.c.  of  the  solution  with  standard  solution  of  potassium 
iodide,  and  without  filtering  estimate  the  remainder  of  the  silver 
by  means  of  iodide  of  starch.  If  the  amount  of  silver  to  be  deter- 
mined is  more  than  0*02  grm.,  it  is  always  better  to  employ  the 
latter  method.  In  the  case  of  a  nitric  acid  solution  containing  sil- 
ver with  lead,  the  latter  metal  is  first  precipitated  with  sulphuric 
acid  and  filtered  off,  calcium  carbonate  is  added  to  the  filtrate  till 
all  free  acid  is  neutralized,  the  fluid  is  filtered  again  (if  necessary), 
and  lastly,  more  calcium  carbonate  is  added,  and  then  the  iodide  of 
starch.  Yery  dilute  solutions  must  be  concentrated,  so  that  one  may 
have  no  more  than  from  50  to  100  c.c.  to  deal  with.  The  method  is 
worthy  of  notice  and  specially  suited  for  the  estimation  of  small 
quantities  of  silver.  With  such  it  has  afforded  me  perfectly  satis- 
factory results.  Instead  of  the  standard  iodide  of  starch,  a  dilute 
standard  solution  of  iodine  in  potassium  iodide  may  be  equally  well 


§  116.]  LEAD.  351 

employed — with  addition  of  starch  solution  (FIELD  *).  If  this 
is  used  you  must  bear  in  mind  that  any  substance  which  decom- 
poses potassium  iodide  with  separation  of  iodine  will  interfere. 

H.  YOGEL  f  has  modified  PISANI'S  method  for  the  conve- 
nience of  photographers.  Nitroso-nitric  acid  (prepared  by  adding 
1  grm.  ferrous  sulphate  to  1000  grm.  of  nitric  acid  of  sp.  gr. 
1-2)  is  added  to  the  silver  solution,  which  may  contain  free  acid; 
starch  solution  is  then  added,  and  also  standard  potassium-iodide 
solution,  until  a  permanent  blue  color.  This  occurs  when  all  the 
silver  is  precipitated,  partly  as  iodide,  partly  as  iodate.  The  pre- 
cipitation depends  upon  the  following  reactions : 

KI  +  AgNO3  =  KNO3  +  Agl ;  and  61  +  6Ag^TOs  =  AglO, 
-f-  5AgI  +  3N2O6.  In  both  cases  1  eq.  of  iodine  precipitates  1  eq. 
of  silver.  The  potassium-iodide  solution  is  made  by  YOGEL  of  such 
strength  that  1  c.  c.  is  the  equivalent  of  O'Ol  grm.  silver  nitrate, 
i.e.)  10  grm.* of  pure,  dry  potassium  iodide  are  dissolved  in  suffi- 
cient liquid  to  measure  1024*1  c.c.  From  my  experience,  the 
method  is  quite  expeditious,  but  is  not  very  accurate,  because 
equal  volumes  of  silver  solution  will  require  distinctly  varying 
quantities  of  potassium-iodide  solution  if  the  conditions  are 
altered,  e.g.^  the  concentration  and  quantity  of  free  acid;  the 
variation  is  evidently  connected  with  the  formation  of  silver 
iodate,  which  is  not  entirely  insoluble  in  the  acid  liquid. 

in.   METHOD  DEPENDING  ON  THE  ACTION  OF  SILVER  NITRATE  ON 

SODIUM  CHLORIDE  IN  THE  PRESENCE  OF  POTASSIUM    CHROMATE. 

This  is  the  reverse  of  the  method  for  the  estimation  of  chlorine, 
§  141  J,  af,  and  will  be  described  in  that  place. 

§116. 

2.   LEAD. 

a.  Solution. 

Few  of  the  lead  salts  are  soluble  in  water.  Metallic  lead,  lead 
oxide,  and  most  of  the  lead  salts  that  are  insoluble  in  water  dis- 
solve in  dilute  nitric  acid.  Concentrated  nitric  acid  effects  neither 
complete  decomposition  nor  complete  solution,  since,  owing  to  the 

*  Chem.  News,  IT,  17. 

f  Pogg.  Ann.,  cxxiv,  347  ;  Zeitschr.  f.  analyt.  Chem.,  v,  227. 


352  DETERMINATION.  [§  116. 

insolubility  of  lead  nitrate  in  concentrated  nitric  acid,  the  first  por- 
tions of  nitrate  formed  protect  the  yet  undecomposed  parts  of  the 
salt  from  the  action  of  the  acid.  For  the  solubility  of  lead  chlo- 
ride and  sulphate,  see  §  83.  As  we  shall  see  below,  the  analysis  of 
these  compounds  may  be  effected  without  dissolving  them.  Lead 
iodide  dissolves  readily  in  moderately  dilute  nitric  acid  upon  ap- 
plication of  heat,  with  separation  of  iodine.  Solution  of  potassa 
is  the  only  menstruum  in  which  lead  chromate  dissolves  without 
decomposition ;  for  analysis,  it  is  best  converted  into  chloride. 

b.  Determination. 

0  Lead  may  be  determined  as  oxide,  sulphate,  chromate,  or  sul- 
phide, chloride,  lead  oxide  -\-  lead,  and  metallic  lead;  also  by 
volumetric  analysis. 

We  may  convert  into 

1.  LEAD  OXIDE: 

a.  .By  Precipitation. 

All  lead  salts  soluble  in  water,  and  those  of  its  salts  which, 
insoluble  in  that  menstruum,  dissolve  in  nitric  acid,  with  separa- 
tion of  their  acid. 

b.  By  Ignition.  , 

a.  Lead  salts  of  readily  volatile  or  decomposable  inorganic  acids. 
/?.   Lead  salts  of  organic  acids. 

2.  LEAD  SULPHIDE  : 

All  lead  salts  in  solution. 

3.  LEAD  SULPHATK  : 

a.  By  Precipitation. 

The  salts  that  are  insoluble  in  water,  but  soluble  in  nitric  acid, 
and  the  acids  of  which  cannot  be  separated  from  the  solution. 

b.  By  Evaporation. 

a.  All  the  oxides  of  lead,  and  also  the  lead  salts  of  volatile 
acids. 

ft.  Many  of  the  organic  compounds  of  lead. 

4.  LEAD  CHROMA TE  : 

The  compounds  of  lead  soluble  in  water  or  nitric  acid. 

5.  LEAD  CHLORIDE: 
Lead  chromate. 

6.  LEAD  OXIDE  -f-  LEAD. 
Many  organic  lead  compounds. 


§  116.]  LEAD.  353 

7.   METALLIC  LEAD  : 

The  oxides  and  most  of  the  lead  salts  (compounds  of  lead  with 
chlorine,  bromine,  and  iodine). 

Lead  may  be  also  determined  volumetricallj,  but  rarely 
advantageously. 

The  application  of  these  several  methods  must  not  be  under- 
stood to  be  rigorously  confined  to  the  compounds  specially  enu- 
merated under  their  respective  heads ;  thus,  for  instance,  all  the 
compounds  enumerated  under  1  may  likewise  be  determined  as  lead 
sulphate ;  and,  as  above  mentioned,  all  soluble  compounds  of  lead 
may  be  converted  into  lead  sulphide  ;  also,  the  lead  in  lead  sulphate 
may  be  without  difficulty  determined  as  sulphide.  Lead  chloride, 
bromide,  and  iodide  may  be  conveniently  reduced  to  the  metallic 
state  in  a  current  of  hydrogen  gas  in  the  manner  described  in  §  115 
(reduction  of  silver  chloride),  if  it  is  not  deemed  preferable  to  dis- 
solve them  in  water,  or  to  decompose  them  by  a  boiling  solution 
of  sodium  carbonate,  in  which  case,  after  cooling,  carbonic-acid 
gas  is  passed  into  the  solution,  when  the  small  quantity  of  lead 
retained  will  be  precipitated.  If  the  reduction  method  is  resorted 
to,  the  heat  applied  should  not  be  too  intense,  since  this  might 
cause  some  lead  chloride  to  volatilize. 

The  higher  oxides  of  lead  are  reduced  by  ignition  to  the  state 
of  lead  monoxide,  and  may  thus  be  readily  analyzed  and  dissolved. 
Should  the  operator  wish  to  avoid  having  recourse  to  ignition,  the 
most  simple  mode  of  dissolving  the  higher  oxides  of  lead  is  to  act 
upon  them  with  dilute  nitric  acid  with  the  addition  of  alcohol. 
For  the  methods  of  analyzing  lead  sulphate,  chromate,  iodide,  and 
bromide,  I  refer  to  the  paragraphs  treating  of  the  corresponding 
acids,  in  the  second  part  of  this  section.  To  effect  the  estimation 
of  lead  in  the  oxide  and  in  many  lead  salts,  especially  also  in  the 
sulphate,  the  compound  under  examination  may  be  fused  with 
potassium  cyanide  and  the  metallic  lead  obtained  well  washed  and 
weighed.  From  the  sulphide  also  the  greater  portion  of  the  lead 
may  be  separated  by  this  method,  but  never  the  whole  (H.  BOSE*). 

1.    Determination  as  Oxide, 
a.   By  Precipitation. 

Mix  the  moderately  dilute  solution  with  ammonium  carbonate 
slightly  in  excess,  add  some  caustic  ammonia,  apply  a  gentle  heat, 

*  Pogg.  Annal.,  91,  144. 


354  DETERMINATION.  [§  116. 

allow  to  cool  and  filter  throng! i  a  small  thin  filter.  "Wash  with 
pure  water,  dry,  and  transfer  the  precipitate  to  a  watch-glass,, 
removing  it  as  completely  as  possible  from  the  filter ;  burn  the 
latter  in  a  weighed  porcelain  crucible.  After  the  crucible  is  cold,, 
moisten  the  ash  with  nitric  acid,  allow  it  to  evaporate,  ignite  gently, 
allow  to  cool,  add  the  precipitate  and  ignite  gently  till  all  the  car- 
bonic acid  is  driven  off.  For  the  properties  of  the  precipitate  and 
residue,  see  §  83.  The  results  are  very  satisfactory,  although  gen- 
erally a  trifle  too  low,  owing  to  lead  carbonate  not  being  absolutely 
insoluble,  particularly  in  fluids  rich  in  ammonium  salts  (Expt.  No.. 
42,  5). 

b.  By  Ignition. 

Compounds  like  lead  carbonate  or  nitrate  are  cautiously  ig- 
nited in  a  porcelain  crucible  until  the  weight  remains  constant. 
Lead  nitrate  must  be  very  completely  dried  before  being  ignited, 
in  order  to  avoid  loss  through  decrepitation.  For  the  manner  of 
converting  lead  salts  of  organic  acids  into  oxide,  see  6. 
2.  Determination  as  Sulphide. 

Lead  may  be  completely  precipitated  fron.  acid,  neutral  and 
alkaline  solutions  by  hydrogen  sulphide,  and  also  from  neutral  and 
alkaline  solutions  by  ammonium  sulphide.  Precipitation  from 
acid  solution  is  usually  employed,  especially  in  separations.  A 
large  excess  of  acid  and  also  warming  should  both  be  avoided. 
The  former  is  prejudicial  to  complete  precipitation  (§  83,  f\  the 
latter  may  readily  occasion  the  re-solution  of  the  sulphide  that  has 
already  been  precipitated.  In  order  to  guard  against  incomplete 
precipitation,  before  filtering,  test  a  portion  of  the  supernatant 
fluid  by  mixing  with  a  relatively  large  quantity  of  strong  hydrogen 
sulphide  water;  the  fluid  must  remain  clear. 

If  the  fluid  contained  no  hydrochloric  acid  or  metallic  chloride, 
the  lead  sulphide  is  pure.  After  it  has  been  filtered  off,  washed 
with  cold  w^ater  and  dried,  it  is  transferred,  together  with  the 
filter-ash,  to  a  porcelain  crucible,  a  little  sulphur  added,  and  ignited 
*.n  hydrogen  at  gentle  redness  till  its  weight  is  constant.  It  should 
always  be  allowed  to  cool  in  a  current  of  the  gas,  before  being 
weighed.  As  regards  the  apparatus,  see  §  108,  2,  Fig.  83.  For  the 
properties  of  the  residue,  see  §  83,. /.  The  results  are  satisfactory 
(H.  HOSE).  The  heat  of  the  ignition  must  not  be  too  low,  or  the 
residue  will  contain  too  much  sulphur,  nor  too  high,  or  the  lead 
sulphide  will  begin  to  volatilize,  and  lead  disulphide  will  also  be 


§  116.]  LEAD.  355 

formed  with  loss  of  hydrogen  sulphide.  Drying  the  precipitate 
at  100°  cannot  be  recommended  (§  83,/).  If  the  fluid,  on  the 
contrary,  contained  hydrochloric  acid  or  a  metallic  chloride,  the 
lead  sulphide  contains  chloride  which  cannot  be  removed  even  by 
boiling  the  precipitate  with  ammonium  sulphide.  If  the  precipi- 
tate were  treated  as  above,  we  should  obtain  a  tolerably  pure 
sulphide,  but  not  without  loss  from  volatilization  of  chloride.  A 
precipitate  of  this  kind  must  therefore  be  decomposed  with  strong 
hydrochloric  acid,  the  solution  evaporated  to  dry  ness,  the  residue 
dissolved  by  heating  with  a  concentrated  solution  of  sodium 
acetate,  and  this  solution  diluted  and  poured  with  stirring  into 
excess  of  strong  hydrogen  sulphide  water.  Or  the  lead  chloride 
obtained  may  be  evaporated,  heated  to  200°,  and  weighed  as  such 

(FlNKENER*). 

3.  Determination  as  Sulphate. 

a.  By  Precipitation. 

a.  Mix  the  solution  (which  should  not  be  over  dilute)  with 
moderately  dilute  pure  sulphuric  acid  slightly  in  excess,  and  add 
to  the  mixture  double  its  volume  of  common  alcohol ;  wait  a  few 
hours,  to  allow  the  precipitate  to  subside ;  filter,  wash  the  precipi- 
tate with  common  alcohol,  dry,  and  ignite  after  the  method 
described  in  §  53.  Though  a  careful  operator  may  use  a  platinum 
crucible,  still  a  thin  porcelain  crucible  is  preferable.  See  also  the 
remarks,  1,  a. 

ft.  In  cases  where  the  addition  of  alcohol  is  inadmissible,  a 
greater  excess  of  sulphuric  acid  must  be  used,  and  the  precipitate, 
which  is  allowed  some  time  to  subside,  filtered,  and  washed  first 
with  water  acidulated  with  a  few  drops  of  sulphuric  acid,  then 
repeatedly  with  alcohol.  The  remainder  of  the  process  is  con- 
ducted as  in  a. 

If  the  fluid  contained  nitric  acid,  whether  alcohol  is  used  or 
not,  it  is  advisable  to  evaporate  on  the  water-bath  after  the 
addition  of  the  sulphuric  acid,  till  the  nitric  acid  has  escaped, 
otherwise  the  precipitation  will  not  be  complete.  If  the  fluid 
contained  hydrochloric  acid  or  a  metallic  chloride,  lead  chloride  is 
thrown  down  with  the  sulphate.  In  this  case  you  must  either 
evaporate  the  fluid  witli  excess  of  sulphuric  acid  and  heat  the 
residue  till  sulphuric  acid  fumes  escape  to  drive  off  the  hydro- 

*  Handb.  der  analyL  Chem.  von  H.  ROSE,  6.  Aufl.  von  FINKENER,  932. 


356  DETERMINATION.  [§  110. 

chloric  acid,  or  you  must  treat  the  precipitate  and  filter-ash  in  the 
crucible  with  concentrated  sulphuric  acid,  evaporate  and  ignite  to 
convert  it  into  pure  lead  sulphate  (FINKENER*). 

For  the  properties  of  the  precipitate,  see  §  83.  The  method  a 
gives  accurate  results ;  those  obtained  by  /?  are  less  exact  (a  little 
too  low),  but  still  however  satisfactory,  if  the  directions  given  are 
adhered  to.  If,  on  the  contrary,  a  proper  excess  of  sulphuric  acid 
is  not  added,  in  the  presence,  for  instance,  of  ammonium  salts,  the 
lead  is  not  completely  precipitated,  and  if  pure  water  is  used  for 
washing,  decided  traces  of  the  precipitate  are  dissolved. 

~b.  By  Evaporation. 

a.  Put  the  substance  into  a  weighed  dish,  dissolve  in  dilute 
nitric  acid,  add  moderately  dilute  pure  sulphuric  acid  slightly  in 
excess,  and  evaporate  at  a  gentle  heat ;  at  last  high  over  the  lamp,,, 
until  the  excess  of  sulphuric  acid  is  completely  expelled.  In  the 
absence  of  organic  substances,  the  evaporation  may  be  effected 
without  fear  in  a  platinum  dish  ;  but  if  organic  substances  are 
present,  a  light  porcelain  dish  is  preferable.  With  due  care  in  the 
process  of  evaporation,  the  results  are  perfectly  accurate. 

/?.  Organic  compounds  of  lead  are  converted  into  the  sulphate 
by  treating  them  in  a  porcelain  crucible,  with  pure  concentrated 
sulphuric  acid  in  excess,  evaporating  cautiously  in  the  well-covered 
crucible,  until  the  excess  of  sulphuric  acid  is  completely  expelled, 
and  igniting  the  residue.  Should  the  latter  not  look  perfectly 
white,  it  must  be  moistened  once  more  with  sulphuric  acid,  and 
the  operation  repeated.  The  method  gives,  when  conducted  with 
great  care,  accurate  results  ;  a  trifling  loss  is,  however,  usually  in- 
curred, the  escaping  sulphur  dioxide  and  carbon  dioxide  gases 
being  liable  to  carry  away  traces  of  the  salt. 

4.  Determination  as  Lead  Chromate. 

If  the  solution  is  not  already  distinctly  acid  render  it  so  with 
acetic  acid?  then  add  potassium  dichrornate  in  excess,  and,  if  free 
nitric  acid  is  present,  add  sodium  acetate  in  sufficient  quantity  to 
replace  the  free  nitric  acid  by  free  acetic  acid ;  let  the  precipitate 
subside  at  a  gentle  heat,  and  collect  on  a  weighed  filter  dried  at 
100°,  wash  with  water,  dry  at  100°,  and  weigh.  The  precipitate 

*  Handb.  der  analyt.  Chem.  von  H.  ROSE,  6.  Aufl.  von  FINKENER,  933. 


§  116.]  LEAD.  357 

may  also  be  ignited  according  to  §  53,  but  in  this  case  care  must  be 
taken  that  hardly  any  of  the  salt  remains  adhering  to  the  paper, 
and  that  the  heat  is  not  too  high.  For  the  properties  of  the 
precipitate,  see  §  93,  2.  The  results  are  accurate  (Expt.  No.  68). 

5.  Determination  as  Lead  Chloride. 

Add  an  excess  of  hydrochloric  acid  to  the  solution,  concentrate 
on  a  water-bath,  treat  the  residue  with  absolute  alcohol  to  which 
a  little  ether  has  been  added,  allow  to  settle,  filter,  and  wash 
with  ether-alcohol.  The  lead  chloride  may  be  either  dried  at 
100°  after  being  collected  on  a  dried  and  weighed  filter,  or  it  may 
be  carefully  treated  as  in  §  53.  In  the  latter  case  use  a  porcelain 
crucible,  and  take  care  to  leave  no  lead  chloride  in  the  filter  and 
to  avoid  a  temperature  above  200°. 

6 .  Determination  as  Lead  Oxide  -f-  Lead. 

Gently  heat  the  organic  lead  compound  (1  to  2  grm.)  in  a  small 
weighed  porcelain  dish,  allowing  the  heat  to  play  first  upon  the 
margins  of  the  dish  so  that  the  decomposition  may  begin  on  one 
side,  and  thence  proceed  slowly.  When  the  entire  mass  is  decom- 
posed, heat  more  strongly  till  no  glowing  particles  are  percepti- 
ble and  the  residue  appears  to  be  a  carbon-free  mixture  of  lead 
oxide  and  lead  globules.  Weigh  the  residue,  then  warm  it  with 
acetic  acid  until  the  oxide  is  completely  dissolved,  which  is 
eoon  effected,  wash  the  lead  by  decantation  with  water,  and  finally 
heat  the  residual  lead  to  remove  all  water,  and  weigh.  On  de- 
ducting the  weight  found  from  that  of  the  residue  first  weighed, 
the  weight  of  the  oxide  contained  is  found.  On  calculating  the 
height  of  metal  contained  in  the  oxide  found,  and  adding  it  to 
the  weight  of  the  metallic  lead  directly  found,  the  total  metal 
Contained  in  the  compound  is  obtained. 

In  carrying  out  this  process,  two  points  must  be  observed. 
First,  the  decomposition  must  be  allowed  to  proceed  very 
fclowly,  because  rapid  combustion  of  the  carbon  and  hydrogen  of 
the  compound  at  the  expense  of  the  oxygen  of  the  lead  oxide 
causes  so  high  a  temperature  to  develop  as  to  volatilize  some  lead, 
which  passes  off  in  visible  fumes.  Secondly,  care  must  be  taken 
that  no  carbon  remains  in  the  residue ;  and  this  can  with  cer- 
tainty be  ascertained  with  acetic  acid.  Neglect  of  the  first  point 
gives  results  which  are  too  low ;  neglect  of  the  second  will  give 


358  DETERMINATION.  [§  116. 

results  too  high.     The  method  is  otherwise  very  convenient)  and 
when  carefully  executed  gives  accurate  results. 

DULK  has  proposed  the  following  modification  of  the  method 
first  proposed  by  BERZELIUS  :  The  compound  is  very  gently- 
heated  in  a  covered  porcelain  crucible  until  the  organic  matter  is 
completely  carbonized.  Then  remove  the  lid  and  stir  the  con- 
tents with  an  iron  wire.  The  mass  begins  to  ignite,  and  a  mix- 
ture of  lead  oxide  with  metallic  lead  results,  which  may  contain 
some  unconsumed  carbon.  Remove  the  crucible  from  the  flame, 
throw  into  it  a  few  pieces  of  recently  fused  ammonium  nitrate 
and  then  replace  the  cover.  The  salt  fuses,  oxidizes  the  lead, 
and  converts  it  partly  into  nitrate.  The  crucible  is  now  ignited 
until  vapors  of  hyponitric  acid  are  no  longer  visible.  The 
residual  oxide  is  then  weighed.  This  rapid  method  insures  com- 
plete combustion  of  all  the  carbon  and  saves  the  time  and  labor 
of  one  weighing.  The  results  are  very  satisfactory. 

7.  Determination  as  Metallic  Lead. 

a.  This  method  is  applicable  to  lead  oxide  and  most  lead  com- 
pounds, such  as  lead  sulphate  and  phosphate,  but  not  chromate ; 
to  lead  sulphide  it  is  applied  with  difficulty.  The  substance  is 
fused  with  4  to  5  times  its  weight  of  potassium  cyanide  (prepared 
according  to  LIEBIG'S  process)  in  a  well-covered,  well-glazed  por- 
celain crucible.  After  cooling,  treat  the  mass  with  water,  rap- 
idly decant  the  solution  from  the  reduced  lead,  .wash  the  latter 
first  with  water,  then  with  diluted  and  finally  strong  alcohol,  dry 
and  weigh.  Sometimes  the  result  is  a  single  lead  globule;  usu- 
ally, however,  there  are  obtained  a  number  of  globules  mixed 
with  lead  powder.  After  weighing,  dissolve  the  lead  in  warmed 
dilute  nitric  acid.  Any  residue  (portions  of  the  glaze  of  the 
crucible)  is  determined,  and  its  weight  deducted  from  that  first 
found  (H.  ROSE).* 

&.  Lead  may  be  precipitated  from  soluble  as  well  as  insoluble 
lead  salts  (lead  chloride  and  sulphate)  by  means  of  zinc  or  cad- 
mium. To  effect  this,  warm  the  lead  compound  with  water  and 
a  little  hydrochloric  acid  on  a  water-bath,  and  add  a  smooth  piece 
of  pure  zinc  or  cadmium  (soluble  without  residue  in  hydrochloric 

*  Pogg.  Annal ,  xci,  104. 


§  116.]  LEAD.  359 

acid).  Eeduction  begins  at  once.  The  lead  precipitating  on  the 
zinc  is  removed  from  time  to  time  with  a  glass  rod,  and  more 
.acid  added  occasionally  if  necessary.  The  operation  is  at  an  end 
when  lead  no  longer  deposits  on  the  zinc,  and  when  a  small  quan- 
tity of  the  solution  gives  no  reaction  with  hydrogen  sulphide. 
The  zinc  or  cadmium  is  then  removed,  the  water  decanted,  and  the 
spongy  lead  rapidly  and  completely  washed  by  decantation.  Dis- 
tilled water  must  not  be  used,  as  this  dissolves  traces  of  lead; 
spring  water  should  be  used,  and  to  prevent  it  from  precipitating 
the  zinc  or  cadmium  add  a  little  tincture  logwood,  and  then  so 
much  very  dilute  sulphuric  acid,  until  the  red  color  just  gives 
place  to  a  yellow.  The  spongy  lead  cannot  be  dried  without  the 
formation  of  some  hydroxide;  hence  it  may  be  dried  at  150°— 
200°,  the  weight  of  the  mixture  of  lead  and  lead  oxide  determined, 
and  the  oxide  estimated  as  under  8  c,  and  the  wreight  of  oxygen 
found  deducted  from  the  weight  of  the  mixture  first  found  ;  or 
the  spongy  lead  may  be  dissolved  in  nitric  acid  and  the  lead  deter- 
mined as  in  3  £,  as  a  sulphate 


8.   Determination  of  Lead  by  Volumetric  Analysis. 

Although  there  is  no  lack  of  proposed  methods  for  the  volu- 
metric estimation  of  lead,  we  are  still  without  a  really  good  method 
for  practical  purposes,  that  is,  a  method  which  can  be  generally 
employed  and  which  is  at  the  same  time  simple  and  exact.  For 
the  present,  therefore,  in  almost  all  cases  the  gravimetric  deter- 
mination of  lead  is  to  be  preferred  to  the  volumetric.  On  my  own 
part  at  least  I  cannot  see  that  it  is  easier  or  any  better,  when  one 
has  the  precipitate  washed,  to  subject  it  to  a  volumetric  process  — 
whereby  the  accuracy  is  necessarily  diminished  —  instead  of  igniting 
it  gently  and  weighing.  For  this  reason  the  better  volumetric 
methods  will  be  but  briefly  described,  the  rest  being  altogether 
omitted. 

a.  The  solution  of  the  normal  lead  salt  must  be  free  from 
alkali  salts,  more  especially  from  ammonium  salts.  It  is  precipi- 
tated with  oxalic  acid  (not  with  ammonium  oxalate),  the  well- 
washed  precipitate  is  dissolved  in  nitric  acid,  sulphuric  acid  added, 


*Joum.f.  prakt.  CJiem.,  ci,  150;  Zeitschr.f.  analyt.  Ghem.,  vu,  102. 


330  DETERMINATION.  [§  116. 

and   the   oxalic   acid   in   the   solution   determined   by  potassium 
permanganate  (§  137)  HEMPEL. 

Z>.  II.  SCHWAKZ'S  method.*  To  the  nitric  acid  solution  add 
ammonia  or  sodium  carbonate,  as  long  as  the  precipitate  redissolves 
on  shaking,  mix  with  sodium  acetate  in  not  too  small  quantity,  and 
then  run  in  from  a  burette  a  solution  of  potassium  dichromate 
(containing  14  '721  grm.  in  the  litre)  till  the  precipitate  begins  to 
settle  rapidly.  Now  place  on  a  porcelain  plate  a  number  of  drops 
of  a  neutral  solution  of  silver  nitrate,  and  proceed  with  the  addition 
of  the  chromate,  two  or  three  drops  at  a  time,  stirring  carefully 
after  each  addition.  When  the  precipitate  has  settled  tolerably 
clear,  which  takes  only  a  few  seconds,  remove  a  drop  of  the  super- 
natant liquid  and  mix  it  with  one  of  the  drops  of  silver  solution 
on  the  plate.  A  small  excess  of  chromate  gives  at  once  a  distinct 
red  coloration  ;  the  precipitated  lead  chromate  does  not  act  on  the 
silver  solution,  but  remains  suspended  in  the  drop.  The  number 
of  c.  c.  of  solution  of  chromate  used  (minus  0*1,  which  SCHWARZ 
deducts  for  the  excess)  multiplied  by  0-0207  =  the  quantity  of  lead. 
If  the  fluid  appear  yellow  before  the  reaction  with  the  silver  salt 
occurs,  sodium  acetate  is  wanting.  In  such  a  case  first  add  more 
sodium  acetate,  then  1  c.  c.  of  a  solution  containing  0'0207  lead  in 
1  c.  c.,  complete  the  process  in  the  usual  way,  and  deduct  1  c.  c. 
from  the  quantity  of  chromate  used  on  account  of  the  extra  lead 
added.  Any  iron  present  must  be  in  the  form  of  a  ferric  salt ; 
metals  the  chromates  of  which  are  insoluble  must  be  removed 
before  the  method  can  be  employed. 

c.  The  lead  is  precipitated  according  to  1 ,  a,  the  carbonate  (its 
composition  is  a  matter  of  indifference  in  the  present  case)  is 
washed,  dissolved  in  a  measured  quantity  of  standard  nitric  acid 
(§  215),  and  a  neutral  solution  of  sodium  sulphate  added,  whereby 
lead  sulphate  is  precipitated  and  an  equivalent  quantity  of  sodium 
nitrate  formed.  If  the  nitric  acid  still  free  is  now  determined 
with  standard  alkali,  we  shall  find  the  quantity  of  acid  that  has 
been  neutralized  by  means  of  the  lead,  from  which  the  amount  of 
lead  may  be  calculated,  each  c.  c.  of  standard  nitric  acid  being  the 
equivalent  of  0'1034()  lead.  You  may  also  determine  the  free  nitric 
acid  by  adding  standard  sodium  carbonate  till,  the  vessel  being  on 

*  Dingl.  polyt.  Journ.,  CLXIX,  284;  Zeitschr.f.  analyt.  Chem.,  n,  378. 


§  117.]  MERCURY   IN   MERCUROUS    COMPOUNDS.  361 

a  black  surface,  a  permanent  turbidity  is  visible.     Besults  good 
(F.  MOHE*). 

§117. 
3.  MERCUKY  IN  MEKCTJKOUS  COMPOUNDS. 

a.  Solution. 

Mercurous  oxide  and  mercurous  salts  may  generally  be  dissolved 
by  means  of  dilute  nitric  acid,  but  without  application  of  heat  if 
conversion  into  mercuric  compounds  is  to  be  avoided.  If  all  that 
is  required  is  to  dissolve  the  mercury,  the  easiest  way  is  to  warm 
the  substance  for  some  time  with  nitric  acid,  then  add  hydrochloric 
acid,  drop  by  drop,  and  continue  the  application  of  a  moderate  heat 
until  a  perfectly  clear  solution  is  produced,  which  now  contains  all 
the  mercury  in  form  of  mercuric  salts.  Heating  the  solution  to 
boiling,  or  evaporating,  must  be  carefully  avoided,  as  otherwise 
mercuric  chloride  may  escape  with  the  steam. 

b.  Determination. 

If  it  is  impracticable  to  produce  a  solution  of  the  mercurous 
compound  without  formation  of  mercuric  salts,  it  becomes  neces- 
sary to  convert  the  mercury  completely  into  mercuric  salts,  when 
it  may  be  determined  as  directed  §  118.  But  if  a  solution  of  a 
mercurous  compound  has  been  obtained,  quite  free  from  mercuric 
salts,  the  determination  of  the  mercury  may  be  based  upon  the 
insolubility  of  mercurous  chloride,  and  effected  either  gravimetri- 
cally  or  volumetrically.  The  process  of  determining  mercury, 
described  §  118,  1,  a,  may,  of  course,  be  applied  equally  well  in  the 
case  of  mercurous  compounds. 

1.  Determination-  as  Mercurous  Chloride. 

Mix  the  cold  highly  dilute  solution  with  solution  of  sodium 
chloride,  as  long  as  a  precipitate  forms ;  let  the  precipitate  subside, 
collect  on  a  weighed  filter,  dry  at  100°,  and  weigh.  For  the 
properties  of  the  precipitate,  see  §  84.  Results  accurate.  If  the 
jnercurons  solution  contains  much  free  nitric  acid,  the  greater  part 
of  this  should  be  neutralized  with  sodium  carbonate  before  adding 
the  sodium  chloride. 

*  Lehrluch  der  Titrirmethode,  3.  Aufl.  115. 


362  DETERMINATION.  [§  117. 

2.  Volumetric  Methods. 

Several  methods  have  been  proposed  under  this  head :  the 
following  are  those  which  are  most  worthy  of  recommendation  :— 

a.  Mix  the  cold  solution  with  decinormal  solution  of  sodium 
chloride  (§  141,  &,  or),  until  this  no  longer 'produces  a  precipitate, 
and  is  accordingly  present  in  excess ;  filter  and  wash  thoroughly, 
taking  care,  however,  to  limit  the  quantity  of  water  used ;  add  a 
few  drops  of  solution  of  potassium  chromate,  then  pure  sodium 
carbonate,  sufficient  to  impart  a  light  yellow  tint  to  the  fluid,  and 
determine  by  means  of  solution  of  silver  nitrate  (§  141,  &,  a)  the 
quantity  of  sodium  chloride  in  solution,  consequently  the  quantity 
which  has  been  added  in  excess ;  this  shows,  of  course,  also  the 
amount  of  sodium  chloride  consumed  in  effecting  the  precipitation. 
One  mol.  of  Hg2O  is  reckoned  for  2  mols.  of  NaCl,  consequently 
for  every  c.c.  of  the  decinormal  solution  of  sodium  chloride,  '0208 
grm.  of  mercurous  oxide.  As  filtering  and  washing  form  indis- 
pensable parts  of  the  process,  this  method  affords  no  great  ad  van 
tage  over  the  gravimetric ;  however,  the  results  are  accurate 
(FR.  MOHK*).  The  two  methods,  1  and  2,  &,  may  also  be  advaii 
tageously  combined. 

J.  Precipitate  the  mercurous  solution,f  according  to  1,  with 
sodium  chloride  in  a  stoppered  bottle,  allow  to  subside,  filter,  wash, 
push  a  hole  through  the  bottom  of  the  filter,  and  rinse  the  precipi- 
tate into  the  bottle,  which  usually  has  some  of  the  washed  mercu- 
rous chloride  adhering  to  its  inside.  Add  a  sufficient  quantity  of 
solution  of  potassium  iodide,  together  with  standard  iodine  solution 
{to  1  grin,  Hg2012  about  2'5  grm.  KI  and  100  c.c.  decinormal  iodine 
.solution;}:),  insert  the  stopper,  and  shake  till  the  precipitate  has 
entirely  dissolved  (Hg2Cl2+  6fel  +  21  =  2[IIgI2(KI)J  +  2KC1). 
As  iodine  is  in  excess,  the  solution  appears  brown.  If  any  mercu- 
ric iodide  separates,  add  potassium  iodide  to  redissolve  it.  Now 
add  from  a  burette  solution  of  sodium  thiosulphate — correspond- 
ing to  decinormal  iodine  solution — till  the  fluid  is  decolorized  and 
appears  like  water,  transfer  to  a  measuring  flask,  rinse  and  fill  up 
to  the  mark,  shake,  take  out  an  aliquot  part,  add  starch  paste  to  it, 
and  determine  the  excess  of  sodium  thiosulphate  with  decinormai 
iodine  solution.  After  multiplying  by  the  proper  number,  add  the 
c.c.  originally  employed,  subtract  the  c.c.  of  thiosulphate  used,  and 

*  Lehrbuch  der  Titrirmethode,  3.  Aufl.  395. 

\  If  mercuric  oxide  is  also  present,  see  §  118,  2.  \  See  §  146,  2. 


§  118.]  MERCURY    IN    MERCUKIC    COMPOUNDS.  363 

calculate  tlie  quantity    of  mercury  from  the  remainder.      2  at. 
iodine  =  1  moL  HgaCla.     Results  good  (!!EMPEL  *). 


§  US. 
4.   MERCURY  IN  MERCUKIC  COMPOUNDS. 

a.  Solution. 

Mercuric  oxide,  and  those  mercuric  compounds  which  are 
insoluble  in  water,  are  dissolved,  according  to  circumstances,  in 
hydrochloric  acid  or  in  nitric  acid.  Mercuric  sulphide  is  heated 
with  hydrochloric  acid,  and  nitric  acid  or  potassium  chlorate  added 
until  complete  solution  ensues ;  it  is,  however,  most  readily  dis- 
solved by  suspending  it  in  dilute  potassa  and  transmitting  chlorine, 
at  the  same  time  gently  warming  (H.  ROSE).  When  a  solution 
of  mercuric  chloride  is  evaporated  on  the  water-bath,  mercuric 
chloride  escapes  with  an  aqueous  vapor.  This  fact  must  not  be 
lost  sight  of  in  effecting  solutions  of  mercuric  compounds.  The 
methods  proposed  by  YouLf  give  on  this  account  inaccurate 
results.  FR.  MOHR  \  and  R.  RIETH  §  also  have  not  given  this 
source  of  error  proper  attention. 

5.  Determination. 

Mercury  may  be  weighed  in  the  metallic  state,  or  as  mercu- 
rous chloride,  mercuric  sulphide,  or  mercuric  oxide  (§  84) ;  in 
separations  it  is  sometimes  determined  as  loss  on  ignition.  It  may 
also  be  estimated  volumetrically. 

The  first  three  methods  may  be  used  in  almost  all  cases ;  the 
determination  as  mercuric  oxide,  on  the  contrary,  is  possible 
only  in  mercurous  or  mecuric  nitrates.  The  methods  by  which 
the  mercury  is  determined  as  mercurous  chloride  or  mercuric  sul- 
phide are  to  be  preferred  before  those  in  which  it  is  separated  in 
the  metallic  form.  The  volumetric  method  5  is  of  very  limited 
application.  The  mercurous  chloride  obtained  by  method  2, 
instead  of  being  weighed,  may  be  determined  volumetrically  as 
in  §  117,  2,  I. 


*Annal  d.  Chem.  u.  Pharm..  ex,  176.         \2bid.,xciv,  230. 
J  Lehrbuch  der  Titrirmelhode,  3.  Aufl.,  208. 
§  RIETH'S  Volumetric,  225. 


364  DETERMINATION.  [§  118. 

1.  Determination  as  Metallic  Mercury. 

a.  In  the  Dry  Way. 

The  process  is  conducted  in  the  apparatus  illustrated  by 
Fig.  88. 

Take  a  tube  45  cm.  long  and  about  12  mm.  wide,  made  of 
difficultly  fusible  glass  and  sealed  at  one  end.  First  put  into  the 
tube  a  mixture  of  sodium  bicarbonate  and  powdered  chalk  6  cm. 
long,  then  a  layer  of  quicklime ;  these  two  will  occupy  the  space 
from  a  to  5.  Then  add  the  intimate  mixture  of  the  substance 
with  an  excess  of  quicklime  (&-<?),  then  the  lime  rinsings  of  the 
mortar  (c-d\  then  a  layer  of  quicklime  (d-e),  and  lastly,  a  loose 
stopper  of  asbestos  (e-f).  The  anterior  end  of  the  tube  is  then 
drawn  out  and  bent  at  a  somewhat  obtuse  angle.  The  manipu- 
lations in  the  processes  of  mixing  and  filling  being  the  same  as  in 
organic  analysis,  they  will  be  found  in  detail  in  the  chapter  on  that 
subject. 

A  few  gentle  taps  upon  the  table  are  sufficient  to  shake  the 
contents  of  the  tube  down  so  as  to  leave  a  free  passage  through  the 
whole  length  of  the  tube.  The  tube,  so  prepared  and  arranged,  is 
now  placed  in  a  combustion  furnace,  the  point  being  inserted  into 
a  flask  containing  water,  the  surface  of  which  it  should  just  touch, 
so  that  the  opening  may  be  just  closed. 

The  tube  is  now  surrounded  with  red-hot  charcoal,  in  the  same 
way  as  in  organic  analysis,  proceeding  slowly  from  e  to  #,  the  last 
traces  of  mercurial  vapor  being  expelled  by  heating  the  mixture  at 


Fig.  88. 

the  sealed  end  of  the  tube.  Whilst  the  tube  still  remains  in  a  state 
of  intense  ignition,  the  neck  is  cut  off  at/*,  and  carefully  and  com- 
pletely rinsed  into  the  receiving  flask,  by  means  of  a  washing-bottle. 
The  small  globules  of  mercury  which  have  distilled  over  are  united 
into  a  large  one,  by  agitating  the  flask,  and,  after  the  lapse  of  some 
time,  the  perfectly  clear  water  is  decanted,  and  the  mercury  poured 


§  118.]  MERCURY   IN    MERCURIC   COMPOUNDS.  365 

into  a  weighed  porcelain  crucible,  where  the  greater  portion  of  the 
water  still  adhering  to  it  is  removed  with  blotting-paper.  The 
mercury  is  then  finally  dried  under  a  bell-jar,  over  concentrated 
sulphuric  acid,  until  the  weight  remains  constant.  Heat  must  not 
be  applied.  For  the  properties  of  the  metal,  see  §  8-1.  In  the 
case  of  sulphides,  in  order  to  avoid  the  presence  of  vapor  of  water 
in  the  tube,  which  would  give  rise  to  the  formation  of  sulphuretted 
hydrogen,  the  mixture  of  sodium  hydrogen  carbonate  and  chalk  is 
replaced  by  magnesite.  Mercuric  iodide  cannot  be  completely 
decomposed  by  lime.  To  analyze  this  in  the  dry  way,  substitute 
finely  divided  metallic  copper  for  the  lime  (II.  ROSE*).  The  accu- 
racy of  the  results  is  entirely  dependent  upon  the  care  bestowed. 
The  most  highly  accurate  results  are,  however,  obtained  by  the 
application  of  the  somewhat  more  complicated  modification  adopted 
by  ERDMANN  and  MARCHAND  for  the  determination  of  the  atomic 
weight  of  mercury  and  of  sulphur.  For  the  details  of  this  modi- 
fied process,  I  refer  to  the  original  essay,  f  simply  remarking  here, 
that  the  distillation  is  conducted,  in  a  combustion-tube,  in  a  cur- 
rent of  carbon  dioxide  gas,  and  that  the  distillate  is  received  in  a 
weighed  bulb  apparatus  with  the  outer  end  filled  with  gold-leaf,  to 
insure  the  condensation  of  every  trace  of  mercury  vapor.  This 
way  of  receiving  and  condensing  may  be  employed  also  in  the 
analysis  of  amalgams  (KoNiG^:). 

I.  In  the  Wet  Way. 

The  solution,  free  from  nitric  acid,  and  mixed  with  free  hy- 
drochloric acid,  is  precipitated,  in  a  perfectly  clean  flask  (best 
previously  washed  with  hot  potassa  lye),  with  an  excess  of  a  clear, 
recently  prepared  solution  of  stannous  chloride  containing  free 
hydrochloric  acid ;  the  mixture  is  boiled  for  a  short  time,  the  flask 
loosely  stoppered,  and  then  allowed  to  cool.  After  some  time 
the  perfectly  clear  supernatant  fluid  is  decanted  from  the  metallic 
mercury,  which,  under  favorable  circumstances,  will  be  found 
united  into  one  globule;  if  this  is  the  case,  the  globule  of  mer- 
cury may  be  washed  at  once  by  decantation,  first  with  water 
acidulated  with  hydrochloric  acid  and  finally  with  pure  water;  it 
is  dried  and  estimated  as  in  a. 

*  Pogg.  AnnaL,  ex,  546. 

"\Journ.  f.  prakt.  C?iem.,  xxxi,  385;  also  Pharm.  CentralbL,  1844,  354. 

\Journ.f.  prakt.  Chem.,  LXX,  64. 


366  DETERMINATION.  [§  118. 

If,  on  the  other  hand,  the  particles  of  the  mercury  have  not 
united,  their  union  into  one  globule  may  as  a  rule  be  readily  ef- 
fected by  boiling  a  short  time  with  some  moderately  dilute  hydro- 
chloric acid  mixed  with  a  few  drops  of  stannous  chloride  (having, 
of  course,  previously  removed  by  decantation  the  supernatant 
clear  fluid).  For  the  properties  of  metallic  mercury,  see  §  84. 

Instead  of  stannous  chloride,  other  reducing  agents  may  be 
used,  especially  phosphorous  acid  at  a  boiling  temperature.  This 
method  gives  accurate  results  only  when  conducted  with  the  great- 
est care.  In  general,  a  little  mercury  is  lost. 

2.  Determination  as  Mercurous  Chloride, 
a.  After  H.  ROSE.'*  Mix  the  mercuric  solution  (which  may 
contain  nitric  acid,  but  which  must  then  be  considerably  diluted) 
with  hydrochloric  acid  and  excess  of  phosphorous  acid  (obtained 
by  the  oxidation  of  phosphorus  in  moist  air),  allow  to  stand  for 
12  hours  in  the  cold  or  at  a  very  gentle  heat  (at  all  events  under 
60°),  collect  the  mercury,  now  completely  separated  as  mercurous- 
chloride,  on  a  weighed  filter,  wash  with  hot  water,  dry  at  100% 
and  weigh.  Results  perfectly  satisfactory. 

3.  Determination  as  Mercuric  Sulphide. 

The  solution  is  sufficiently  diluted,  acidulated  with  hydrochloric 
acid,  and  precipitated  with  clear  saturated  hydrogen  sulphide  water 
(or  in  the  case  of  large  quantities,  by  passing  the  gas) ;  filter  after 
allowing  the  precipitate  a  short  time  to  deposit,  wash  quickly  with 
cold  water,  dry  at  100°,  and  weigh.  Results  very  satisfactory. 

If  from  any  cause  (e.g.  presence  of  ferric  salts,  free  chlorine,  or 
the  like)  the  precipitate  should  contain  free  sulphur,  the  filter  is 
spread  out  on  a  glass  plate,  the  precipitate  removed  to  a  porcelain 
dish  by  the  aid  of  a  jet  from  the  wash-bottle,  and  warmed  for  some 
time  with  a  moderately  strong  solution  of  sodium  sulphite.  The 
filter,  having  been  in  the  mean  while  somewhat  dried  on  the  glass 
plate,  is  replaced  in  the  funnel,  the  supernatant  fluid  is  poured  on 
to  it,  the  treatment  with  sodium  sulphite  is  repeated,  and  the  pre- 
cipitate (now  free  from  sulphur)  is  finally  collected  on  the  filter, 
washed,  dried,  and  weighed.  Results  very  good  (J.  LowEf). 

Should  the  quantity  of  sulphur  mixed  with  the  precipitate  be 

*  Pogg.  AnnaL,  ex,  529. 
\Journ.f.prakt.  Chem.,  LXXVII,  73. 


§  118.]  MERCURY   IN   MERCURIC    COMPOUNDS.  367 

not  very  large,  it  may  be  removed  also  as  follows :  The  precipi- 
tate is  first  washed  with  water,  then  fully  dried,  then  repeatedly 
washed  with  carbon  disulphide  (which  must  leave  no  residue  on 
evaporation),  till  a  few  drops  of  the  washings  evaporate  on  a 
watch-glass  without  leaving  a  residue.  (The  precipitate  is  re- 
tained on  the  filter  throughout  this  operation.) 
Properties  of  mercuric  sulphide,  §  84, 

4.  Determination  as  Oxide. 

In  the  mercurous  and  mercuric  salts  of  the  nitrogen  acids,  the 
metal  may  be  very  conveniently  determined  in  the  form  of  mer- 
curic oxide  (MARION  AC*).  For  this  purpose  the  salt  is  heated  in 
a  bulb-tube,  of  which  the  one  end,  drawn  out  to  a  point,  dips 
under  water,  the  other  end  being  connected  with  a  gasometer,  by 
means  of  which  dry  air  is  transmitted  through  the  tube  as  long 
as  the  application  of  heat  is  continued.  In  this  way  complete 
decomposition  of  the  salt  is  readily  effected,  without  reaching  the 
temperature  at  which  the  oxide  itself  would  be  decomposed. 

5.  Volumetric  Methods. 

a.  Precipitate  as  mercurous  chloride  as  in  2,  and  treat  the 
washed  precipitate  as  in  §  117,  2,  I. 

~b.  According  to  LIEBIG  f :  This  method  depends  upon  the 
fact  that  sodium  phosphate  precipitates  mercury  from  solutions  of 
mercuric  nitrate,  but  not  from  mercuric  chloride,  in  the  form  of 
fiocculent,  white  mercuric  phosphate,  which  soon  becomes  crys- 
talline ;  and  that  therefore  sodium  chloride  readily  dissolves  the 
precipitate  (as  long  as  it  is  still  amorphous),  sodium  phosphate 
and  mercuric  chloride  being  formed.  On  knowing  the  quantity 
of  sodium  chloride  necessary  to  effect  the  solution  of  the  precipi- 
tate, that  of  the  mercury  is  known  also,  since  2  eq.  of  sodium 
phosphate  are  the  equivalent  of  1  eq.  of  mercuric  oxide  (as 
phosphate). 

a.  Sodium-chloride  Solution:  Decinormal  sodium  chloride 
may  be  used.  Every  c.  c.  of  this  containing  0*00585  grm.  NaCl 
is  the  equivalent  of  0-0108  HgO. 

/?.  Preparation  of  Mercuric-oxide  Solution :  This  solution 
must,  of  course,  be  free  from  all  compounds  of  chlorine,  iodine, 
and  bromine,  and  the  mercury  must  be  present  as  a  mercuric 

*  Jahresber.  von  LIEBIG  u.  KOPF,  1849,  594. 
f  Annal.  de  Chem  et  PJiarm.,  LXXXV,  307 


368  DETERMINATION.  [§  118. 

salt;  and  it  should  be  of  proper  dilution  also.  According  to 
LIEBIG,  10  c.  c.  should  contain  not  more  than  about  0'2  grin,  of 
mercuric  oxide ;  hence,  if  a  preliminary  experiment  shows  it  to 
be  too  strong,  the  solution  must  be  diluted.  The  solution  should, 
further,  be  free  from  metals,  and  should  not  contain  too  much 
free  acid ;  only  so  much  should  be  present  as  to  afford  a  non-acid 
solution  on  adding  the  quantity  of  sodium -phosphate  solution 
required  in  the  experiment  (say  3  or  4:  c.  c.).  A  solution  which  is 
too  acid  is  treated  with  sodium  carbonate  until  basic  salt  precipi- 
tates, and  this  is  then  redissolved  by  adding  a  drop  or  two  of 
nitric  acid, 

y.  Performance  of  the  Analytical  Process:  This  may  be 
carried  out  in  two  ways.  It  is  best  to  employ  both,  since  the  first 
yields  results  a  little  too  high,  the  second  a  little  too  low,  hence 
with  both  the  errors  compensate  each  other.  Method  1 :  To 
10  c.  c.  of  the  mercury  solution  in  a  beaker  add  3  or  4  c.  c.  of  a  satu- 
rated sodium- phosphate  solution,  and  then,  before  the  precipitate 
has  time  to  become  crystalline,  add  sodium-chloride  solution,  the 
last  portions  being  added  very  cautiously,  until  the  precipitate  has 
entirely  disappeared. 

Suppose  20*5  c.  c.  of  sodium-chloride  solution  have  been  re- 
quired to  effect  this.  We  then  measure  off  (Method  2)  20*5  c.  c.  of 
the  same  sodium-chloride  solution,  add  to  it  3  or  4:  c.  c.  of  sodium- 
phosphate  solution,  and  run  in  from  a  burette  sufficient  of  the  same 
mercury  solution  to  just  afford  a  permanent  precipitate.  If  10 '25 
c.  c.  of  mercury  solution  have  been  required  to  effect  this,  then 
20'5-)- 20*5  —  41  c.c.  sodium-chloride  solution  have  been  required 
for  10  -|-  10-25  =  20*25  c.  c.  of  mercuric-oxide  solution,  from 
which  it  follows  that,  as  1  c.  c.  of  sodium-chloride  solution  is  the 
equivalent  of  0*0108  grin,  of  mercuric  oxide,  41  c.  c,  of  the  solu- 
tion will  correspond  to  0'4428  grin,  mercuric  oxide — the  quantity 
contained  in  20 '25  of  the  mercury  solution. 

LIEBIG  has  shown  by  numerous  experiments  that  this  method 
gives  very  close  results,  e.g. ,  0*1878  grm.  instead  of  0-1870; 
0-174:  grm.  instead  of  0*1748;  0*1668  grm.  instead  of  0*1664, 
etc.  The  method  is,  however,  susceptible  of  only  very  limited 
application.  For  this  reason  I  omit  giving  FR.  MOHR'S  modifi- 
cation of  the  method,*  which  consists  in  replacing  the  sodium 
phosphate  by  potassium  ferri cyanide. 

*  LeJirbudi  der  Titrirmethode,  3.  Aufl  .  396. 


§  118.]  MEKCURY    IN    MERCURIC    COMPOUNDS.  369 

c.  Regarding  PERSONNE'S  *  method,  which  depends  on  adding 
mercuric-chloride  solution  to  standard  potassium-iodide  solution 
until  incipient,  permanent  precipitation,  see  Zeitschr.  f.  analyt. 
Chem.,  n,  381. 

d.  After  J.  J.  ScHERER.f    Mercuric  nitrate  or  chloride  may  be 
directly  determined  with  sodium  thiosulphate.     The  reactions  are 
as  follows :  2II2O  +  3Hg(NO,)a  +  2Na2S2O3  =  (HgS)2 .  Hg(NO>),+ 
2JSTa2S04  +  4HN O3;  or,  2H2O  +  3HgClf  +  2tfa,SaO.  =  (HgS), .  Hg 
Cla  +  2N~a2SO4  -f-  4.HC1.     The  process  is  conducted  as  follows  in 
the  case  of  mercuric  nitrate :  Mix  the  highly  dilute  solution  with 
a  little  free  nitric  acid  in  a  tall  glass,  and  add  drop  by  drop  solution 
of  sodium  thiosulphate — 12*4  grm.  in  a  litre.     Each  drop  produces 
an  intense  yellow  cloud,  which  on  shaking  quickly  subsides  in  the 
form  of  a  heavy  flocculent  precipitate  (HgS)2  •  Iig(NO3)2.    In  order- 
to    distinguish    clearly  the   exact  end  of   the   reaction,   SCHEBEB 
recommends  to  transfer  the  fluid  towards  the  end  to  a  measuring 
flask,  to  take  out  -J  or  %  of  the  clear  fluid  and  to  finish  with  this. 
The  portion  of  thiosulphate  last  used. is  multiplied  by  3  or  2,  as 
the  case  may  be,  and  added  to  the  quantity  first  used.      1  c.c.  of 
the  solution  corresponds  to  0'015  mercury,  or  0'0162  mercuric 
oxide.     The  relation  is  not  changed  even  when  the  fluid  contains 
another  acid  (sulphuric,  phosphoric). 

In  the  case  of  mercuric  chloride,  the  highly  dilute  solution  is 
mixed  with  a  little  hydrochloric  acid  and  warmed,  nearly  to  boil- 
ing, before  beginning  to  add  the  sodium  thiosulphate.  At  first  a 
white  turbidity  is  formed,  then  the  precipitate  separates  in  thick 
flocks.  When  the  solution  begins  to  appear  transparent,  the  pre- 
cipitant is  added  more  slowly.  In  order  to  hit  the  end  of  the 
reaction  exactly,  small  portions  must  be  filtered  off  towards  the 
close.  The  precipitate  must  be  completely  white ;  if  too  much 
thiosulphate  has  been  added,  it  is  gray  or  blackish,  and  the  experi- 
ment must  be  repeated.  SCHEEEB  obtained  very  accurate  results. 
Of  course  no  other  metals  must  be  present  that  exert  a  decompos- 
ing action  on  sodium  thiosulphate. 


*  Journ.  dePharm.  et  de  Chem.,  XLIII,  477. 
f  Lehrbuch  der  Ghemie,  i,  513. 


370  DETERMINATION.  [§  119. 


5.  COPPER. 

a.  Solution 

Many  cupric  salts  dissolve  in  water.  Metallic  copper  is  best 
dissolved  in  nitric  acid.  Cupric  oxide,  and  those  cupric  salts  which 
are  insoluble  in  water,  may  be  dissolved  in  nitric,  hydrochloric,  cr 
sulphuric  acid.  Cupric  sulphide  is  treated  with  fuming  nitric  acid, 
or  it  is  heated  with  moderately  dilute  nitric  acid,  until  the  separated 
sulphur  exhibits  a  pure  yellow  tint  ;  addition  of  a  little  hydro- 
chloric acid  or  potassium  chlorate  greatly  promotes  the  action  of 
the  dilute  acid. 

l>.   Determination. 

Copper  may  be  weighed  in  the  form  of  cupric  oxide,  or  in  the 
'metallic  state,  or  as  cuprous  sulphide  (§  85).  Into  the  form  of 
cupric  oxide  it  is  converted  by  precipitation,  or  ignition,  sometimes 
with  previous  precipitation  as  sulphide.  The  determination  as 
cuprous  sulphide  is  preceded  usually  by  precipitation  either  a& 
cupric  sulphide  or  as  cuprous  sulphocyanate.  Copper  may  be  deter- 
mined also  by  various  volumetric  and  indirect  methods. 

We  may  convert  into 

1.  CUPRIC  OXIDE  : 

a.  By  Precipitation  as  hydrated  cupric  oxide  and  subsequent 
ignition  :  All  cupric  salts  soluble  in  water,  and  also  those  insoluble 
salts,  the  acids  of  which  may  be  removed  upon  solution  in  nitric 
acid,  provided  no  non-volatile  organic  substances  be  present. 

1}.  By  Precipitation,  preceded  by  Ignition  of  the  compound  : 
Such  of  the  salts  enumerated  under  a  as  contain  a  n  on-  volatile 
organic  substance,  thus  more  particularly  cupric  salts  of  non-vola- 
tile organic  acids. 

c.  By  Ignition  :  Cupric  salts  of  oxygen  acids  that  are  readily 
volatile  or  decomposable  at  a  high  temperature  (cupric  carbonate, 
cupric  nitrate). 

2.  METALLIC  COPPER  :   Copper  in  all  solutions  free  from  other 
metals  precipitable  by  zinc  or  the  galvanic  current,  also  the  oxides 
of  copper. 

3.  CUPROUS  SULPHIDE  :  Copper  in  all  cases  in  which  no  other 
metals  are  present  that  are  precipitable  by  hydrogen  sulphide  or 
potassium  sulphocyanate. 


§  119.]  COPPER.  371 

Of  the  several  methods  of  effecting  the  estimation  of  copper, 
~No.  3  is  particularly  to  be  recommended  for  use  in  laboratories ; 
method  2  is  also  very  convenient,  and  well  adapted  for  assaying. 
Of  the  volumetric  methods,  one  is  suited  for  technical  purposes, 
the  other  for  the  estimation  of  small  quantities  of  copper.  For 
technical  purposes  there  are,  besides,  also  several  col ori metric 
methods,  proposed  by  HEINE,  VON  HUBERT,  JACQUELAIN,  A.  MUL- 
LER,  and  others,  which  are,  all  of  them,  based  upon  the  comparison 
of  an  ammoniacal  solution  of  copper,  of  unknown  strength,  with 
others  of  known  strength.* 

LEVOL'S  indirect  method  of  estimating  copper,  which  is  based 
upon  the  diminution  of  weight  suffered  by  a  strip  of  copper  when 
digested  in  a  close-stoppered  flask  with  ammoniacal  solution  of 
copper  till  decolorization  is  effected,  takes  too  much  time,  and  is 
apt  to  give  false  results  (PmLLiPs,t  ERDMANN^:).  The  latter  remark 
applies  also  to  the  indirect  method  proposed  by  KUNGE,  which  con- 
sists in  boiling  the  solution  of  copper,  free  from  nitric  acid  and 
ferric  salts,  in  presence  of  some  free  hydrochloric  acid,  in  a  flask, 
with  a  weighed  strip  of  copper,  and,  after  decolorization  of  the 
fluid,  determining  the  loss  of  weight  suffered  by  the  copper. 

1.  Determination  as  Cupric  Oxide. 

a.  By  direct  Precipitation  as  Oxide. 

Heat  the  rather  dilute  neutral  or  add  solution  in  a  platinum  or 
porcelain  dish,  to  incipient  ebullition,  add  a  somewhat  dilute  solu- 
tion of  pure  soda  o*  potassa  until  the  formation  of  a  precipitate 
ceases,  and  keep  the  mixture  a  few  minutes  longer  at  a  tempera- 
ture near  boiling.  Allow  to  subside,  filter,  wash  by  decantation 
twice  or  thrice,  boiling  up  each  time,  then  collect  it  on  the  filter, 
wash  thoroughly  with  hot  water,  dry,  and  ignite  in  a  porcelain  or 
platinum  crucible,  as  directed  §  53.  Do  not  use  the  blow-pipe. 
After  ignition,  and  having  added  the  ash  of  the  filter,  let  the 
crucible  cool  in  the  desiccator,  and  weigh.  The  action  of  reducing 
gases  must  be  carefully  guarded  against  in  the  process  of  ignition. 

It  will  sometimes  happen,  though  mostly  from  want  of  proper 
attention  to  the  directions  here  given,  that  particles  of  the  precipi- 

*This  subject  hardly  comes  within  the  scope  of  the  present  work.     I  there- 
fore refer  to  AL.  Mr LLKH.  das  Complementiircolorimeter,  Chemnitz,  1854;  Bo- 
DEMANN'S  Probirkunst  von  KERL,  222;  also  to  DKHMS,  Zeitschr.f.analyt.  Chem.t 
in,  218,  and  GUSTAV  BISCHOF,  jun.,  jf>  ,  vr,  459. 
\Annal.  d.  Chem.  u.  Pharm.,  LXXXI,  208.      \Journ.f  prakt.  Chem.,  LXXV,  211. 


372  DETERMINATION.        .  [§  110. 

tate  adhere  so  tenaciously  to  the  dish  as  to  be  mechanically  irremov- 
able. In  a  case  of  this  kind,  after  washing  the  dish  thoroughly, 
dissolve  the  adhering  particles  with  a  few  drops  of  nitric  acid,  and 
evaporate  the  solution  over  the  principal  mass  of  the  precipitated 
oxide,  before  you  proceed  to  ignite  the  latter.  Should  the  solution 
be  rather  copious,  it  must  first  be  concentrated  by  evaporation, 
until  only  very  little  of  it  is  left.  For  the  properties  of  the  pre- 
cipitate, see  §  85. 

With  proper  attention  to  the  directions  here  given,  the  results 
obtained  by  this  method  are  quite  accurate,  otherwise  they  may  be 
either  too  high  or  too  low.  Thus,  if  the  solution  be  not  sufficiently 
dilute,  the  precipitant  will  fail  to  throw  down  the  whole  of  the 
copper ;  or  if  the  precipitate  be  not  thoroughly  washed  with  hot 
water,  it  will  retain  a  portion  of  the  alkali ;  or  if  the  ignited  pre- 
cipitate be  allowed  to  stand  exposed  to  the  air  before  it  is  weighed, 
an  increase  of  weight  will  be  the  result ;  and  so,  on  the  other  hand, 
a  diminution  of  weight,  if  the  oxide  be  ignited  with  the  filter  or 
under  the  influence  of  reducing  gases,  as  thereby  cuprous  oxide 
would  be  formed.  Should  a  portion  of  the  oxide  have  suffered 
reduction,  it  must  be  reoxidized  by  moistening  with  nitric  acid, 
evaporating  cautiously  to  dryness,  and  exposing  the  residue  to  a 
gentle  heat,  increasing  this  gradually  to  a  high  degree  of  intensity. 

Let  it  be  an  invariable  rule  to  test  the  filtrate  for  copper  with 
hydrogen  sulphide  water.  If,  notwithstanding  the  strictest  compli- 
ance with  the  directions  here  given,  the  addition  of  this  reagent 
produces  a  precipitate,  or  imparts  a  brown  tint  to  the  fluid,  this  is 
to  be  attributed  to  the  presence  of  organic  matter ;  in  that  case, 
concentrate  the  filtrate  and  wash-water  by  evaporation,  acidify, 
precipitate  with  hydrogen  sulphide  water,  filter,  incinerate  the 
filter,  heat  with  nitric  acid,  dilute,  filter,  concentrate,  precipitate 
with  soda,  and  add  the  oxide  obtained  to  the  main  quantity. 

Never  neglect  to  test  the  cupric  oxide  after  weighing  for  alkali 
or  alkali  salt  by  boiling  it  with  water.  If  either  is  present,  the 
oxide  must  be  exhausted  with  hot  water,  and  then  reignited  and 
reweighed.  Finally,  dissolve  the  oxide  in  hydrochloric  acid  to 
detect  and  if  necessary  to  estimate  any  silicic  acid  it  may  contain. 

In  default  of  sufficiently  pure  potash  or  soda,  the  carbonate 
may  be  used,  but  the  solution  must  not  contain  more  than  1  grm. 
copper  in  the  litre ;  the  alkali  carbonate  must  only  be  added 
slightly  in  excess,  and  the  mixture  must  be  boiled  for  half  an  hour. 


§  119.]  COPPER.  373 

The  bluish-green  precipitate  will  then  turn  dark  brown  and  gran- 
ular, and  may  b«  easily  washed  (GIBBS*). 

From  ammwiiacal  solutions,  also,  copper  may  be  precipitated 
by  soda  or  potassa.  In  the  main,  the  process  is  conducted  as  above. 
After  precipitation  the  mixture  is  heated,  until  the  supernatant 
fluid  has  become  perfectly  colorless ;  the  fluid  is  then  filtered  off 
with  the  greatest  possible  expedition.  If  allowed  to  cool  with  the 
precipitate  in  it,  a  small  portion  of  the  latter  would  redissolve. 

b.  By  Precipitation  as  Oxide,  preceded  by  Ignition  of  the 
Substance. 

Heat  the  substance  in  a  porcelain  crucible,  until  the  organic 
matter  present  is  totally  destroyed  ;  dissolve  the  residue  in  dilute 
nitric  acid,  filter  if  necessary,  and  treat  the  clear  solution  as 
directed  in  a. 

c.  By  Ignition. 

The  salt  is  put  into  a  platinum  or  porcelain  crucible,  and 
exposed  to  a  very  gentle  heat,  which  is  gradually  increased  to 
intense  redness ;  the  residue  is  then  weighed.  As  cupric  nitrate 
spirts  strongly  when  ignited,  it  is  always  advisable  to  put  it  into  a 
small  covered  platinum  crucible,  and  to  place  the  latter  in  a  large 
one,  also  covered.  With  proper  care,  the  results  are  accurate. 
Cupric  salts  of  organic  acids  may  also  be  converted  into  cupric  oxide 
by  simple  ignition.  To  this  end,  the  residue  flrst  obtained,  which 
contains  cuprous  oxide,  is  completely  oxidized  by  ignition  with 
mercuric  oxide  (which  leaves  no  residue  on  ignition),  or,  with  less 
advantage,  by  repeated  moistening  with  nitric  acid,  and  ignition. 
A  loss  of  substance  is  generally  incurred  by  the  use  of  nitric  acid 
from  the  difficulty  of  avoiding  spirting. 

2.  Determination  as  Metallic  Copper, 
a.  By  Precipitation  with  Zinc  or  Cadmium.^ 
Introduce  the  solution  of  copper,  after  having,  if  required,  first 
freed  it  from  nitric  acid,  by  evaporation  with  hydrochloric  acid  or 

*  Zeitschr.  f.  analyt.  Cliem.,  vir,  258. 

f  The  method  of  precipitating  copper  by  iron  or  zinc  and  weighing  it  in  Uie 
metallic  form  was  proposed  long  ago;  see  PFAFP'S  Handbuch  der  analytischen 
Chemie,  Altona,  1822,  u,  269;  where  the  reasons  are  given  for  preferring  zinc  as 
a  precipitant,  and  hydrogen  sulphide  is  recommended  as  a  test  for  ascertaining 
whether  the  precipitation  is  complete.  I  mention  this  with  reference  to 
F.  MOHR'S  paper  in  the  Annal.  d.  Chem.  u.  Pharm.,  xcvi,  215,  and  BODE- 
MANN'S  Probirkunst  von  KERL,  220. 


374  DETERMINATION.  [§  119. 

sulphuric  acid,  into  a  weighed  platinum  dish ,  dilute,  if  necessary 
with  some  water,  throw  in  a  piece  of  zinc  (soluble  in  hydrochloric 
acid  without  residue),  and  add,  if  necessary,  hydrochloric  acid  in 
sufficient  quantity  to  produce  a  moderate  evolution  of  hydrogen. 
If,  on  the  other  hand,  this  evolution  should  be  too  brisk,  owing  to 
too  large  excess  of  acid,  add  a  little  water.  Cover  the  dish  with  a 
watch-glass,  which  is  afterwards  rinsed  into  the  dish  with  the  aid 
of  a  washing-bottle.  The  separation  of  the  copper  begins  imme- 
diately ;  a  large  proportion  of  it  is  deposited  on  the  platinum  in 
form  of  a  solid  coating;  another  portion  separates,  more  particu- 
larly from  concentrated  solutions,  in  the  form  of  red  spongy  masses. 
Application  of  heat,  though  "it  promotes  the  reaction,  is  not  abso- 
lutely necessary ;  but  there  must  always  be  sufficient  free  acid 
present  to  keep  up  the  evolution  of  hydrogen.  After  the  lapse  of 
about  an  hour  or  two,  the  whole  of  the  copper  has  separated.  To 
make  sure  of  this,  test  a  small  portion  of  the  supernatant  fluid 
with  hydrogen  sulphide  water ;  if  this  fails  to  impart  a  brown  tint 
to  it,  you  may  safely  assume  that  the  precipitation  of  the  copper  is 
complete.  Ascertain  now,  also,  whether  the  zinc  is  entirely  dis- 
solved, by  feeling  about  for  any  hard  lumps  with  a  glass  rod,  and 
observing  whether  renewed  evolution  of  hydrogen  will  take  place 
upon  addition  of  some  hydrochloric  acid.  If  the  results  are  satis- 
factory in  this  respect  also,  press  the  copper  together  with  the  glass 
rod,  decant  the  clear  fluid,  which  is  an  easy  operation,  pour,  with- 
out loss  of  time,  boiling  water  into  the  dish,  decant  again,  and 
repeat  this  operation  until  the  washings  are  quite  free  from  hydro- 
chloric acid.  Decant  the  water  now  as  far  as  practicable,  rinse  the 
dish  with  strong  alcohol,  dry  at  100°,  let  it  cool,  .and  weigh.  If 
you  have  no  platinum  dish,  the  precipitation  may  be  effected  also 
in  a  porcelain  crucible  or  glass  dish ;  but  it  will,  in  that  case,  take 
a  longer  time,  because  of  the  lack  of  the  galvanic  action  between 
the  platinum  and  zinc;  and  the  whole  of  the  copper  will  be 
obtained  in  loose  masses,  and  not  firmly  adhering  to  the  sides  of  the 
crucible  or  dish,  as  in  the  case  of  precipitation  in  platinum  vessels. 
«,  The  results  are  very  accurate.  The  direct  experiment,  No. 
69,  gave  100  and  100-06,  instead  of  100.  FK.  Moim  (loo.  cit.) 
obtained  equally  satisfactory  results  by  precipitating  in  a  porce- 
lain crucible.* 

*  STOKER  (On  the  alloys  of  copper  and  zinc,  Cambridge,  1860,  p.  47)  says  that 
the  precipitated  copper  retains  water,  but  I  have  not  found  this  to  be  the  case. 


§  119  ]  COPPER.  375 

Zinc  being  sometimes  difficult  to  obtain  of  sufficient  purity, 
cadmium  may  be  used  instead;  it  dissolves  with  less  violence  in 
strongly  acid  copper  solutions.  It  may  be  used  in  the  form  of  rod 
in  which  it  usually  occurs  in  commerce  (CLASSEN*). 

h.  By  Precipitation  with  the  Galvanic  Current. 

This  method  makes  us  independent  of  pure  zinc  or  cadmium, 
and  yields  the  copper  in  a  compact  form,  readily  washed  and  deter- 
mined. It  is  now  largely  used  in  copper  works,  constant  batteries 
have  been  employed  for  it,  and  the  whole  process  has  been  organ- 
ized  for  use  on  a  large  scale  by  LTJCKOW,  and  adopted  by  the  Mans- 
feld  Ober-Berg-und  Hiitten-Direction  in  Eisleben.f  A  small  elec- 
trolytic apparatus  without  separate  battery,  for  single  precipitations, 
has  been  described  by  ULLGKEN.^: 

c.  By  Ignition  in  Hydrogen. 

The  oxides  of  copper  when  ignited  in  a  current  of  pure  hydro 
gen  are  converted  into  metallic  copper,  and  may  thus  be  convex 
ientiy  analyzed.    Occasionally  the  cupric  oxide  obtained  by  1,  a  <,, 
J,  is  reduced  either  at  once,  or  after  weighing ;  in  the  latter  casr, 
the  reduction  serves  as  a  control. 

3.  Determination  as  Cuprous  Sulphide. 

a.  By  Precipitation  as  Cupric  Sulphide. 

Precipitate  the  solution — which  is  best  moderately  acid,  bu, 
should  not  contain  a  great  excess  of  nitric  acid — according  to  th«? 
quantity  of  copper  present,  either  by  the  addition  of  strong  hydro- 
gen sulphide  water,  or  by  passing  the  gas.  In  the  absence  of  nitric 
acid  it  is  well  to  heat  nearly  to  boiling  while  the  gas  is  passing,  as 
this  makes  the  precipitate  denser,  and  it  is  more  easily  washed. 
When  the  precipitate  has  fully  subsided,  and  you  have  made  sura 
that  the  supernatant  fluid  is  no  longer  colored  or  precipitated  by 
strong  hydrogen  sulphide  water,  filter  quickly,  wash  the  precipi- 
tate without  intermission  with  water  containing  hydrogen  sulphide, 
and  dry  on  the  filter  with  some  expedition.  Transfer  to  a  weighed 
porcelain  crucible,  add  the  filter-ash  and  some  pure  powdered  sul-* 
phur,  and  ignite  strongly  in  a  stream  of  hydrogen  (§  108,  Fig. 
83).  It  is  advisable  to  use  a  glass  blow-pipe.  The  results  are 
very  accurate  (II.  KOSE§). 

*  Journ.  f.  prakt.  Chem.,  xcvi,  259. 

\Zeiischr.  f.  analyt.  Chem.,  vm,  23  and  xi,  1.  Compare  also  G^BBS,  ib.t 
in,  334,  and  LECOQ  DE  BOISBAUDAN,  ib.,  vn,  253. 

$lb. ,  vii,  442.  §  Pogg.  Annal.  ex,  138. 


370  DETERMINATION.  [§119-. 

This  method,  which  was  recommended  by  BERZELIUS,  and 
afterwards  by  BRUNNER,  has  only  lately  received  a  very  practical 
form  from  the  apparatus  introduced  by  II.  ROSE.  I  feel  great 
pleasure  in  recommending  it.  In  my  own  laboratory  it  is  in 
frequent  use. 

If  the  precipitated  cupric  sulphide  is  ignited  instead  in  a 
current  of  hydrogen  in  a  covered  porcelain  crucible,  from  which 
the  heat  as  well  as  the  cover  are  removed  occasionally  for  a  few 
seconds,  the  contents  will  be  converted  into  a  variable  mixture  of 
CuaS  and  CuO,  which  may  contain,  according  to  circumstances, 
cupric  oxide  or  cuprous  sulphide.  Since,  however,  the  percent- 
age content  of  cupric  oxide  and  cuprous  sulphide  in  copper  is 
the  same,  the  copper  content  in  the  residue  may  also  be  deter- 
mined (ULRICI  *).  This  method  is  simpler  than  the  one  detailed 
above,  but  is  not  quite  as  accurate. 

1).  By  Precipitation  as  Cuprous  /Sulphocyanate,  after  Rivox.f 
The  solution  should  be  as  free  as  possible  from  nitric  acid  and 
free  chlorine,  and  should  contain  little  or  no  free  acid.  Add  sul- 
phurous or  hypophosphorous  acid  in  sufficient  quantity,  and  then 
solution  of  potassium  sulphocyanate  in  the  least  possible  excess. 
The  copper  precipitates  as  white  cuprous  sulphocyanate.  It  is 
filtered  after  standing  some  time,  washed  and  dried,  mixed  with 
sulphur,  ignited  in  hydrogen  in  the  apparatus  mentioned  in  «,  and 
this  ignition  with  sulphur  is  repeated  till  the  weight  is  constant. 
The  precipitate  may  also  be  collected  on  a  weighed  filter,  dried  at 
100°,  and  then  weighed.  The  experiment,  No.  71,  conducted  in 
the  latter  way,  gave  99*66  instead  of  100.  The  process  yields 
satisfactory  results,  but  they  are  always  inclined  to  be  a  little  too 
low,  as  the  cuprous  sulphocyanate  is  not  absolutely  insoluble.  The 
loss  is  larger  in  the  presence  of  much  free  acid. 

c.  Cuprous  and  cupric  oxide,  cupric  sulphate,  and  many  other 
salts  of  copper  (but  not  chloride,  bromide,  or  iodide)  may  be  directly 
converted  into  cuprous  sulphide,  by  mixing  with  sulphur  and 
igniting  in  hydrogen  as  in  a  (II.  ROSE,  loc.  cit.).  The  results  are 
thoroughly  satisfactory. 

*Journ.f.  prakt.  Chem.,  cvn,  110. 

f  Compt.  Rend.,  xxxvni,  868;  Journ.  f.  prakt.  Chem.,  LXII,  252. 


§119.]  COPPER.  377 

4.     Volumetric  Methods. 

a.  DE  HAEN'S  METHOD.* 

I  recommend  this  method,  which  was  devised  in  my  own 
laboratory,!  as  more  especially  applicable  in  cases  where  small 
quantities  of  copper  are  to  be  estimated  in  an  expeditious  way. 
The  method  is  based  upon  the  fact  that,  when  a  cupric  salt  in 
solution  is  mixed  with  potassium  iodide  in  excess,  cuprous  iodide 
and  free  iodine  are  formed,  the  latter  remaining  dissolved  in  the 
solution  of  potassium  iodide  :  CuSO4  +  2KI  =  Cul  +  K2SO4  +  I. 
Now,  by  estimating  the  iodine  by  BUNSEN'S  method,  or  with  sodium 
thiosnlphate  (§  146),  we  learn  the  quantity  of  copper,  as  1  aL. 
iodine  (126'85)  corresponds  to  1  at.  copper  (63 -6).  The  following 
is  the  most  convenient  way  of  proceeding :  Dissolve  the  compound 
of  copper  in  sulphuric  acid,  best  to  a  neutral  solution  ;  a  moderate- 
excess  of  free  sulphuric  acid,  however,  does  not  injuriously  affect 
the  process.  Dilate  the  solution,  in  a  measuring  flask,  to  a  defi- 
nite volume ;  100  c.c.  should  contain  from  1  to  2  grm.  of  copper. 
Introduce  now  about  10  c.c.  of  potassium  iodide  solution  (1  in  10)» 
into  a  stoppered  bottle,  add  10  c.c.  of  the  copper  solution,  mix, 
allow  to  stand  10  minutes,  and  then  determine  the  separated 
iodine,  either  with  sulphurous  acid  and  iodine  (§  146,  1),  or  with 
sodium  thiosnlphate  (§  146,  2).  The  copper  solution  must  be  free 
from  ferric  salts  and  other  bodies  which,  decompose  potassium 
iodide,  also  free  nitric  acid,  and  free  hydrochloric  acid; -and  the* 
solution  must  not  be  allowed  to  stand  too  long  before  titration.. 
With  strict  attention  to  these  rules,  the  results  are  quite  accurate. 
DE  HAEN  obtained,  for  instance  0*3567  instead  of  O3566  of  cupric 
sulphate,  99 -89  and  100 -1  instead  of  100  of  metallic  copper- 
Further  experiments  (No.  72)  have  convinced  me,  however,  that, 
though  the  results  attainable  by  this  method  are  satisfactory,  they 
are  not  always  quite  so  accurate  as  would  be  supposed  from  the  above 
figures  given  by  DE  HAEN.  Acting  upon  FR.  MOHR'S  suggestion 
I  tried  to  counteract  the  injurious  influence  of  the  presence  of 

*  Annal.  d.  Chem.  u.  Pharm.,  xci.  237. 

f  BROWN  (Quart.  Journ.  of  the  Chem.  Soc.,  x,  65),  who  published  this  as  a 
new  method  iu  1857,  appears  to  have  been  ignorant  of  its  publication  in  1854. 
Even  the  slight  variation  of  determining  the  iodine  with  sodium  hyposulphite 
(SCHWAKZ)  instead  of  with  sulphurous  acid  (BUNSEN)  was  given  by  MOHR 
(Lehrbuch  der  Titrirmethode,  i,  387)  in  1855.  The  same  may  be  said  of  RUMP- 
LER,  who  in  1868  (Journ.  f  prakt.  Chem.,  cv,  193)  published  the  method,  with 
a  slight  modification,  as  new. 


378  DETERMINATION.  [§  119. 

nitric  acid,  by  adding  to  the  solution  containing  nitric  acid,  first, 
ammonia  in  excess,  then  hydrochloric  acid  to  slight  excess ;  the 
result  was  by  no  means  satisfactory.  The  reason  of  this  is  that  a 
solution  of  ammonium  nitrate,  mixed  with  some  hydrochloric  acid, 
will,  even  after  a  short  time,  begin  to  liberate  iodine  from  solution 
of  potassium  iodide. 

5.   PARKES'  *  METHOD;   AND  H.  FLECK' sf  MODIFICATION. 

PARKES'  expeditious  method  is  based  on  the  action  of  potassium 
cyanide  on  ammoniacal  copper  solution.  On  adding  potassium 
cyanide  to  the  azure-blue  fluid,  the  color  disappears,  CuCy, 
OTI4Cy,  and  KOIi  being  formed,  while  one  equivalent  of  cyanogen 
is  liberated,  and  acting  on  the  free  ammonia  present,  yields  urea, 
urea  oxalate,  ammonium  cyanide,  and  ammonium  formate  (LIE- 
BIG-  :{;).  The  decomposition  is  not  always  uniform,  however,  tlrj 
quantity  and  strength  of  the  ammonia  having  considerable  in- 
fluence ;  see  LIEBIG  (loo.  cit.'),  as  also  my  experiments  (No.  73,  &), 
from  which  it  appears  that  the  neutral  ammonium  salts  present 
modify  the  results.  See  also  FLECK  (loo.  cit.\  v.  WOLFSKRON,§ 
STEINBECK,|  and  KIRPITSCHOW.^ 

FLECK  proposed  the  following  modification :  Instead  of  am- 
monia, a  1 :  10  solution  of  ammonium  sesquicarbonate  is  used  at  a 
temperature  of  60°,  the  end  of  the  reaction  being  rendered  more 
readily  determined  by  adding  2  drops  of  a  1 :  20  potassium-fer- 
rocyanide  solution,  neither  the  blue  color  nor  its  transparency  be- 
ing affected  by  this  addition.  The  potassium-cyanide  solution  is 
standardized  against  a  copper  solution  of  known  strength,  before 
being  employed  for  solutions  of  unknown  strength.  On  adding 
the  potassium-cyanide  solution  by  drops  to  the  blue  solution 
warmed  to  60°,  the  odor  of  cyanogen  becomes  quite  distinct, 
while  the  color  of  the  solution  becomes  gradually  paler.  As  soon 
as  the  copper  double  salt  is  decomposed,  the  red  color  of  copper 
ferrocyanide  becomes  visible  without  any  precipitate  forming, 
and  on  adding  the  last  drop  of  the  potassium-cyanide  solution 
this  color  fades  away  also  and  leaves  a  perfectly  colorless  liquid. 

*  Mining  Journal,  1851.  \Polytechn.  Centralbl.,  1859,  1313. 

\  Annal.  d-  CTiem.  u.  Pharm,,  xcv.  118. 

$Zeitschr.f.  analyL  Chem.,  v.  403.  ||  Ibid.,  vin,  16. 

1  Zeilschr.  f.  Chem.  (II),  vn,  207. 


§  119.]  COPPER.  379 

This  modification  yields  results  which  while  concordant  are  yet 
only  approximate.*  Where  such  suffice,  the  method  may  be  used, 
as  it  is  quite  convenient. 

I  have  found  that  in  this  method  also,  ammonium  salts,  if 
present,  have  an  influence  on  the  results  (see  Exp.  ]STo.  73,  5), 
hence  the  method  appears  to  he  useful  only  when  the  standard- 
ization of  the  potassium-cyanide  solution  and  the  analytical  proc- 
esses are  performed  under  similar  circumstances. 

On  this  principle  is  based  STEINBECK' sf  method,  which  was 
devised  for  estimating  the  copper  in  the  Mansfeld  shales,  and 
which  received  a  premium  from  the  Mansfeld  Ober-Berg-und 
Hiitten-Direction.  It  depends  upon  the  precipitation  of  metallic 
copper  from  a  hydrochloric-acid  solution  by  zinc  in  contact  with 
platinum.  After  being  washed,  the  metallic  copper  is  dissolved 
in  a  definite  quantity  of  nitric  acid,  a  definite  quantity  of  am- 
monia added,  and  the  standard  solution  of  potassium  cyanide 
then  added  until  decolorization  is  effected.  Since,  in  this 
method,  only  definite  quantities  of  ammonia  and  ammonium 
nitrate  are  present,  the  results  obtained  are  very  concordant,  and 
also  very  nearly  correct  if  the  cyanide  solution  is  standardized 
against  a  copper  solution  the  strength  of  which  is  approximately 
like  that  of  solution  to  be  operated  upon.  The  cyanide  solution 
should  be  made  of  such  strength  that  1  c.  c.  of  it  is  the  equiva- 
lent of  0*005  grin,  of  copper. 

c.   METHODS  DEPENDING  UPON  THE  PRECIPITATION  OF  COPPER 
BY  SODIUM  SULPHIDE. 

PELOUZE  supersaturates  the  neutral  or  acid  copper  solution  with 
ammonia,  heats  the  solution  to  between  60°  and  80°,  and  adds  so- 
dium sulphide  until  the  blue  color  just  disappears.  The  precipitate 
that  forms  at  this  temperature  has  the  composition  5CuS  +  CuO. 
As  the  temperature  is  not  without  influence  on  the  composition 
of  the  precipitate,  and  as  the  moment  of  disappearance  of  the 
blue  color  is  not  very  marked,  FR.  MOHR  J  and  KUNZEL  §  have 

*  FLECK  used  in  6  tests,  in  which  varying  quantities  of  ammonium  carbonate 
were  purposely  taken  for  100  c.  c.  of  copper  solution,  a  minimum  of  15'2  c.  c. 
and  a  maximum  of  15  75  c.  c.,  an  average  of  15*46  c.  c.  of  potassium-cyanide 
solution. 

\  Zeitsckr.  f.  analyt.  CJiem.,  vui,  8. 

i  Lehrbuch  der  Titrirmethode ,  3.  Ann.,  429. 

§  Jour.f.  prakt.  Chem.t  LXXXVIII,  486;  Zeitschr.  f.  analyt.  C7iem.,  n,  373. 


380  DETERMINATION.  [§  119. 

modified  the  method.  The  former  precipitates  in  the  cold 
(whereby  cupric  sulphide  is  formed),  and  ascertains  the  incipient 
excess  of  sodium  sulphide  by  using  alkaline-lead  solution.  The 
latter  precipitates  at  the  boiling  temperature  (the  oxysulphide 
formed  in  this  case  rapidly  settles),  and  ascertains  when  the  pre- 
cipitation of  copper  is  complete  by  bringing  a  drop  of  the 
fluid  into  contact  with  freshly  precipitated  hydrated  zinc  sulphide 
(it  should  not  be  colored  brown).  The  sodium-sulphide  solution 
should  be  diluted  so  that  1  c.  c.  will  precipitate  about  O'Ol  grm. 
copper.  It  may  be  standardized  by  using  a  solution  containing 
10  grm.  of  copper  per  litre.  20  c.  c.  are  taken,  representing  0'2 
grin,  of  copper,  supersaturated  with  ammonia,  then  diluted  with 
water,  heated  to  boiling,  and  sodium- sulphide  solution  then  added 
until  the  reaction  is  complete.  The  zinc  sulphide  required  is  pre-< 
pared  by  dissolving  ordinary  zinc  in  hydrochloric  acid,  adding  an 
excess  of  ammonia,  and  boiling  with  a  small  quantity  of  sodium- 
sulphide  solution,  whereby  any  lead  present  is  precipitated.  Suf- 
ficient sodium-sulphide  solution  is  now  added  to  precipitate  nearly 
all  of  the  zinc  (leaving  a  small  quantity  unprecipitated) ;  the 
magma  obtained  is  uniformly  spread  out  over  several  layers  of 
blotting-paper. 

According  to  KUNZEL  the  method,  if  carefully  carried  out, 
gives  errors  not  exceeding  0*25  per  cent.;  hence  it  is  perfectly 
suitable  for  technical  purposes. 

d.    METHODS    DEPENDING  UPON   THE    REDUCTION  OF    CUPKIO 
CHLORIDE  BY   STANNOUS  CHLORIDE. 

E.  MULDER  *  was  the  first  to  base  upon  this  reaction  a  method 
of  estimating  copper,  using  indigo-carmine  as  an  indicator.  FR. 
WEIL  f  found  that  if  sufficient  hydrochloric  acid  is  present,  the 
end  of  the  reaction  is  indicated  by  the  decolorization  of  the  hot 
liquid.  He  prepared  the  stannous- chloride  solution  by  dissolving 
6  grm.  of  tinfoil  in  200  c.  c.  of  hot  hydrochloric  acid  and  diluting 
the  solution  with  boiled  water  to  make  1  litre.  The  copper  solu- 
tion, against  which  the  stannous-chloride  solution  must  be  stand- 
ardized before  every  fresh  series  of  estimations,  is  prepared  by 

*Jakresber.  von  KOPP  u.  WILL,  1860,  613. 
\Zeitschr.f.  analyt   Chem.,  ix,  297. 


§  119.]  COPPER.  381 

dissolving  7'854  grm.  of  copper  sulphate  (=  2  grm.  Cu.),  pow- 
dered and  dried  by  pressure  between  blotting-paper,  in  water  to 
make  500  c,  c.  25  c.  c.  of  this  solution  (containing  O'l  grm.  cop- 
per) are  then  introduced  into  a  100-c.  c.  flask,  5  c.  c.  of  pure, 
concentrated  hydrochloric  acid  added,  the  whole  heated  to  gentle 
boiling,  and  stannous-chioride  solution  added  to  the  boiling  liquid, 
rapidly  at  first,  but  towards  the  last  by  drops,  until  the  fluid 
is  perfectly  colorless.  5  c.  c.  of  hydrochloric  acid  are  again 
added ;  if  a  slight  color  develops,  it  is  discharged  by  adding  a 
few  drops  of ,  stannous-chioride  solution.  The  further  certainty 
that  the  reaction  is  complete  is  afforded  on  adding  a  few  drops  of 
mercuric-chloride  solution  to  a  small  quantity  of  the  cooled  solu- 
tion ;  if  no  turbidity  is  noticeable  there  is  no  excess  of  stannous 
chloride  present,  hence  a  little  of  the  latter  may  be  added  until 
a  faint  precipitate  of  mercurous  chloride  is  developed.  In  this 
case,  however,  there  must  be  deducted  0-05  c.  c.  from  the  quantity 
of  stannous-chioride  solution  used.  In  titrating  a  copper  solu- 
tion proceed  similarly.  Any  nitric  acid  present  must  be  evap- 
orated off  after  adding  an  excess  of  sulphuric  acid.  If  any  fer- 
ric salt  is  present,  it  will  be  reduced  with  the  cupric  chloride. 
In  such  a  case  precipitate  the  copper  in  a  second  portion  of  the 
solution  with  zinc  and  platinum  wire  in  the  heat,  determine  the 
ferrous  salt  with  potassium  permanganate  or  chrornate  (§  112), 
and  calculate  how  much  of  the  stannous-chioride  solution  had 
been  used  to  reduce  the  ferric  salt :  the  remainder  will  be  that 
used  up  for  the  cupric  chloride ;  or  wash  the  precipitated  cop- 
per, dissolve  it  in  sulphuric  acid,  and  then  reduce  it  with  stan- 
nous chloride.  The  test  analyses  cited  by  WEIL  show  very  satis- 
factory results. 

e.  SCHWARZ  *  precipitates  cuprous  oxide  from  the  solution  of 
potassio-cupric  tartrate  by  heating  with  grape  sugar,  filters  off 
the  precipitate,  washes  it,  warms  it  with  ferric  chloride  and  hy- 
drochloric acid,  and,  according  to  the  equation  Cu2O  -f-  Fe3Cl6  + 
.2IICl  =  2CuCl1  +  2FeClf  +  H1O,  estimates  the  ferrous  chloride 
formed  by  means  of  potassium  permanganate.f 


*  A/Dial   d.  Chem.  n.  Pharm  .  LXXXIV,  84. 

f  Potassium  Hiromate  is  not  eligible  for  use  because  the  cupric  chloride  im- 
pairs the  distinctness  of  the  end  reaction. 


382  DETERMINATION.  [§  120. 

f.  E.  FLEISCHER  *  precipitates  the  copper  as  cuprous  sulplio- 
cyanate  (§  119,  3,  5),  boils  the  washed  precipitate  with  potassa 
lye,  arid  thus  obtains  cuprous  oxide ;  or  he  adds  stannous  chloride 
and  potassium  iodide  and  obtains  a  precipitate  of  cuprous  iodide. 
In  either  case  the  precipitate  is  brought  into  contact  with  ferric- 
sulphate  solution,  the  ferrous  salt  formed  estimated,  and  from 
this  the  copper  calculated. 

g.  F.  FLEITMANN  f  precipitates  the  copper  with  zinc,  brings 
the  washed  precipitate  into  contact  with  ferric  chloride  and  hy- 
drocholoric  acid,  and  estimates  the  ferrous  chloride  formed 
(Cu  +  Fe.01.  =  CuCl2  +  2FeCla). 

h.  H.  SCHWARZ  £  adds  potassium  xanthogenate  to  the  acetic- 
acid  solution  of  copper  until  no  further  precipitate  forms.  Since 
the  other  heavy  metals,  excepting  zinc,  are  also  precipitated  by 
the  reagent  from  acetic-acid  solutions,  the  copper  must  be  separated 
from  the  precipitate. 

As  noted,  the  methods  e  to  li  require  the  previous  precipita- 
tion or  isolation  of  the  copper  in  one  way  or  another ;  they  can- 
not therefore  be  preferred  to  gravimetric  methods,  excepting  in 
very  special  cases. 

§  120. 
6.   BISMUTH. 

a.  Solution. 

Metallic  bismuth,  bismuth  trioxide,  and  all  other  compounds 
of  that  metal,  are  dissolved  best  in  nitric  acid  more  or  less  diluted. 
It  must  be  borne  in  mind  that  hydrochloric-acid  solutions  of  bis- 
muth, if  concentrated,  cannot  be  evaporated  without  loss  of 
bismuth  chloride. 

1).   Determination. 

Bismuth  is  weighed  in  the  form  of  trioxide,  chromate,  sul- 
pJiide,  or  arsenate,  or  in  the  metallic  state.    The  compounds  of  bis* 
muth  are  converted  into  trioxide  by  ignition,  by  precipitation  as 
basic  carbonate,  or  by  repeated  evaporation  of  the  nitric-acid  solu- 


*  ZeitscJir.  /.  analyt.  Chem.,  ix,  255. 
f  Annal.  d.  Chem.  u.  Pharm.,  xcvm,  141. 

\Dingl.  polyt.  Journ.,  cxc,  220  and  295;  also  Zeitschr.  f.  analyt.   Chem.,. 
vin,  462. 


§  120.]  BISMUTH.  383 

tion,      These  are  sometimes  preceded  by  separation  as  sulphide. 
The  determination  as  metallic  bismuth  is  frequently  preceded  by 
precipitation  as  sulphide  or  as  basic  chloride. 
We  may  convert  into 

1.  BISMUTH  TRIOXIDE: 

a.  By  Precipitation  as  basic  Bismuth  Carbonate.     All  com- 
pounds of  bismuth  which  dissolve  in  nitric  acid  to  nitrate,  no  other 
acid  remaining  in  the  solution. 

b.  By  Ignition. 

a.  Bismuth  salts  of  readily  volatile  oxygen  acids, 
/?.  Bismuth  salts  of  organic  acids. 

c.  By  Evaporation.     Bismuth  in  nitric-acid  solution. 

d.  By  Precipitation  as  Bismuth  Trisulphide.    All  compounds 
of  bismuth  without  exception. 

2.  BISMUTH  CHROMATE.     All  compounds  named  in  1,  a. 

3.  BISMUTH  TRISULPHIDE.     The  compounds  of  bismuth  without 
exception. 

4.  METALLIC    BISMUTH  :    The  trioxide  and  oxygen   salts,   the 
sulphide,  the  basic  chloride,  in  which  latter  form  the  bismuth  may 
be  precipitated  out  of  allots  solutions. 

1.  Determination  of  Bismuth  as  Trioxide. 

a.  By  Precipitation  as  Bismuth  Carbonate. 

If  the  solution  is  concentrated  add  water,  taking  no  notice  of 
any  precipitate  of  basic  nitrate  that  may  be  formed.  Mix  with 
ammonium  carbonate  in  very  slight  excess,  and  heat  for  some  time 
nearly  to  boiling ;  filter,  dry  the  precipitate,  and  ignite  in  the  man- 
ner directed  §  116,  1  (Ignition  of  lead  carbonate) ;  the  process  of 
ignition  serves  to  convert  the  carbonate  into  bismuth  trioxide.  For 
the  properties  of  the  precipitate  and  residue,  see  §  86.  The  method 
gives  accurate  results,  though  generally  a  trifle  too  low,  owing  to 
the  circumstance  that  bismuth  carbonate  is  not  absolutely  insoluble 
in  arumonium  carbonate.  Were  you  to  attempt  to  precipitate 
bismuth,  by  means  of  ammonium  carbonate,  from  solutions  con- 
taining sulphuric  acid  or  hydrochloric  acid,  you  would  obtain 
incorrect  results,  since  with  the  basic  carbonate,  basic  sulphate  or 
basic  chloride  would  be  precipitated,  which  are  not  decomposed  by 
excess  of  ammonium  carbonate.  Were  you  to  filter  off  the  precipi- 
tate without  warming,  a  considerable  loss  would  be  sustained,  as 


384  DETERMINATION.  [§  120. 

the  whole  of  the  basic  carbonate  would  not  have  been  separated 
(Expt.  No.  74). 

l>.  By  Ignition. 

a.  Compounds  like  bismuth  carbonate  or  nitrate  are  ignited  in 
a  porcelain  crucible  until  their  weight  remains  constant. 

ft.  Salts  of  organic  acids  are  treated  like  the  corresponding 
compounds  of  copper  (§  119,  1,  c). 

c.  By  Evaporation. 

The  solution  of  the  nitrate  is  evaporated,  in  a  porcelain  dish  on 
the  water-bath,  till  the  neutral  salt  remains  in  syrupy  solution ; 
add  water,  loosen  the  white  crust  that  is  formed  with  a  glass  rod 
from  the  sides,  evaporate  again  on  a  water-bath,  reprecipitate  with 
water,  and  repeat  the  whole  operation  three  or  four  times.  After 
the  dry  mass  on  the  water-bath  has  ceased  to  smell  of  nitric  acid, 
it  is  allowed  to  cool  thoroughly,  and  then  treated  with  cold  water 
containing  a  little  ammonium  nitrate  (1  in  500) ;  after  the  residue 
and  fluid  have  been  a  short  time  together,  filter,  wash  with  the 
weak  solution  of  ammonium  nitrate,  dry  and  ignite  (§  53).  Results 
very  satisfactory  (J.  LOWE*). 

d.  By  Precipitation  as  Bismuth  Trfeulphide. 

Dilute  the  solution  with  water  slightly  acidulated  with  acetic 
acid  (to  prevent  the  precipitation  of  a  basic  salt),  and  precipitate 
with  hydrogen  sulphide  water  or  gas ;  allow  the  precipitate  to 
subside,  and  test  a  portion  of  the  supernatant  fluid  with  hydrogen 
sulphide  water:  if  it  remains  clear,  which  is  a  sign  that  the 
bismuth  is  completely  precipitated,  filter  (the  filtrate  should  smell 
strongly  of  HaS),  and  wash  the  precipitate  with  water  containing 
hydrogen  sulphide.  Or  mix  with  ammonia  until  the  free  acid  is 
neutralized,  then  add  ammonium  sulphide  in  excess,  and  allow  to 
digest  for  some  time. 

The  washed  precipitate  may  now  be  weighed  in  three  different 
forms,  viz.,  as  trisulphide,  as  metal,  or  as  trioxide.  The  treatment 
in  the  two  former  cases  will  be  described  in  3  and  4 :  in  the*  latter 
case  proceed  as  follows  : 

Spread  the  filter  out  on  a  glass  plate  and  remove  the  precipitate 
to  a  vessel  by  means  of  a  jet  of  water  from  the  wash-bottle — or,  if 
this  is  not  practicable,  put  the  precipitate  and  filter  together  into 
the  vessel — and  heat  gently  with  moderately  strong  nitric  acid 

*  Journ.f.  prakt.  Chem.,  LXXIV,  344. 


§  120.]  BISMUTH.  385 

until  complete  decomposition  is  effected ;  the  solution  is  then 
•diluted  with  water  slightly  acidulated  with  acetic  or  nitric  acid, 
and  filtered,  the  filter  being  washed  with  the  acidulated  water;  the 
titrate  is  then  finally  precipitated  as  directed  in  a. 

2.  Determination  of  Bismuth  as  Chromate  (J.  LOWE*). 
Pour  the  solution  of  bismuth,  which  must  be  as  neutral  as 

possible,  and  must,  if  necessary,  be  first  freed  from  the  excess  of 
nitric  acid  by  evaporation  on  the  water-bath,  into  a  warm  solution 
of  pure  potassium  dichromate  in  a  porcelain  dish,  with  stirring, 
and  take  care  to  leave  the  alkali  chromate  slightly  in  excess. 
Rinse  the  vessel  which  contained  the  solution  of  bismuth  with 
water  containing  nitric  acid  into  the  porcelain  dish.  The  precipi- 
tate formed  must  be  orange-yellow,  and  dense  throughout ;  if  it  is 
fiocculent,  and  has  the  color  of  the  yolk  of  an  egg,  this  is  a  sign 
that  there  is  a  deficiency  of  potassium  dichromate ;  in  which  case 
add  a  fresh  quantity  of  this  salt,  taking  care,  however,  to  guard 
against  too  great  an  excess,  and  boil  until  the  precipitate  presents 
the  proper  appearance.  Boil  the  contents  of  the  dish  for  ten 
minutes,  with  stirring  ;  then  wash  the  precipitate,  first  by  repeated 
boiling  with  water  and  decantation  on  to  a  weighed  filter,  at  last 
thoroughly  on  the  latter  with  boiling  water ;  dry  at  about  120°, 
and  weigh.  For  the  properties  and  composition  of  the  precipitate, 
.see  §  86.  Results  very  satisfactory. 

3.  Determination  of  Bismuth  as  Trisulphide. 
Precipitate  the  bismuth  as  trisulphide  according  to  1,  d.    If 

the  precipitate  contains  free  sulphur,  extract  the  latter  by  boiling 
with  solution  of  sodium  sulphite,  or  by  treatment  with  carbon 
disulphide  (compare  the  determination  of  mercury  as  sulphide, 
§  118,  3),  collect  on  a  weighed  filter,  dry  at  100°,  and  weigh. 

The  drying  must  be  conducted  with  caution.  At  first  the 
precipitate  loses  weight,  by  the  evaporation  of  wrater,  then  it  gains 
weight,  from  the  absorption  of  oxygen.  Hence  you  should  weigh 
every  half  hour,  and  take  the  lowest  weight  as  the  correct  one. 
Compare  Expt.  No.  52.  Properties  and  composition,  §  86,  y. 

The  bismuth  sulphide  cannot  be  conveniently  converted  into 
the  metallic  state  by  ignition  in  hydrogen,  as  its  complete  decom- 
position is  a  work  of  considerable  time.  As  regards  reduction 
with  potassium  cyanide,  see  4. 

*  Journ.f.  prakt.  CJiem.,  LXVII,  464 


386  DETERMINATION.  [§  120. 

4.   Determination  of  Bismuth  as  Metal. 

The  oxide,  sulphide,  or  basic  chloride  to  be  reduced  is  fused 
in  a  porcelain  crucible  with  five  times  its  quantity  of  ordi- 
nary potassium  cyanide.  The  crucible  must  be  large  enough 
In  the  case  of  oxide  and  basic  chloride,  the  reduction  is  completed 
in  a  short  time  at  a  gentle  heat ;  sulphide,  on  the  other  hand, 
requires  longer  fusion  and  a  higher  temperature.  The  operation 
has  been  successful  if  on  treatment  with  water  metallic  grains  are 
obtained.  These  grains  are  first  washed  completely  and  rapidly 
with  water,  then  with  weak  and  lastly  with  strong  alcohol,  dried 
and  weighed.  If  you  have  been  reducing  the  sulphide,  and  on 
treating  the  fused  mass  with  water  a  black  powder  (a  mixture  of 
bismuth  with  bismuth  sulphide)  is  visible,  besides  the  metallic  grains,, 
it  is  necessary  to  fuse  the  former  again  with  potassium  cyanide. 

It  sometimes  happens  that  the  crucible  is  attacked,  and  particles 
of  porcelain  are  found  mixed  with  the  metallic  bismuth  ;  to  prevent 
this  from  spoiling  the  analysis,  weigh  the  crucible  together  with  a 
small  dried  filter  before  the  experiment,  collect  the  metal  on  the 
filter,  dry  and  weigh  the  crucible  with  the  filter  and  bismuth  again. 
Results  good  (H.  HOSE*). 

The  precipitation  of  bismuth  as  basic  chloride,  and  the  reduc- 
tion of  the  latter  with  potassium  cyanide,  has  been  recommended 
by  H.  RosE.f  The  process  is  conducted  as  follows :  Nearly  neu- 
tralize any  large  excess  of  acid  that  may  be  present  with  potassar 
soda,  or  ammonia,  add  ammonium  chloride  in  sufficient  quantity 
(if  hydrochloric  acid  is  not  already  present),  and  then  a  rather  large 
quantity  of  water.  After  allowing  to  stand  some  time,  test  whether 
a  portion  of  the  clear  supernatant  fluid  is  rendered  turbid  by  a. 
further  addition  of  water ;  and  then,  if  required,  add  water  to  the 
whole  till  the  precipitation  is  complete.  Finally  filter,  wash  com- 
pletely with  cold  water,  dry  and  fuse  according  to  the  directions 
just  given  with  potassium  cyanide.  It  is  less  advisable  to  dry  the 
precipitate  at  100°,  weigh  and  calculate  the  metal  present  from  the 
formula  BiOCl,  as  washing  causes  a  slight  alteration  in  its  com- 
position (unless  a  little  hydrochloric  acid  is  added  to  the  wash- 
water,  which  is  inconvenient  when  the  precipitate  is  collected  on 
a  weighed  filter),  and  if  precipitated  in  the  presence  of  sulphuric, 
phosphoric  acids,  &c.,  it  is  liable  to  contain  small  quantities  of 
these  acids.  Results  accurate. 

*Pogg.  Annal.,  xci,  104,  and  ex,  136.  \  lb  ex,  425. 


§  121.]  CADMIUM.  387 

5.  Determination  of  Bismuth  as  Arsenate. 
II.  SALKOWSKI  *  recommends  the  determination  of  bismuth  as 
a  arsenate  (BiAsO4.H,O)  which  is  dried  at  100°  to  120°,  the 
method  being  based  on  the  observation  by  SCHEELE  that  bismuth 
arsenate  is  perfectly  insoluble  in  nitric  acid.  The  solution  of 
bismuthic  nitrate  is  acidulated  with  nitric  acid(but  must  be  free  from 
other  acids),  precipitated  by  a  moderate  excess  of  arsenic  acid, 
and  stirred,  avoiding  touching  the  sides  of  the  beaker  with  the 
rod  (otherwise  the  crystalline  precipitate  will  adhere  fast  to  the 
parts  touched).  The  whole  is  then  allowed  to  stand,  without 
wanning,  for  a  few  hours,  then  the  precipitate  is  collected  on  a 
filter  previously  dried  at  120°,  and  washed  until  the  washings 
begin  to  pass  slightly  turbid.  Then  dry  at  120°  and  weigh. 
Ignition  of  the  precipitate  is  not  advisable,  as  the  carbon  of  the 
filter  exercises  a  reducing  action  even  when  ammonium  nitrate  is 
used.  The  test  analyses  made  by  SALKOWSKI  gave  99*88  to 
100-02  instead  of  100. 

§121. 

7.  CADMIUM. 
a.  Solution. 

Cadmium,  its  oxide,  and  all  the  other  compounds  insoluble  in 
water,  are  dissolved  in  hydrochloric  acid  or  in  nitric  acid. 

1).  Determination. 

Cadmium  is  weighed  either  in  the  form  of  oxide,  or  in  that  of 
sulphide  (§  87).  It  may  also  be  weighed  as  sulphate,  and  in  the 
absence  of  other  bases  precipitable  by  oxalic  acid,  it  may  be  esti- 
mated volumetrically. 

We  may  convert  into 

1.  CADMIUM  OXIDE: 

a.  By  Precipitation.  The  compounds  of  cadmium  which  are 
soluble  in  water ;  the  insoluble  compounds,  the  acid  of  which  is 
removed  upon  solution  in  hydrochloric  acid ;  cadmium  salts  of 
organic  acids. 

1).  By  Ignition.  Cadmium  salts  of  readily  volatile  or  easily 
decomposable  inorganic  oxygen  acids. 

2.  CADMIUM  SULPHIDE  :  All   compounds  of   cadmium   without 
exception.  

*  Journ.  f.  prakt.  Chem.,  civ,  170;  Zeitechr.  f.   analyt.   Chem.,  vin,  205. 


388  DETERMINATION.  [§  121. 

3.  CADMIUM  SULPHATE  :  All  compounds  of  cadmium,  in  the 
absence  of  other  non-volatile  substances. 

1.  Determination  as  Cadmium  Oxide. 

a.  By  Precipitation. 

Precipitate  with  potassium  carbonate,  wash  the  precipitated 
cadmium  carbonate,  and  convert  it,  by  ignition,  into  oxide.  The 
precipitation  is  conducted  as  in  the  case  of  zinc,  §  108,  1,  a.  The 
cadmium  oxide  which  adheres  to  the  filter  may  easily  be  reduced 
and  volatilized ;  it  is  therefore  necessary  to  be  cautious.  In-  the 
first  place  choose  a  thin  filter,  transfer  the  dried  precipitate  as  com- 
pletely as  possible  to  the  crucible,  replace  the  filter  in  the  funnel, 
and  moisten  it  with  ammonium  nitrate  solution,  allow  to  dry,  and 
then  burn  carefully  in  a  coil  of  platinum  wire.  Let  the  ash  fall 
into  the  crucible  containing  the  mass  of  the  precipitate,  ignite 
carefully,  avoiding  the  action  of  reducing  gases,  and  finally  weigh. 
It  is  difficult  to  remove  the  last  portions  of  carbonic  acid  ;  you  must 
therefore  repeat  the  ignition  till  the  weight  remains  constant. 
Properties  of  precipitate  and  residue,  §  87.  Results  generally  a 
little  too  low. 

b.  By  Ignition. 

Same  process  as  for  zinc,  §  108,  1,  c. 

2.  Determination  as  Cadmium  Sulphide. 

It  is  best  to  precipitate  the  moderately  acid  solution  with  hydro- 
gen sulphide  water  or  gas,  which  must  be  used  in  sufficient  excess. 
The  presence  of  a  considerable  quantity  of  free  hydrochloric  or 
nitric  acid  may — especially  if  the  solution  is  not  enough  diluted — 
prevent  complete  precipitation,  hence  such  an  excess  should  be 
avoided,  and  the  clear  supernatant  fluid  should  in  all  cases  be  tested, 
by  the  addition  of  a  relatively  large  amount  of  hydrogen  sulphide 
water  to  a  portion,  before  being  filtered.  Alkaline  solutions  of 
cadmium  may  be  precipitated  with  ammonium  sulphide.  If  the 
cadmium  sulphide  is  free  from  admixed  sulphur,  it  may  be  at  once 
collected  on  a  weighed  filter,  washed  first  with  diluted  hydrogen 
sulphide  water  mixed  with  a  little  hydrochloric  acid,  then  with 
pure  water,  dried  at  100°,  and  weighed ;  if,  on  the  contrary,  it  con- 
tains free  sulphur,  it  may  be  purified  by  boiling  with  a  solution  of 
sodium  sulphite,  or  by  treatment  with  carbon  disulphide  (see  Mer- 
curic Sulphide,  §  118,  3).  Results  accurate.  The  precipitation  of 
sulphur  may  occasionally  be  obviated  by  adding  to  the  cadmium 


§  122.]  PALLADIUM.  389 

solution  potassium  cyanide  till  the  precipitate  first  formed  is  redis- 
solved,  and  then  precipitating  this  solution  with  hydrogen  sulphide. 
If  the  cadmium  sulphide  is  not  to  be  weighed  as  such,  warm  it, 
together  with  the  filter,  with  moderately  strong  hydrochloric  acid, 
till  the  precipitate  has  dissolved  and  the  odor  of  hydrogen  sulphide 
is  no  longer  perceptible,  filter  and  precipitate  the  solution  as  in 
1,  a,  after  having  removed  the  excess  of  free  acid  for  the  most  part 
by  evaporation. 

3.  Determination  as  Cadmium  Sulphate. 

Same  process  as  for  magnesium  (§  104,  1).  The  CdSO4  may 
be  rather  strongly  ignited  without  decomposition. 

4.  W.  GIBBS*  determines  cadmium  volumetrically  by  mixing 
the  concentrated  solution  of  the  sulphate,  nitrate,  or  chloride  with 
excess  of  oxalic  acid  and  a  quantity  of  strong  alcohol,  filtering, 
washing  with  alcohol,  dissolving  in  hot  hydrochloric  acid  and 
determining  the  oxalic  acid  with  permanganate  (§  13  Y).  W.  Gr. 
LEisoNf  obtained  satisfactory  results  by  this  process. 

Supplement  to  the  Fifth  Group. 

§  122. 
8.  PALLADIUM. 

Palladium  is  converted,  for  the  purpose  of  estimation,  into  the 
metallic  state  •  or — in  many  separations — into  potassium  palladia 
chloride. 

1.  Determination  as  Palladium. 

a.  Neutralize  the  solution  of  palladious  chloride  almost  com- 
pletely with  sodium  carbonate,  mix  with  solution  of  mercuric 
cyanide ;  and  heat  gently  for  some  time,  until  the  odor  of  hydro- 
cyanic acid  has  gone  off.  A  yellowish-white  precipitate  of  palladi- 
ous cyanide  will  subside  ;  from  dilute  solutions,  only  after  the  lapse 
of  some  time.  Wash  first  by  decantation,  then  on  the  filter,  dry 
thoroughly,  ignite  cautiously,  finally  over  the  gas  blowpipe  till  the 
palladium  paracyanide  first  formed  is  decomposed,  then  ignite  in 
hydrogen,  since  the  palladium  has  been  slightly  oxidized  As  soon 
as  the  lamp  is  removed,  stop  the  hydrogen  to  prevent  absorption, 
and  weigh  the  metal.  If  the  solution  contains  palladious  nitrate, 
evaporate  it  first  with  hydrochloric  acid  to  dryness;  as  otherwise 
*  Zeitschr.f.  analyt.  Chem.,  vn,  259.  f  lb.,  x,  343. 


390  DETERMINATION.  [§  122. 

the  precipitate  obtained  deflagrates  upon  ignition  ("WOLLASTOIST). 
Results  exact. 

5.  Mix  the  solution  of  palladious  chloride  or  nitrate  with 
sodium  or  potassium  formate,  and  warm  until  no  more  carbonic 
acid  escapes.  The  palladium  precipitates  in  brilliant  scales  (DoBE- 
KEINEK). 

€.  Precipitate  the  acid  solution  of  palladium  with  hydrogen 
sulphide,  filter,  wash  with  boiling  water,  roast,  dissolve  in  hydro' 
chloric  acid  and  nitric  acid,  and  precipitate  as  in  a. 

Exposed  to  a  moderate  red  heat  metallic  palladium  becomes 
covered  with  a  film  varying  from  violet  to  blue,  but  at  a  higher 
temperature  it  recovers  its  lustre,  which  it  keeps  after  being  sud- 
denly cooled,  for  instance,  with  cold  water.  This  tarnishing  and 
recovery  of  the  metallic  lustre  is  not  attended  with  any  percepti- 
ble difference  of  weight.  Palladium  which  has  taken  up  oxygen 
is  immediately  reduced  in  hydrogen ;  when  cooled  in  the  current 
of  gas,  it  retains  some  absorbed  hydrogen.  Palladium  requires  the 
very  highest  degree  of  heat  for  its  fusion.  It  dissolves  readily  in 
nitrohydrochloric  acid,  with  difficulty  in  pure  nitric  acid,  more 
easily  in  nitric  acid  containing  nitrous  acid,  with  difficulty  in  boil- 
ing concentrated  sulphuric  acid. 

2.  Determination  as  Potassium  Palladia  Chloride. 

Evaporate  the  solution  of  palladic  chloride  with  potassium 
chloride  and  nitric  acid  to  dryness,  and  treat  the  mass  when  cold 
with  alcohol  of  0'833  sp.  gr.,  in  which  the  double  salt  is  insoluble. 
Collect  on  a  weighed  filter,  dry  at  100°,  and  weigh.  Results  a 
little  too  low,  as  traces  of  the  double  salt  pass  away  with  the  alcohol 
washings  (BEKZELIUS).  Instead  of  weighing  the  double  salt  you 
may  ignite  in  hydrogen,  remove  the  potassium  chloride  with  water 
and  weigh  the  metal  obtained.  This  method  is  indeed  to  be  pre- 
ferred, as  it  prevents  any  potassium  chloride  in  the  precipitate  from 
affecting  the  result. 

POTASSIUM  PALLADIC  CHLORIDE  consists  of  microscopic  octa- 
hedra;  it  presents  the  appearance  of  a  vermilion  or,  if  the  crystals 
are  somewhat  large,  of  a  brown  powder.  It  is  very  slightly  solu- 
ble in  cold  water ;  it  is  almost  insoluble  in  cold  alcohol  of  the  above 
strength.  It  contains  26*89  per  cent,  palladium. 


§  123.]  GOLD.  391 

Sixth  Group. 

OOLD PLATESTUM ANTIMONY TIN   IN    STANNIC   COMPOUNDS TIN     IN 

STANNOUS      COMPOUNDS ARSENOTJS     AND     AKSENIC    ACIDS (  MO- 

LYBDIC  ACID). 

§123. 

1.  GOLD. 

a.  Solution. 

Metallic  gold,  and  all  compounds  of  gold  insoluble  in  water, 
are  warmed  with  hydrochloric  acid,  and  nitric  acid  is  gradually 
added  until  complete  solution  is  effected ;  or  they  are  repeatedly 
digested  with  strong  chlorine  water.  The  latter  method  is  resorted 
to  more  especially  in  cases  where  the  quantity  of  gold  to  be  dis- 
solved is  small,  and  mixed  with  foreign  oxides  which  it  is  wished 
to  leave  undissolved.  According  to  "W".  SKEY*  tincture  of  iodine, 
or,  for  larger  quantities  of  gold,  bromine  water,  is  better  than  chlo- 
rine water.  They  give  solutions  freer  from  other  metals  than  the 
chlorine  water  gives. 

b.  Determination. 

Gold  is  always  weighed  in  the  metallic  state.  The  compounds 
are  brought  into  this  form,  either  by  ignition  or  by  precipitation, 
as  gold,  or  auric  sulphide. 

We  convert  into 

METALLIC  GOLD  : 

a.  By  Ignition.  All  compounds  of  gold  which  contain  no  fixed 
acid,  or  other  body. 

1).  By  Precipitation  as  metallic  gold.  All  compounds  of  gold 
without  exception  in  cases  where  a  is  inapplicable. 

c.  By  Precipitation  as  auric  sulphide.     This  method  serves  to 
effect  the  separation  of  gold  from  certain  other  metals  which  may 
be  mixed  with  it  in  a  solution. 

Determination  as  Metallic  Gold, 
a.  By  Ignition. 

Heat  the  compound,  in  a  covered  porcelain  crucible,  very  gently 
at  first,  but  finally  to  redness,  and  weigh  the  residuary  pure  gold. 
For  properties  of  the  residue,  see  §  88.  The  results  are  most 
accurate. 

*Z*itschr.f.  analyt.  Chem.,  x,  221. 


392  DETERMINATION.  [§ 

I.  By  Precipitation  as  Metallic  Gold. 

a.  The  solution  is  free  from  Nitric  Acid.  Mix  the  solution 
with  a  little  hydrochloric  acid,  if  it  does  not  already  contain  some 
of  that  acid  in  the  free  state,  and  add  a  clear  solution  of  ferrous 
sulphate  in  excess;  heat  gently  for  a  few  hours  until  the  precipi- 
tated fine  gold  powder  has  completely  subsided ;  filter,  wash,  dry, 
and  ignite  according  to  §  52.  A  porcelain  dish  is  a  more  appro- 
priate vessel  to  effect  the  precipitation  in  than  a  beaker,  as  the 
heavy  fine  gold  powder  is  more  readily  rinsed  out  of  the  former 
than  out  of  the  latter.  There  are  no  sources  of  error  inherent  in 
the  method. 

fi.  The  solution  of  Gold  contains  Nitric  Acid.  Evaporate  the 
solution,  on  a  water-bath,  to  the  consistence  of  syrup,  adding  from 
time  to  time  hydrochloric  acid ;  dissolve  the  residue  in  water  con- 
taining hydrochloric  acid,  and  treat  the  solution  as  directed  in  a* 
It  will  sometimes  happen  that  the  residue  does  not  dissolve  to  a. 
clear  fluid,  in  consequence  of  a  partial  decomposition  of  auric  chlo- 
ride into  aurous  chloride  and  metallic  gold ;  however,  this  is  a  mat- 
ter of  perfect  indifference. 

y.  In  cases  where  it  is  wished  to  avoid  the  presence  of  iron  in 
the  filtrate,  the  gold  may  be  reduced  by  means  of  oxalic  acid.  To 
this  end,  the  dilute  solution — freed  previously,  if  necessary,  from 
nitric  acid,  in  the  manner  directed  in  /?— is  mixed,  in  a  beaker, 
with  oxalic  acid,  or  with  ammonium  oxalate  in  excess,  some  sul- 
phuric acid  added  (if  that  acid  is  not  already  present  in  the  free 
state),  and  the  vessel,  covered  with  a  glass  plate,  is  kept  standing 
for  two  days  in  a  moderately  warm  place.  At  the  end  of  that 
time,  the  whole  of  the  gold  will  be  found  to  have  separated  in 
small  yellow  scales,  which  are  collected  on  a  filter,  washed  first 
with  dilute  hydrochloric  acid,  then  with  water,  dried,  and  ignited. 
If  the  gold  solution  contains  a  large  excess  of  hydrochloric  acid, 
the  latter  should  be  for  the  most  part  evaporated,  before  the  solu- 
tion is  diluted  and  the  oxalic  acid  added.  If  the  gold  solution  con- 
tains chlorides  of  alkali  metals,  it  is  necessary  to  dilute  largely,  and 
allow  to  stand  for  a  long  time,  in  order  to  effect  complete  precipi- 
tation (H.  ROSE). 

d.  The  gold  may  also  be  thrown  down  in  the  metallic  form  by 
chloral  hydrate  *  in  the  presence  of  potassa.  Warm  the  solution, 

*  HAGEK'S  Pharmac.  Centralhalle,  xi,  393. 


§  124.]  PLATINUM.  393 

add  the  chloral,  then  pure  potassa  in  excess,  and  boil  for  a  minute 
or  so.     The  gold  is  precipitated  with  evolution  of  chloroform. 

£.  Finally,  gold  may  be  thrown  down  by  many  metals,  such  as 
zinc,  cadmium,  magnesium,  &c.  The  latter  has  been  recommended 
by  SCHEIBLEB*  for  the  analysis  of  the  gold  salts  of  organic  bases. 
The  precipitate  is  first  washed  with  hydrochloric  acid,  then  with 
water. 

c.  By  Precipitation  as  Auric  Sulphide. 

.  Hydrogen  sulphide  gas  is  transmitted  in  excess  through  the 
dilute  solution  containing  some  free  acid  ;  the  precipitate  formed 
is  speedily  filtered  off,  without  heating,  washed,  dried,  and  ignited 
in  a  porcelain  crucible.  For  the  properties  of  the  precipitate,  see 
§  88.  'No  sources  of  error. 


2.  PLATINUM. 

a.  Solution. 

Metallic  platinum,  and  the  compounds  of  platinum  which  are 
insoluble  in  water,  are  dissolved  by  digestion,  at  a  gentle  heat,  with 
nitrohydrochloric  acid. 

b.  Determination. 

Platinum  is  invariably  weighed  in  the  metallic  state,  to  which 
condition  its  compounds  are  brought,  either  by  precipitation  as 
ammonium  platinic  chloride,  potassium  platinic  chloride,  or  pla- 
tiiric  sulphide,  or  by  ignition,  or  by  precipitation  with  reducing 
agents.  All  compounds  of  platinum,  without  exception,  may,  in. 
most  cases,  be  converted  into  platinum  by  either  of  these  methods. 
"Which  is  the  most  advantageous  process  to  be  pursued  in  special 
instances,  depends  entirely  upon  the  circumstances.  The  reduc- 
tion to  the  metallic  state,  by  simple  ignition  is  preferable  to  the 
other  methods,  in  all  cases  where  admissible.  The  precipitation  as 
platinic  sulphide  is  resorted  to  exclusively  to  effect  the  separation 
of  platinum  from  other  metals. 

Determination  as  Metallic  Platinum. 
a.  By  Precipitation  as  Ammonium  Platinic  Chloride. 
The  solution  must  be  concentrated  if  necessary  by  evaporation 

*Ber.  der  dcutsch.  chem.  Gesellsch.,  1869,  295. 


394  DETERMINATION.  [§  124. 

on  a  water-bath.  Mix  in  a  beaker  with  ammonia  until  the  excess 
of  acid  (that  is,  supposing  an  excess  of  acid  to  be  present)  is  nearly 
saturated ;  add  ammonium  chloride  in  excess  and  mix  the  fluid 
with  a  pretty  large  quantity  of  strong  alcohol.  Cover  the  beaker 
now  with  a  glass  plate  and  let  it  stand  for  twenty-four  hours,  after 
which  collect  in  a  weighed  asbestos  filter,  or  in  an  unweighed 
paper  filter,  wash  the  precipitate  with  alcohol  of  about  80  per 
cent.,  till  the  substances  to  be  separated  are  Removed,  dry  carefully, 
ignite  according  to  §  99,  2,  and  weigh.  In  the  case  of  large  quan- 
tities the  final  ignition  is  advantageously  conducted  in  a  stream  of 
hydrogen  (§  108,  Fig.  83),  in  order  to  be  quite  sure  of  effecting 
complete  decomposition.  For  the  properties  of  the  precipitate  and 
residue,  see  §  89.  The  results  are  satisfactory,  though  generally  a 
little  too  low,  as  the  ammonium  platinic  chloride  is  not  altogether 
insoluble  in  alcohol  of  the  above  strength  (Expt.  No.  16),  and  as 
the  fumes  of  ammonium  chloride  are  liable  to  carry  away  traces  of 
the  yet  uiidecomposed  double  chloride,  if  the  application  of  heat  is 
not  conducted  with  the  greatest  care. 

If  the  precipitated  ammonium  platinic  chloride  were  weighed 
in  that  form,  the  results  would  be  inaccurate,  since,  as  I  have  con- 
vinced myself  by  direct  experiments,  it  is  impossible  to  completely 
free  the  double  chloride,  by  washing  with  alcohol,  from  all  traces 
of  the  ammonium  chloride  thrown  down  with  it,  without  dissolving 
at  the  same  time  a  notable  portion  of  the  double  chloride.  As  a 
general  rule,  the  results  obtained  by  weighing  the  ammonium  pla- 
tinic chloride  in  that  form  are  one  or  two  per  cent,  too  high. 

~b.  By  Precipitation  as  Potassium  Platinic  Chloride. 

Mix  the  solution,  in  a  beaker,  with  potassa,  until  the  greater 
part  of  the  excess  of  acid  (if  there  be  any)  is  neutralized;  add 
potassium  chloride  slightly  in  excess,  and  finally  a  pretty  large 
quantity  of  strong  alcohol ;  should  your  solution  of  platinum  be 
very  dilute,  you  must  concentrate  it  previously  to  the  addition  of 
the  alcohol.  After  twenty- four  hours  collect  the  precipitate 
weighed  asbestos  filter,  wash  with  alcohol  of  80  per  cent. ,  dry 
thoroughly  at  100°,  convert  into  platinum  according  to  §  97,  4,  a, 
and  weigh.  For  the  properties  of  the  precipitate  and  residue, 
see  §  89. 

The  results  are  more  accurate  than  those  obtained  by  method  #, 
since,  on  the  one  hand,  the  potassium  platinic  chloride  is  more 


§  125.]  ANTIMONY.  395 

insoluble  in  alcohol  than  the  corresponding  ammonium  salt ;  and, 
on  the  other  hand,  loss  of  substance  is  less  likely  to  occur  during 
ignition.  To  weigh  the  potassium  platinic  chloride  in  that  form 
would  not  be  practicable,  as  it  is  impossible  to  remove,  by  washing 
with  alcohol,  all  traces  of  the  potassium  chloride  thrown  down 
with  it,  without,  at  the  same  time,  dissolving  a  portion  of  the 
double  chloride. 

c.  By  Precipitation  as  Plcutinic  Sulphide. 

Precipitate  the  solution  with  hydrogen- sulphide  water  or  gas, 
according  to  circumstances,  heat  the  mixture  to  incipient  ebulli- 
tion, filter,  wash  the  precipitate,  dry,  and  ignite  according  to  §  52. 
For  the  properties  of  the  precipitate  and  residue,  see  §  89.  The 
results  are  accurate. 

d.  By  Ignition. 

Same  process  as  for  gold,  §  123.  For  the  properties  of  the 
residue,  see  §  89.  The  results  are  most  accurate. 

e.  By  Precipitation  with' Reducing  Agents. 

Various  reducing  agents  may  be  employed  to  precipitate  plati- 
num from  its  solutions  in  the  metallic  state.  The  reduction  is 
very  promptly  effected  by  ferrous  sulphate  and  potassa  or  soda 
(the  protosesquioxide  of  iron  being  removed  by  subsequent  addi- 
tion of  hydrochloric  acid,  HEMPEL),  or  by  pure  zinc  or  magnesium 
(the  excess  of  which  is  removed  by  hydrochloric  acid) ;  somewhat 
more  slowly,  and  only  with  application  of  heat,  by  alkali  formates. 
Mercurous  nitrate  also  precipitates  the  whole  of  the  platinum  from 
solution  of  platinic  chloride ;  upon  igniting  the  brown  precipitate 
obtained,  fumes  of  inercurous  chloride  escape,  and  metallic  plati- 
num remains. 

§125. 

3.  ANTIMONY. 

a.  Solution. 

Antirnonous  oxide,  and  the  compounds  of  antimony  which  are 
insoluble  in  water,  or  are  decomposed  by  that  agent,  are  dissolved 
in  more  or  less  concentrated  hydrochloric  acid.  Metallic  antimony 
is  dissolved  best  in  nitrohydrochloric  acid.  The  ebullition  of  a 
hydrochloric  acid  solution  of  antimonous  chloride  is  attended  with 
volatilization  of  traces  of  the  latter ;  the  concentration  of  a  solution 


396  DETEEMINATIONo  [§  125. 

of  the  kind  by  evaporation  involves  accordingly  loss  of  substance. 
Solutions  so  highly  dilute  as  to  necessitate  a  recourse  to  evapora- 
tion must  therefore  previously  be  supersaturated  with  potassa. 
Solutions  of  antimonous  chloride,  which  it  is  intended  to  dilute 
with  water,  nrast  previously  be  mixed  with  tartaric  acid,  to  prevent 
the  separation  of  basic  salt.  In  diluting  an  acid  solution  of  anti- 
monic  acid  in  hydrochloric  acid,  the  water  must  not  be  added 
gradually  and  in  small  quantities  at  a  time,  which  would  make  the 
fluid  turbid,  but  in  sufficient  quantity  at  once,  which  will  leave  the 
fluid  clear. 

1).  Determination. 

Antimony  may  be  weighed  as  antimonous-  sulphide  or  anti- 
mony tetroxide:  in  separations  it  is  sometimes  weighed  as  metallic 
antimony  j  or  it  is  estimated  volumetrically.  Antimony  oxides 
and  the  salts,  with  readily  volatile  or  decomposable  oxygen  acids, 
may  be  converted  into  antimony  tetroxide  by  simple  ignition. 

Antimony  in  solution  is  almost  invariably  first  precipitated  as 
sulphide,  which  is  then,  with  the  view  of  estimation,  converted 
into  anhydrous  sulphide  or  determined  volumetrically. 

Of  the  volumetric  methods  the  first  two  are  applicable  only 
when  the  antimony  is  present  as  a  pure  tetroxide  or  as  anti- 
monous chloride. 

1.  Precipitation  as  Antimonous  Sulphide. 

Add  to  the  antimony  solution  hydrochloric  acid,  if  not  already 
present,  then  tartaric  acid,  and  dilute  with  water,  if  necessary. 
Introduce  the  clear  fluid  into  a  flask,  closed  with  a  doubly  perfo- 
rated cork;  through  one  of  the  perforations  passes  a  tube,  bent 
outside  at  a  right  angle,  which  nearly  extends  to  the  bottom  of  the 
flask ;  through  the  other  perforation  passes  another  tube,  bent  out- 
side twice  at  right  angles,  which  reaches  only  a  short  way  into  the 
flask ;  the  outer  end  of  this  tube  dips  slightly  under  water.  Con- 
duct through  the  first  tube  hydrogen  sulphide  gas,  until  it  pre- 
dominates strongly ;  put  the  flask  in  a  moderately  warm  place,  and 
after  some  time  conduct  carbon  dioxide  into  the  fluid,  until  the 
excess  of  the  other  gas  is  almost  completely  removed.  If  there  is 
no  reason  against  it,  from  the  presence  of  a  large  quantity  of 
hydrochloric  acid,  or  from  the  presence  of  nitric  acid,  it  is  well  to 
heat  the  solution  during  the  passing  of  the  gas,  finally  even  boiling. 


§  125.]  ANTIMONY.  397 

• 

The  precipitate  is  then  denser,  and  may  be  very  easily  washed. 

(SllARPLES*). 

If  the  amount  of  the  precipitate  is  at  all  considerable,  filter 
without  intermission  through  a  weighed  filter,  wash  rapidly  and 
thoroughly  with  water  mixed  with  a  few  drops  of  hydrogen  sul- 
phide water,  dry  at  100°,  and  weigh.  The  precipitate  so  weighed 
.always  retains  some  water,  and  may,  besides,  contain  free  sulphur; 
in  fact,  it  always  contains  the  latter  in  cases  where  the  antimony 
solution,  besides  antimonous  salts,  contains  antimonic  acid  or 
antimony  pentachloride,  since  the  precipitation  under  these  cir- 
cumstances is  preceded  by  a  reduction  of  antimonic  to  antimo- 
nous compounds,  accompanied  by  separation  of  sulphur  (H.  ROSE). 
A  further  examination  of  the  precipitate  is  accordingly  indis- 
pensable. To  this  end  treat  a  sample  of  the  weighed  precipitate 
with  strong  hydrochloric  acid.  If 

a.  The  sample  dissolves  to  a  clear  fluid,  this  is  a  proof  that  the 
precipitate  only  contains  Sb,S3 ;  but  if 

I).  Sulphur  separates,  this  shows  that  free  sulphur  is  present. 

In  case  a  (in  order  to  remove  the  water  retained  at  100°)  the 
greater  portion  of  the  dried  precipitate  is  weighed  in  a  porcelain 
boat,  which  is  then  inserted  into  a  glass  tube,  about  2  decimetres 
long ;  a  slow  current  of  dry  carbon  dioxide  is  transmitted  through 
the  latter,  and  the  boat  cautiously  heated  by  means  of  a  lamp, 
moved  to  and  fro  under  it,  until  the  orange  precipitate  becomes 
black.  The  precipitate  is  then  allowed  to  cool  in  the  current  of 
•carbon  dioxide,  and  weighed ;  from  the  amount  found,  the  total 
quantity  of  anhydrous  antimonous  sulphide  contained  in  the  entire 
precipitate  is  ascertained  by  a  simple  calculation.  The  results  are 
accurate.  Expt.  No.  75  gave  99 -24  instead  of  100.  But  if  thu 
precipitate  is  simply  dried  at  100°,  the  results  are  about  2  per 
cent,  too  high — see  the  same  experiment.  For  the  properties  of 
the  precipitate,  see  §  90. 

In  case  ~b,  the  precipitate  is  subjected  to  the  same  treatment  as 
in  a,  with  this  difference  only,  that  the  contents  of  the  boat  are 
heated  much  more  intensely,  and  the  process  is  continued  until  no 
more  sulphur  is  expelled.  This  removes  the  whole  of  the  admixed 
sulphur ;  the  residue  consists  of  pure  antimonous  sulphide.  It 
must  be  completely  soluble  in  fuming  hydrochloric  acid  on  heating. 

*  Zeilschr.  /.  analyt.  Chem.,  x,  343. 


398  DETERMINATION.  [§  125, 

If  the  amount  of  the  precipitate  is  small,  collect  it  in  a  weighed 
asbestos  filtering  tube,  dry  in  a  slow  current  of  carbon  dioxide  at  a 
gentle  heat,  heat  finally  rather  more  strongly  till  the  sulphide  has 
turned  black  and  any  free  sulphur  present  has  volatilized,  allow  to 
cool,  replace  the  gas  in  the  tube  by  air,  and  weigh.  Results  quite 
satisfactory.45' 

BUNSEN  recommends  converting  the  antimonous  sulphide  into 
antimony  tetroxide. 

For  the  method  of  estimating  the  antimony  in  the  sulphide 
volumetrically  and  indirectly,  see  3,  c. 

2.  Determination  as  Antimony  Tetroxide. 

a.  In  the  case  of  antimonous  oxide  or  a  compound  of  the  same 
with  an  easy  volatile  or  decomposable  oxygen  acid,  evaporate 
carefully  with  nitric  acid  and  ignite  finally  for  some  time  till  the 
weight  is  constant.  The  experiment  may  be  safely  made  in  a 
platinum  crucible.  With  antimonic  acid,  the  evaporation  with 
nitric  acid  is  unnecessary. 

5.  If  antimonous  sulphide  is  to  be  converted  into  anntimony 
tetroxide,  one  of  the  two  following  methods  given  by  BUNSEN  f  is 
employed : 

a.  Moisten  the  dry  antimony  sulphide  with  a  few  drops  of 
nitric  acid  of  1*42  sp.  gr.,  then  treat,  in  a  weighed  porcelain 
crucible  with  concave  lid,  with  8-10  times  the  quantity  of 
fuming  nitric  acid,J  and  let  the  acid  gradually  evaporate  on  the 
water-bath.  The  sulphur  separates  at  first  as  a  fine  powder,  which, 
however,  is  readily  and  completely  oxidized  during  the  process  of 
evaporation.  The  white  residual  mass  in  the  crucible  consists  of 
antimonic  acid  and  sulphuric  acid,  and  may  by  ignition  be  con- 
verted without  loss  -into  antimony  tetroxide.  If  the  antimony 
sulphide  contains  a  large  excess  of  free  sulphur,  this  must  be 
removed  by  washing  with  carbon  disulphide. 

/?.  Mix  the  antimony  sulphide  with  30-50  times  its  quantity 


*Zeitsckr.f.  analyt.  Chem.,  vm,  155. 

f  Annal.  d.  Chem.  u.  Pharm.,  cvi,  3. 

\  Nitric  acid  of  l'42sp.  gr.  is  not  suitable  for  this  purpose,  as  its  boiling-point 
is  almost  10°  above  the  fusing-point  of  sulphur,  whereas  fuming  nitric  acid  boils 
at  86°,  consequently  below  the  fusing-point  of  sulphur.  With  nitric  acid  of  1  -42 
sp.  gr.,  therefore,  the  separated  sulphur  fuses  and  forms  drops, which  obstinately 
resist  oxidation. 


§  125.]  ANTIMONY.  399 

of  pure  mercuric  oxide,*  and  heat  the  mixture  gradually  in  an 
open  porcelain  crucible.  As  soon  as  oxidation  begins,  which  may 
be  known  by  the  sudden  evolution  of  gray  mercurial  fumes, 
moderate  the  heat.  When  the  evolution  of  mercurial  fumes 
diminishes  raise  the  temperature  again,  always  taking  care,  how- 
ever, that  no  reducing  gases  come  in  contact  with  the  contents  of 
the  crucible.  Remove  the  last  traces  of  mercuric  oxide  over  the 
blast  gas-lamp,  then  weigh  the  residual  fine  white  powder  of  anti- 
mony tetroxide.  As  mercuric  oxide  generally  leaves  a  trifling  fixed 
residue  upon  ignition,  the  amount  of  this  should  be  determined 
once  for  all,  the  mercuric  oxide  added  approximately  weighed,  and 
the  corresponding  amount  of  fixed  residue  deducted  from  the 
antimony  tetroxide.  The  volatilization  of  the  oxide  of  mercury 
proceeds  much  more  rapidly  when  effected  in  a  platinum  crucible 
instead  of  a  porcelain  one.  But,  if  a  platinum  crucible  is  employed, 
it  must  be  effectively  protected  from  the  action  of  antimony  upon 
it,  by  a  good  lining  of  mercuric  oxide,  f  If  the  antimony  sulphide 
contains  free  sulphur,  this  must  first  be  removed  by  washing 
with  carbon  disulphide  before  the  oxidation  can  be  proceeded 
with,  since  otherwise  a  slight  deflagration  is  avoidable. 

According  to  later  experiments  made  by  B  ONSEN,  \  it  is  some- 
what difficult  to  obtain  good  results  by  this  method,  because  a 
temperature  a  little  above  that  required  to  reduce  Sb,O6  to  Sb,O4 
will  reduce  the  latter  SbaO8.  Ignition  over  a  blast-lamp  in  a  very 
large  covered  platinum  or  rather  large  open  porcelain  crucible, 
keeping  only  the  bottom  at  a  full  red  heat,  is  recommended  as  a 

*  Prepared  by  precipitation  from  mercuric  chloride  by  excess  of  soda  solution 
and  thorough  washing. 

f  This  is  effected  best,  according  to  BUNSEN,  in  the  following  way:  Soften  the 
sealed  end  of  a  common  test-tube  before  the  glass-blower's  lamp  ;  place  the 
softened  end  in  the  centre  of  the  platinum  crucible,  and  blow  into  it,  which  will 
cause  it  to  expand  and  assume  the  exact  form  of  the  interior  of  the  crucible. 
Crack  off  the  bottom  of  the  little  flask  so  formed,  and  smooth  the  sharp  edges 
cautiously  by  fusion.  A  glass  is  thus  obtained,  open  at  both  ends,  which  exactly 
fits  the  crucible.  To  effect  the  lining  by  means  of  this  instrument,  fill  the  crucible 
loosely  with  mercuric  oxide  up  to  the  brim,  then  force  the  glass  gradually  and 
slowly  down  to  the  bottom  of  the  crucible,  occasionally  shaking  out  the  oxide 
of  mercury  from  the  interior  of  the  glass.  The  inside  of  the  crucible  is  thus 
covered  with  a  layer  of  oxide  of  mercury  \ — 1  line  thick,  which,  after  the  removal 
of  the  glass,  adheres  with  sufficient  firmness,  even  upon  ignition. 

\  Zeitschr.  f.  anal.  Chem.,  xvm,  268. 


400  DETERMINATION.  [§  125. 

method  bj  which  it  is  possible  to  drive  off  just  one  atom  O  from 

6b.O.. 

3.    Volumetric  Methods. 

a.  Oxidation  of  Antimonous  Oxide  to  Antimonic  Oxide  by 
Iodine  (Fn.  MOHR  *). 

The  oxidation  is  effected  in  alkaline  solution  and  proceeds  as 
follows :  Sb2O3  +  41  +  4NaOH  =  Sb9OB  +  4NaI  +  2H3O.  The 
method  gives  results  which  are  serviceable  only  under  very  defi- 
nite conditions,  because  the  antimonous  oxide  has  not  always  an 
equal  tendency  to  change  to  antimonic  oxide  in  alkaline  solution, 
but  this  tendency  is  greater  in  the  presence  of  much  alkali  car- 
bonate than  when  little  is  present,  and  becomes  constant  only  with 
a  certain  excess  of  carbonate.  According  to  my  investigations  it 
is  best  to  proceed  thus : 

Dissolve  a  quantity  of  the  compound  containing  about  O'l 
grm.  of  antimonous  oxide  in  about  10  c.  c.  of  an  aqueous 
solution  of  tartaric  acid,  then  add  sufficient  sodium-carbonate 
solution  to  about  neutralize  the  liquid.  Add  now  20  c.  c.  of 
a  cold,  saturated  solution  of  sodium  bicarbonate  and  (the  liquid 
remaining  clear)  some  starch  paste,  then  titrate  with  iodine 
(§  146)  until  the  fluid  just  remains  blue  on  being  stirred.  The 
fact  that  the  blue  color  soon  disappears,  however,  must  not  induce 
the  operator  to  add  more  iodine.  4  eq.  of  iodine  corresponds  to 
1  eq.  of  antimonous  oxide. 

The  results  so  obtained  are  entirely  satisfactory  (Expt.  No.  76). 
I  cannot  recommend  the  use  of  sodium  carbonate  as  employed 
by  FR.  MOHR  in  his  experiments,  because  it  itself  has  the  prop- 
erty of  fixing  a  considerable  quantity  of  iodine,  varying,  moreover, 
according  to  the  quantity  of  water  used  (Expt.  No.  77),  whereas 
this  is  not  the  case  with  sodium  bicarbonate  (Expt.  No.  78). 
Compare  also  §  127,  5,  #,  1,  and  Expt.  No.  79. 

a.  Conversion  of  Antimonous  Chloride  to  Antimonic  Chlo- 
ride by  Hydrochloric  Acid  and  Potassium  Chromate  or  Perman- 
ganate. 

F.  KES&LER'sf  first  description  of  this  method  was  so  wanting 
in  precision,  that  it  could  not  be  depended  upon.  However,  he 
has  sincej  determined  most  accurately  the  conditions  under  which 
antimony  in  acid  solution  may  be  satisfactorily  titrated  either  with 
potassium  chromate  (the  excess  of  the  standard  solution  being 
determined  with  ferrous  sulphate)  or  with  potassium  permanganate. 

*  Lehrbuch  der  Titrir method e,  3.  Aufl.,  276.         \Pogg   AnnaL,  xcv,  204. 
%lb.,  cxvin,  17,  and  Zeitschr  f.  analyt.  Chem.,  n   3*3. 


§  124.]  ANTIMONY.  401 

I.   Titration  with  Potassium  Dichr ornate. 

1.  REQUISITES. 

a.  Standard  Solution  of  Arsenous  Acid.  Dissolve  exactly 
5  grm.  pure  arsenous  oxide  by  the  aid  of  some  soda  solution,  add 
hydrochloric  acid  till  slightly  acid,  then  100  c.c.  more  of  hydro- 
chloric acid  of  1-12  sp.  gr.,  and  dilute  to  1000  c.c.  Each  c.c.  con- 
tains 0-005  grm.  arsenous  oxide  and  corresponds  to  0*007293 
antimonous  oxide. 

ft.  Solution  of  Potassium  Dichromate.  Dissolve  about  2*5  grm. 
to  1  litre. 

y.  Solution  of  Ferrous  Sulphate.  Dissolve  about  1*1  grm.  iron 
wire  in  20  c.c.  dilute  sulphuric  acid  (1  to  4),  filter,  and  dilute  to 
1  litre. 

#.  Solution  of  Potassium  Ferricyanide.  Should  be  tolerably 
dilute  and  freshly  prepared. 

2.  DETERMINATION  OF  THE  SOLUTIONS. 

a.  .Relation  between  the  Solution  of  Chr ornate  and  the  Solution 
of  Ferrous  Sulphate.  Run  into  a  beaker  10  c.c.  of  the  chromate 
solution  from  the  burette,  add  5  c.c.  of  hydrochloric  acid  and  50 
c.c.  water,  and  then  add  iron  solution  from  a  burette  till  the  fluid 
is  green.  Continue  adding  the  iron  solution,  a  c.c.  at  a  time,  test- 
ting  after  each  addition  whether  a  drop  of  the  fluid,  when  brought 
in  contact  with  a  drop  of  the  potassium  f erricyanide,  on  a  porcelain 
plate,  manifests  a  distinct  reaction  for  ferrous  iron.  As  soon  as 
this  point  is  attained,  add  0 '5  c.c.  of  chromate  solution  and  then  iron 
solution  two  drops  at  a  time,  till  the  blue  reaction  just  occurs.- 
Now  read  off  both  burettes,  and  calculate  how  much  chromate 
solution  corresponds  to  10  c.c.  of  iron  solution.  This  experiment 
is  to  be  repeated  before  every  fresh  series  of  analyses,  as  the  iron 
solution  gradually  oxidizes. 

ft.  Delation  between  the  Chromate  Solution  and  the  Solution  of 
Arsenous  Acid.  Transfer  10  c.c.  of  the  arsenous  solution  to  a 
beaker,  add  20  c.c.  hydrochloric  acid  of  1/2  sp.  gr.,  and  80 — 100 
c.c.*  water,  run  in  chromate  solution  till  the  yellow  color  of  the 
tin  id  shows  an  excess,  wait  a  few  minutes,  add  excess  of  iron  solu- 
tion, then  again  0-5  chromate  solution,  and  finally  again  iron  solu- 
tion till  the  end-reaction  appears  (see  above).  Deduct  from  the 

*  The  water  must  be  measured,  for  the  action  of  chromic  acid  on  arsenous 
acid  (and  also  on  antimonous  chloride)  is  normal  only  if  the  fluid  contains  at 
least  one  sixth  of  its  volume  of  hydrochloric  acid  of  1'12  sp.  gr. 


402  DETERMINATION.  [§  125. 

total  quantity  of  chromate  solution  employed,  the  amount  corre- 
sponding to  the  iron  used,  and  from  the  datum  thus  afforded  calcu- 
late how  much  antimony  corresponds  to  100  c.c.  of  chromate  solu- 
tion ;  in  other  words,  how  much  antimony  is  converted  by  the 
quantity  of  chromate  mentioned  from  SbCl3  into  SbCl5. 

3.  THE  ACTUAL  ANALYSIS. 

In  the  absence  of  organic  matter,  heavy  metallic  oxides,  and  other 
bodies  which  are  detrimental  to  the  reaction,  dissolve  the  antimo- 
nous  compound  at  once  in  hydrochloric  acid.  The  solution  should 
contain  not  less  than  -J-  of  its  volume  of  hydrochloric  acid  of  T12< 
sp.  gr.  It  is  not  advisable,  on  the  other  hand,  that  it  should  con- 
tain more  than  ^,  otherwise  the  end-reaction  with  potassium  fern- 
cyanide  is  slower  in  making  its  appearance  and  loses  its  nicety. 
Tartaric  acid  cannot  be  employed  as  a  solvent,  since  it  interferes 
with  the  action  of  chromic  acid  on  ferrous  salts.  Now  proceed  as 
directed  in  2.  If  the  direct  determination  of  antimony  in  the 
hydrochloric  acid  solution  is  not  practicable,  precipitate  it  with 
hydrogen  sulphide.  Wash  the  precipitate,  transfer  it,  together 
with  the  filter,  to  a  small  flask  ;  treat  it  with  a  sufficiency  of  hydro- 
chloric acid,  dissolve  by  digestion  on  the  water-bath,  add  a  suffi- 
cient quantity  of  a  nearly  saturated  solution  of  mercuric  chloride- 
in  hydrochloric  acid  of  1*12  sp.  gr.  to  remove  the  hydrogen  sul- 
phide, and  then  proceed  as  directed. 

II.  Titration  with  Potassium  Permanganate. 

Here  also  the  fluid  must  contain  at  least  -J-  of  its  volume  of 
hydrochloric  acid  of  1.12  sp.  gr.  The  permanganate  solution,., 
which  may  contain  about  1*5  gnu.  of  the  crystallized  salt  in  a  litre, 
is  added  to  permanent  reddening.  The  end-reaction  is  exact,  and 
the  conversion  of  antiinonous  to  antimonic  chloride  goes  on  uni- 
formly, although  the  degree  of  dilution  may  vary,  provided  the 
above  relation  between  hydrochloric  acid  and  water  is  kept  up. 
It  is  not  well  that  the  hydrochloric  acid  should  exceed  -J  of  the 
volume  of  the  fluid,  as  in  that  case  the  end-reaction  would  be  too 
transitory.  Tartaric  acid,  at  least  in  the  proportion  to  antimony  in 
which  it  exists  in  tartar  emetic,  does  not  interfere  with  the  reac- 
tion. Hence  the  permanganate  may  be  standardized  by  the  aid  of 
solution  of  tartar  emetic  of  known  strength. 

If  you  have  to  analyze  antimonous  sulphide,  proceed  as  directed 
I.  3 ;  make  the  fluid  mixed  with  mercuric  chloride  up  to  a  certain 


§  126.]      TIN  IN   STANNOUS   AND  STANNIC    COMPOUNDS.          403 

volume,  allow  to  settle,  and  use  a  measured  portion  of  the  perfectly 
c!<  ,ir  solution  for  the  experiment. 

My  own  experiments*  have  shown  that  KESSLER'S  methods  are 
also  suitable  for  the  estimation  of  very  small  quantities  of  anti- 
mony. 

o.  Volumetric  Estimation  ~by  determining  the  Hydrogen  Sul- 
phide yielded  by  the  Sulphide  (R.  SCHNEIDER  f). 

Both  antimonous  and  antimonic  sulphides  yield  under  the 
fcctiou  of  boiling  hydrochloric  acid  3  mol.  hydrogen  sulphide  for 
every  2  atoms  of  antimony.  Hence,  if  the  amount  of  the  gas 
evolved  under  such  circumstances  is  estimated,  the  amount  of  anti- 
mony is  known. 

For  decomposing  the  sulphide  and  absorbing  the  gas,  the  same 
apparatus  serves  as  BUNSEN  employs  for  his  iodimetric  analyses 
(§  130).  The  size  of  the  boiling-flask  should  depend  on  the  quan- 
tity of  sulphide ;  for  quantities  up  to  0*4  grm.  SbaS3 ,  a  flask  of  100 
c.c.  is  large  enough;  for  0'4 — 1  grm.,  use  a  200  c.c.  flask.  The 
body  of  the  flask  should  be  spherical,  the  neck  rather  narrow, 
•  long,  and  cylindrical.  If  the  sulphide  of  antimony  is  on  a  filter, 
put  both  together  into  the  flask.  The  hydrochloric  acid  should 
not  be  too  concentrated. 

The  determination  of  the  hydrogen  sulphide  is  best  conducted 
according  to  the  method  given  in  §  148,  h.  The- results  obtained 
by  SCHNEIDER  are  satisfactory.  If  the  precipitate  contains  anti- 
monious  chloride,  the  results  are  of  course  false,  and  this  would 
actually  be  the  case  if  on  precipitation  with  hydrogen  sulphide  the 
addition  of  the  tartaric  acid  were  omitted. 

§126. 
4.  TIN  IN  STANNOUS  COMPOUNDS,  and  5.  TIN  IN  STANNIC  COMPOUNDS. 

a.  Solution. 

In  dissolving  compounds  of  tin  soluble  in  water,  a  little  hydro- 
chloric acid  is  added  to  insure  a  clear  solution.  Nearly  all  the 
compounds  of  tin  insoluble  in  water  dissolve  in  hydrochloric  acid, 
or  in  aqua  regia.  The  hydrate  of  metastannic  acid  may  be  dissolved 
by  boiling  with  hydrochloric  acid,  decanting  the  fluid,  and  treating 
the  residue  with  a  large  proportion  of  water.  Ignited  stannic  oxide, 

*  ZeitscJir.  f.  analyt.  Chem.,  vm,  155.  f  Pogg.  Annal.,  ex,  634. 


404  DETERMINATION.  [§  12 

and  stannic  compounds  insoluble  in  acids,  are  prepared  for  solution 
in  hydrochloric  acid,  by  reducing  them  to  the  state  of  a  fine  pow- 
der, and  fusing  in  a  silver  crucible  with  potassium  or  sodium 
hydroxide,  in  excess.  Metallic  tin  is  dissolved  best  in  aqua  regia ; 
the  solution  frequently  contains  metastannic  chloride  mixed  with 
the  stannic  chloride  (Tn.  SCHEERER*).  It  is  generally  determined, 
liowever,  by  converting  it  into  stannic  oxide,  without  previous 
solution.  Acid  solutions  of  stannic  salts,  which  contain  hydrochlo- 
ric acid,  or  a  chloride,  cannot  be  concentrated  by  evaporation,  not 
even  after  addition  of  nitric  acid  or  sulphuric  acid,  without  volatili- 
zation of  stannic  chloride  taking  place. 

b.  Determination. 

Tin  is  weighed  in  the  form  of  stannic  oxide,  into  which  it  is 
converted,  either  by  the  agency  of  nitric  acid,  or  by  precipitation 
as  stannic  (or  metastannic)  acid,  or  by  precipitation  as  sulphide.  A 
great  many  volumetric  methods  of  estimating  tin  have  been  pro- 
posed. They  all  depend  on  obtaining  the  tin  in  solution  in  the 
condition  of  stannous  chloride,  and  converting  this  into  stannic 
chloride  either  in  alkaline  or  acid  solution.  A  few  only  yield  satis- 
factory results. 

We  may  convert  into 

STANNIC  OXIDE: 

a.  By  Treatment  with  Nitric  Acid.    Metallic  tin,  and  those 
compounds  of  tin  which  contain  no  fixed  acid,  provided  no  com- 
pounds of  chlorine  be  present. 

b.  By  Precipitation  as  Stannic  (or  Metastannic]  Acid.    All  tin 
salts  of  volatile  acids,  provided  no  non- volatile  organic  substances 
nor  ferric  salts  be  present. 

c.  By  Precipitation  as  Sulphide.    All  compounds  of  tin  with- 
out exception. 

In  methods  a  and  c,  it  is  quite  indifferent  whether  the  tin  is 
present  as  a  stannous  or  a  stannic  compound.  The  method  b 
requires  the  tin  to  be  present  as  a  stannic  salt.  The  volumetric 
methods  may  be  employed  in  all  cases  ;  but  the  estimation  is  simple 
and  direct  only  where  the  tin  is  in  solution  as  stannous  chloride 
and  free  from  other  oxidizable  bodies,  or  can  readily  be  brought 
Into  this  state.  For  the  methods  of  determining  stannous  and 
stannic  tin  in  presence  of  each  other,  I  refer  to  Section  Y. 

*Joum.f.  prakt.  Chem.  N.  F.,  in,  472. 


§   126.]      TIN   IN   STANNOUS   AND   STANNIC    COMPOUNDS.       405 

1.  Determination  of  Tin  as  Stannic  Oxide. 

a.  By  Treatment  with  Nitric  Acid. 

This  method  is  resorted  to  principally  to  convert  the  metallic 
tin  into  stannic  oxide.  For  this  purpose  the  finely-divided  metal 
is  put  into  a  capacious  flask,  and  moderately  concentrated  pure 
nitric  acid  (about  1'3  sp.  gr.)  gradually  poured  over  it ;  the  flask  is 
overed  with  a  watch  glass.  When  the  first  tumultuous  action  of 
the  acid  has  somewhat  abated,  a  gentle  heat  is  applied  until  the 
metastannic  acid  formed  appears  of  a  pure  white  color,  and  further 
action  of  the  acid  is  no  longer  perceptible.  The  contents  of  the 
flask  are  then  transferred  to  a  porcelain  dish  and  evaporated  on  a 
water-bath  nearly  to  dry  ness,  water  is  then  added,  and  the  precipi- 
tate is  collected  on  a  filter,  washed,  till  the  washings  scarcely  red- 
den litmus  paper,  dried,  ignited,  and  weighed.  The  ignition  is 
effected  best  in  a  small  porcelain  crucible,  according  to  §  53  ;  still 
a  platinum  crucible  may  also  be  used.  A  simple  red  heat  is  not 
sufficient  to  drive  off  all  the  water  ;  the  ignition  must  therefore  be 
finished  over  a  gas  blowpipe.  Compounds  of  tin  which  contain  no 
fixed  substances  may  be  converted  into  stannic  oxide  by  treating 
them  in  a  porcelain  crucible  with  nitric  acid,  evaporating  to  dry- 
ness,  and  igniting  the  residue.  If  sulphuric  acid  be  present,  the 
expulsion  of  that  acid  may  be  promoted,  in  the  last  stages  of  the 
process,  by  ammonium  carbonate,  as  in  the  case  of  acid  potassium 
sulphate  (§  97) ;  here  also  the  heat  must  be  increased  as  much  as 
possible  at  the  end.  For  the  properties  of  the  residue,  see  §  91. 
There  are  no  inherent  sources  of  error. 

l>.  By  Precipitation  as  Stannic  (or  Metastannic)  Acid. 

The  application  of  this  method  presupposes  the  whole  of  the 
tin  to  be  present  in  the  state  of  stannic  salts.  Therefore,  if  a  solu- 
tion contains  stannous  salts,  either  mix  with  chlorine  water,  or  con- 
duct chlorine  gas  into  it,  or  heat  gently  with  chlorate  of  potassa, 
until  the  conversion  of  the  stannous  into  stannic  salts  is  effected. 
When  this  has  been  done,  add  ammonia  until  a  permanent  precipitate 
just  begins  to  form,  and  then  hydrochloric  acid,  drop  by  drop,  until 
this  precipitate  is  completely  redissolved ;  by  this  means  a  large 
excess  of  hydrochloric  acid  in  the  solution  will  be  avoided.  Add 
to  the  fluid  so  prepared  a  concentrated  solution  of  ammonium 
nitrate  (or  sodium  sulphate),  and  apply  heat  for  some  time,  where- 
upon the  whole  of  the  tin  will  precipitate  as  stannic  acid.  Decant 
three  times  on  to  a  filter,  then  collect  the  precipitate  on  the  latter, 


406  DETERMINATION.  [§  126. 

wash  thoroughly,  dry,  and  ignite.  To  make  quite  sure  that  the 
whole  of  the  tin  has  separated,  you  need  simply,  before  proceeding 
to  filter,  add  a  few  drops  of  the  clear  supernatant  fluid  to  a  hot 
solution  of  ammonium  nitrate,  or  sodium  sulphate,  when  the  for- 
mation or  non-formation  of  a  precipitate  will  at  once  decide  the 
question.  The  tin  is  also  precipitated  from  metastannic  chloride 
by  the  above  reagents. 

This  method,  which  we  owe  to  J.  LOWENTHAL,  has  been  repeat- 
edly tested  by  him  in  iny  own  laboratory,*  is  easy  and  convenient, 
and  gives  very  accurate  results.  The  decomposition  is  expressed 
by  the  equation,  SnCl4  +  4Na2SO4  +  3H2O  =  H2SnO3  +  4NaCl 
-f- 4NaHSO4,  or  in  precipitating  with  ammonium  nitrate:  SnCl4 
+  4NH4NO3  +  3H2O  =  H2SnO3  +  4KB4C1  +  4HNO3. 

Tin  may  also,  according  to  H.  RosE,f  be  completely  precipi- 
tated from  stannic  solutions  by  sulphuric  acid.  If  the  solution 
contains  metastannic  acid  or  metastannic  chloride,  the  precipitation 
is  effected  without  extraordinary  dilution ;  the  other  stannic  com- 
pounds, however,  require  very  considerable  dilution.  If  free 
hydrochloric  acid  is  absent,  the  precipitation  is  rapid  ;  in  other 
cases  12  or  24  hours  at  least  are  required  for  perfect  precipitation. 
Allow  to  settle  thoroughly,  before  filtering,  wash  well  (if  hydro- 
chloric acid  was  present,  till  the  washings  give  no  turbidity  with 
silver  nitrate),  dry  and  ignite,  at  last  intensely  with  addition  of 
some  ammonium  carbonate.  The  results  obtained  by  OESTEN,  and 
communicated  by  H.  ROSE,  are  exact. 

c.  By  Precipitation  as  Stannous  or  Stannic  Sulphide. 

Precipitate  the  dilute  moderately  acid  solution  with  hydrogen 
sulphide  water  or  gas.  If  the  tin  was  present  in  the  solution  as  a 
stannous  salt,  and  the  precipitate  consists  accordingly  of  the  brown 
stannous  sulphide,  keep  the  solution,  supersaturated  with  hydrogen 
sulphide,  standing  for  half  an  hour  in  a  moderately  warm  place, 
and  then  filter.  If,  on  the  other  hand,  the  solution  contain  a  stan- 
nic salt,  or  metastannic  acid,  and  the  precipitate  is  yellow  and  consists 
of  stannic  sulphide  mixed  with  stannic  oxide,  or  yellowish  brown 
and  consists  of  hydrated  metastannic  sulphide  mixed  with  meta- 
stannic acid  (BARFOED,  TH.  SCHEERER  :{:),  put  the  fluid,  loosely 
covered,  in  a  warm  place,  until  the  odor  of  hydrogen  sulphide 

*  Journ.  f.  prakt.  Chem.t  LVI,  366.  \Pogg.  Annal.,  cxn,  164. 

J  Journ.  f.  prakt.  Chem.  N.  F.,  in.  472. 


§  126.]      TIN  IN   STANNOUS   AND    STANNIC    COMPOUNDS.         407 

Las  nearly  gone  off,  and  then  filter.  The  washing  of  the  stan- 
nic-sulphide precipitate,  which  has  a  great  inclination  to  pass 
through  the  filter,  is  best  effected  with  a  concentrated  solution  of 
sodium  chloride,  the  remains  of  the  latter  being  got  rid  of  by  a 
solution  of  ammonium  acetate  containing  a  small  excess  of  acetic 
acid.  If  there  is  no  objection  to  having  the  latter  salt  in  the  fil- 
trate, the  washing  maybe  entirely  effected  by  its  means  (BUNSEN*). 
Transfer  the  dry  precipitate  as  completely  as  possible  to  a  watch 
glass,  burn  the  filter  carefully  in  a  weighed  porcelain  crucible, 
moisten  the  ash  with  nitric  acid,  ignite,  allow  to  cool,  add  the  pre- 
cipitate, cover  the  crucible,  heat  gently  for  some  time  (slight  decrep- 
itation often  occurs),  remove  the  lid  and  heat  gently  with  access  of 
air,  till  sulphur  dioxide  has  almost  ceased  to  be  formed.  (If  too 
much  heat  is  applied  at  first,  stannic  sulphide  volatilizes,  the  fumes 
•of  which  give  stannic  oxide.)  Now  heat  strongly,  allow  to  cool, 
and  heat  repeatedly  with  pieces  of  ammonium  carbonate  to  a  high 
degree,  to  drive  out  the  last  portions  of  sulphuric  acid.  When  the 
weight  remains  constant  the  experiment  is  ended  (H.  HOSE).  For 
•the  properties  of  the  precipitates,  see  §  91.  The  results  are  accu- 
rate. 

2.  Volumetric  Methods. 

The  determination  of  tin  by  the  conversion  of  stannous  into 
stannic  chloride  with  the  aid  of  oxidizing  agents  (potassium  dichro- 
mate, iodine, potassium  permanganate, etc. )offers peculiar  difficulties, 
inasmuch  as  on  the  one  hand  the  stannous  chloride  takes  up  oxygen 
from  the  air  and  from  the  water  used  for  dilution,  with  more  or 
less  rapidity,  according  to  circumstances ;  and  on  the  other  hand, 
the  energy  of  the  oxidizing  agent  is  not  always  the  same,  being 
influenced  by  the  state  of  dilution  and  the  presence  of  a  larger  or 
.smaller  excess  of  acid. 

In  the  following  methods,  these  sources  of  error  are  avoided  or 
limited  in  such  a  manner  as  to  render  the  results  satisfactory. 

*  Annal.  d.  CJiem.  u.  Pharm.,  cvi,  13. 


408  DETERMINATION.  [§  126. 

1.  Determination  of  Stannous   Chloride  ly  Iodine  in 
Alkaline  Solution  (after  LENSSEN  *). 

Dissolve  the  stannous  salt  or  the  metallic  tin  f  in  hydrochloric 
acid  (preferably  in  a  stream  of  carbon  dioxide),  add  sodium-potas- 
sium tartrate,  then  sodium  bicarbonate  in  excess.  To  the  clear 
slightly  alkaline  solution  thus  formed  add  some  starch-solution, 
and  afterwards  the  iodine  solution  of  §  146,  till  a  permanent  blue 
coloration  appears.  2  at.  free  iodine  used  corresponds  to  1  at.  tin. 
LENSSEN'S  results  are  entirely  satisfactory. 

.2.  Determination  of  Stannous  Chloride  after  addition 
of  Ferric  Chloride. 

The  fact  that  stannous  chloride  in  acid  solution  can  be  far  more 
accurately  converted  into  stannic  by  oxidizing  agents  after  being 
mixed  with  ferric  chloride  (or  even  with  cupric  chloride)  than 
without  this  addition,  was  first  settled  by  LOWENTHAL.  \  Sub- 
sequently STROMEYER  §  published  some  experiments  leading  to  the 
same  results,  together  with  practical  remarks  on  the  best  way  of 
carrying  out  the  method  in  different  cases.  The  processes  thus 
originated,  and  which  have  been  well  tested,  are  as  follows : 

a.  The  given  substance  is  a  stannous  salt.  Dissolve  in  pure 
ferric  chloride  (free  from  ferrous  chloride)  with  addition  of  hydro- 
chloric acid,  dilute  and  add  standard  permanganate  from  the 
burette.  Now  make  another  experiment  with,  the  same  quantity 
of  water  similarly  colored  with  ferric  chloride  to  ascertain  how 
much  permanganate  is  required  to  tinge  the  liquid,  and  subtract 
the  quantity  so  used  from  the  amount  employed  in  the  actual 
analysis,  and  from  the  remainder  calculate  the  tin. 

The  reaction  between  the  tin  salt  and  the  iron  solution  is  SnCl, 
-(-Fe2Cl6=SnCl4+2FeCl2.  The  solution  thus  contains  ferrous, 
chloride  in  the  place  of  stannous  salt,  the  former  being,  as  is  well 
known,  far  less  susceptible  of  alteration  from  the  action  of  free 
oxygen  than  the  latter.  £  at.  iron  found  correspond  to  1  at.  tin. 

*  Journ.  f.prakt.  Chem.,  LXXVIII,  200;  Annal.  d.  Chem.  u.  Pharm.,  cxiv, 
113. 

f  The  solution  of  metallic  tin  is  much  assisted  by  the  presence  of  platinum 
foil,  which  is  accordingly  added.  LENSSEN  found  this  addition  of  platimirn  ta 
be  objectionable  ;  but  no  other  experimenter  has  observed  that  it  interferes, 
with  the  accuracy  of  the  results. 

\  Journ.  f.  prakt-  Chem.,  LXXVI,  484. 

§  Annal  d.  Chem.  u.  Pharm.,  cxvii,  261. 


§  127.]  ARSENOUS   AND   ARSENIC  ACIDS.  409 

It  must  not  be  forgotten  that  the  titration  takes  place  in  presence 
of  hydrochloric  acid,  and  that  hence  the  inconveniences  mentioned 
under  §  112,  2,  yy  may  arise  and  impair  the  accuracy  of  the 
method.  The  results  cannot  be  considered  accurate  unless  the 
standardizing  of  the  permanganate  and  the  analysis  take  place 
under  similar  conditions  as  regards  dilution  and  amount  of  hydro- 
chloric acid. 

I.  The  given  substance  is  metallic  tin.  Either  dissolve  in 
hydrochloric  acid — preferably  with  addition  of  platinum  and  in  an 
atmosphere  of  carbon  dioxide — and  treat  the  solution  according  to 
a,  or  place  the  substance  at  once  in  a  concentrated  solution  of  ferric 
chloride  mixed  with  a  little  hydrochloric  acid ;  under  these  cir- 
cumstances it  will,  if  finely  divided,  dissolve  quickly  even  in  the 
cold  and  without  evolution  of  hydrogen.  Gentle  warming  is 
unobjectionable.  Now  add  the  permanganate.  The  reaction  is 
Sn  +  2Fe2Cl6=SnCl4  +  4FeCl2,  therefore  every  4  at.  iron  found 
reduced  correspond  to  1  at.  tin.  The  results  are  of  course  only 
correct  when  iron  is  not  present.  Where  this  is  the  case,  proceed 
with  the  impure  tin  solution  according  to  c. 

c.  The  given  substance  is  stannic  chloride  or  stannic  oxide,  or  a 
compound  of  tin  containing  iron.     Dissolve  in  water  with  addition 
of  hydrochloric  acid,  place  a  plate  of  zinc  in  the  solution  and  allow 
to  stand  twelve  hours,  then  remove  the  precipitated  tin  with  a 
brush,  wash  it,  dissolve  in  ferric  chloride,  and  proceed  as  in  b. 

d.  The  given  substance  is  pure  stannic  sulphide,  precipitated 
out  of  an  acid  stannic  solution  containing  no  stannous  salt.     Mix 
with  ferric  chloride,  heat  gently,  filter  off  the  sulphur,  and  then 
add  the  permanganate.      4   at.  iron  correspond  to   1  at.  tin,  for 
SnS,  +  2Fe3Cl8  =  SnCl4  +  4FeCl,  +  2S.    The  results  obtained  by 
STROMEYER  are  quite  satisfactory.     As  regards  the  precipitated 
stannic  sulphide,  see  BARFOED,  §  91,  c. 

§12T. 

6.  ARSENOUS  ACID,  and  7.  ARSENIC  ACID. 

a.  Solution. 

The  compounds  of  arsenous  and  arsenic  acids  which  are  not 
soluble  in  water  are  dissolved  in  hydrochloric  acid  or  in  nitrohydro- 
chloric  acid.  Some  native  arsenates  require  fusing  with  sodium 
carbonate.  Metallic  arsenic,  arsenous  sulphide,  and  metallic  arsen- 


410  DETERMINATION.  [§  127. 

ides  are  dissolved  in  fuming  nitric  acid  or  nitrohydrochloric  acid, 
or  a  solution  of  bromine  in  hydrochloric  acid ;  those  metallic 
arsenides  which  are  insoluble  in  these  menstrua  are  fused  with 
sodium  carbonate  and  potassium  nitrate,  by  which  means  they  are 
converted  into  soluble  alkali  arsenates  and  insoluble  metallic  oxides ; 
or  they  may  be  suspended  in  potassa  solution  and  treated  with 
chlorine  (§  164,  B,  7).  In  this  last  manner,  too,  arsenous  sul- 
phide dissolved  in  concentrated  potassa  may  be  very  easily  ren- 
dered soluble.  All  solutions  of  compounds  of  arsenic  which  have 
been  effected  by  long  heating  with  fuming  nitric  acid,  or  by  warm- 
ing with  excess  of  nitrohydrochloric  acid,  or  chlorine,  contain 
arsenic  acid.  A  solution  of  arsenous  acid  in  hydrochloric  acid 
cannot  be  concentrated  by  evaporation,  since  arsenous  chloride 
would  escape  with  the  hydrochloric-acid  fumes.  This,  however, 
less  readily  takes  place  if  the  solution  contains  arsenic  acid ;  in 
fact,  it  only  occurs  in  the  presence  of  a  large  proportion  of  hydro- 
chloric acid  (for  instance,  half  the  volume  of  hydrochloric  acid  of 
1*12  sp.  gr.*).  It  is  therefore  advisable  in  most  cases  where  a 
hydrochloric-acid  solution  containing  arsenic  is  to  be  concentrated 
to  previously  render  the  solution  alkaline. 

&.   Determination. 

Arsenic  is  weighed  as  lead  arsenate,  as  ammonium  magnesium 
arsenate,  as  magnesium  py  roar  senate,  as  uranyl  py  roar  senate,  or 
as  arsenous  sulphide.  The  determination  as  ammonium  mag- 
nesium arsenate  is  sometimes  preceded  by  precipitation  as  am- 
monium arseno-molybdate.  The  method  recommended  by  BER- 
THIER  and  modified  by  v.  KOBELL,  of  separating  the  arsenic  as 
basic-ferric  arsenate,  is  only  used  in  separations.  Arsenic  may  be 
estimated  also  in  an  indirect  way,  and  by  volumetric  methods. 

We  may  convert  into 

1.  LEAD  ARSENATE:  Arsenous  and  arsenic  acids  in  aqueous  or 
nitric-acid  solution.     (Acids  or  halogens  forming  fixed  salts  with 
lead,  and  also  ammonium  salts,  must  not  be  present.) 

2.  AMMONIUM  MAGNESIUM  ARSENATE,  or  MAGNESIUM  PYRO- 

ARSENATE : 

a.  By  direct  Precipitation.  Arsenic  acid  in  all  solutions 
free  from  bases  or  acids  precipitable  by  magnesia  or  ammonia. 

*  Zeitschr.  f.  Chem.,  i,  448. 


§127.]  ARSENOUS   AND   ARSENIC   ACIDS.  411 

1).  Preceded  by  Precipitation  as  Ammonium  Arseno-molyb- 
date.  Arsenic  acids  in  all  cases  where  no  phosphoric  acid  is 
present,  little  or  no  hydrochloric  acid,  nor  any  substance  which 
decomposes  molybdic  acid. 

3.  URANYL  PYKO  ARSENATE  :  Arsenic  acid  in  all  combinations 
soluble  in  water  and  acetic  acid. 

4.  ARSENOUS  SULPHIDE  :  All  compounds  of  arsenic  without 
exception. 

Arsenic  may  be  determined  volumetrically  in  a  simple  and 
exact  manner,  whether  present  in  the  form  of  arsenous  acid  or  an 
alkali  arsenite,  or  as  arsenic  acid  or  an  alkali  arsenate.  The  volu- 
metric methods  have  now  almost  entirely  superseded  the  indirect 
gravimetric  methods  formerly  employed  to  effect  the  determina- 
tion of  arsenous  acid. 

1.  Determination  as  Lead  Arsenate. 

a.  Arsenic  Acid  in  Aqueous  Solution. 

A  weighed  portion  of  the  solution  is  put  into  a  platinum  or 
porcelain  dish,  and  a  weighed  amount  of  recently  ignited  pure  lead 
oxide  added  (about  five  or  six  times  the  supposed  quantity  of  arse- 
nic acid  present) ;  the  mixture  is  cautiously  evaporated  to  dryness, 
and  the  residue  heated  to  gentle  redness,  and  maintained  some 
time  at  this  temperature.  The  residue  is  lead  arsenate  +  lead 
oxide.  The  quantity  of  arsenic  acid  is  now  readily  found  by  sub- 
tracting from  the  weight  of  the  residue  that  of  the  oxide  of  lead 
added.  For  the  properties  of  lead  arsenate,  see  §  92.  The  results 
are  accurate,  provided  the  residue  be  not  heated  beyond  gentle  red- 
ness. 

5.   Arsenous  Acid  in  Solution. 

Mix  the  solution  with  nitric  acid,  evaporate  to  a  small  bulk, 
add  a  weighed  quantity  of  lead  oxide  in  excess,  evaporate  to  dry- 
ness,  and  ignite  the  residue  most  cautiously  in  a  covered  crucible, 
until  the  whole  of  the  lead  nitrate  is  decomposed.  The  residue 
consists  here  also  of  arsenic  acid  +  ^ea(i  oxide.  This  method 
requires  considerable  care  to  guard  against  loss  by  decrepitation 
upon  ignition  of  the  lead  nitrate. 


412  DETERMINATION.  [§  127. 

•  2.  Estimation  as  Ammonium  Magnesium  Arsenate,  or 
Magnesium  Pyroar  senate. 

a.  By  direct  Precipitation. 

This  method,  which  was  first  recommended  by  LEVOL,  presup- 
poses the  whole  of  the  arsenic  in  the  form  of  arsenic  acid.  Where 
this  is  not  the  case,  the  solution  is  gently  heated,  in  a  capacious 
flask,  with  hydrochloric  acid,  and  potassium  chlorate  added  in 
small  portions,  until  the  fluid  emits  a  strong  smell  of  chlorous  acid ; 
it  is  then  allowed  to  stand  at  a  gentle  heat  until  the  odor  of  this 
gas  has  nearly  disappeared. 

The  arsenic-acid  solution  is  now  mixed  with  ammonia  in  ex- 
cess,  which  must  not  produce  turbidity,  even  after  standing  some 
time ;  magnesia  mixture  is  then  added  (§  62,  6).  The  fluid,  which 
smells  strongly  of  ammonia,  is  allowed  to  stand  24  or  48  hours  in  »< 
the  cold,  well  covered,  and  then  filtered  through  a  weighed  filter. 
The  precipitate  is  then  transferred  to  the  filter,  with  the  aid  of 
portions  of  the  filtrate,  so  as  to  use  no  more  washing  water  than 
necessary,  and  washed  with  small  quantities  of  a  mixture  of  three 
parts  water  and  one  part  ammonia,  till  the  washings,  on  being 
mixed  with  nitric  acid  and  silver  nitrate,  show  no  opalescence.  The 
precipitate  is  dried  at  102°  to  103°,  and  weighed.  It  has  the  for- 
mula (MgNH4AsO4)a-|-H2O.'*  As  the  drying  of  ammonium  mag- 
nesium arsenate  till  its  weight  is  constant,  requires  much  time  and 
repeated  weighings,  it  is  a  great  advantage  that  we  can  now  con- 
vert it  without  loss  of  arsenic  into  magnesium  pyroarsenate  (Mga 
As2O7),  thanks  to  the  researches  of  H.  RosE,f  WITTSTEINJ:  and 
PULLER.§  For  this  purpose  first  transfer  the  dried  precipitate  as 
completely  as  possible  to  a  watch-glass,  saturate  the  filter  with 
a  solution  of  ammonium  nitrate,  dry  and  burn  it  cautiously  in  a 
porcelain  crucible.  After  cooling,  transfer  the  precipitate  to  the 
crucible,  heat  in  an  air-bath  to  about  130°,  continue  heating  for  2 
hours  on  a  sand-bath,  then  heat  for  an  hour  or  two  on  an  iron  plate 
a  little  more  strongly,  and  when  the  ammonia  has  been  thus  entirely 

*  If  it  is  dried  in  a  water-bath,  the  drying  must  be  extremely  prolonged,  or 
otherwise  more  than  1  eq.  will  be  left.  After  brief  drying  in  the  water-bath 
the  compound  contains  between  1  and  3  eq.  water.  If  it  is  dried  between  105° 
and  110°  part  of  the  1  eq.  water  is  lost. 

\  Handbuch  der  anal.  Chem.  6.  Aufl.,  n,  390. 

$Zeitschr,f,  anal,  Chem.,  n,  19.  §15.,  x,  63. 


§  127.]  ARSENOUS  AND  ARSENIC   ACIDS.  413 

expelled  ignite  strongly  for  some  time  over  the  lamp.  The  pro- 
cess may  be  shortened  by  conducting  the  heating  in  a  ROSE'S  cruci- 
ble in  a  slow  current  of  oxygen.  The  ammonia  may  then  be 
driven  off  in  10  minutes,  and  after  the  precipitate  has  been  at  last 
strongly  heated  it  will  be  ready  to  weigh.  For  the  properties  of 
the  ammonium  magnesium  arsenate  and  magnesium  pyroarsenate, 
see  §  92.  The  method  yields  satisfactory  results,  since  the  small 
loss  of  precipitate  dissolved  in  the  filtrate  and  washings  is  coun- 
terbalanced by  the  presence  of  a  trace  of  basic  magnesium  sulphate 
(PULLER).  PULLER  with  a  quantity  of  0*37  grm.  ammonium  mag- 
nesium arsenate  lost  only  a  fraction  of  a  milligramme ;  on  the  ad- 
dition of  a  large  proportion  of  ammonium  chloride  the  loss  rose  to 
about  0-002  grm.  The  correction  for  the  solubility  of  the  pre- 
cipitate in  the  ammoniacal  filtrate  containing  excess  of  magnesia 
mixture  is  0-001  grm.  of  (Mg]STH4AsO4)a  +  HaO  for  30  c.c. 

b.  Preceded  ly  Precipitation  as  Ammonium  Arseno-molyl- 
date. 

Mix  the  acid  solution,  which  must  be  free  from  phosphoric  and 
silicic  acids,  with  an  excess  of  solution  of  ammonium  molybdate. 
The  ammonium  molybdate  solution  should  have  been  previously' 
mixed  with  nitric  acid  in  excess,  and  the  whole  process  is  con- 
ducted exactly  as  in  the  case  of  phosphoric  acid — see  §  134,  J,  ft. 
After  dissolving  the  ammonium  arseno-molybdate  in  ammonia, 
neutralize  the  latter  partially  with  hydrochloric  acid.  Treat  th^ 
ammonium  magnesium  arsenate  as  in  a.  Results  satisfactory. 

3.  Estimation  as  Uranyl  Pyroarsenate. 

This  method  was  first  proposed  by  WERTHER.*  It  lias  been 
carefully  studied  by  PuLLERf  in  my  laboratory,  and  gives  thor- 
oughly  satisfactory  results.  Mix  the  arsenic  acid  solution  with 
potash  or  ammonia  in  excess,  and  then  a  good  excess  of  acetic  acid. 
(If  a  precipitate  of  ferric  or  aluminium  arsenate  here  remains 
insoluble,  the  method  would  be  inapplicable.)  Add  uranyl  acetate 
in  excess,  and  boil.  Wash  the  slimy  precipitate  of  uranyl  arsenat^ 
or  of  ammonium  uranyl  arsenate  by  decantation  with  boiling  water, 
and  then  transfer  to  a  filter.  The  addition  of  a  few  drops  of  chta 
roform  to  the  partly  cool  fluid  will  hasten  the  deposition  of  the  pre^ 
cipitate.  Dry,  transfer  the  precipitate  to  a  watch-glass,  cleaning 

*Journ.f.  prakt.  Chcm.,  XLIIT,  346.         \Zeitschr.f.  analyt.  Chem.,  x,  72. 


414  DETERMINATION.  [§  127. 

the  filter  as  much  as  possible  ;  saturate  the  latter  with  ammonium 
nitrate,  dry  it,  incinerate  in  a  porcelain  crucible,  and  add  the  pre- 
cipitate. If  the  precipitate  contains  ammonium,  heat  very  cau- 
tiously, finally  adding  nitric  acid,  or  ignite  in  oxygen.  (See  2,  a.) 
If  the  precipitate  is  free  from  ammonium,  ignite  in  the  ordinary 
way.  Ammonium  salts  do  not  interfere.  Properties  of  the  pre- 
cipitate and  residue,  §  92,  e. 

4.   Estimation  as  Arsenous  Sulphide. 

a.  In  solutions  of  Arsenous  Acid  or  Arsenites  free  from 
Arsenic  Acid. 

The  solution  should  be  strongly  acid  with  hydrochloric  acid. 
Precipitate  with  hydrogen  sulphide  and  expel  the  excess  with  car- 
bon dioxide.  Pass  the  latter  through  the  solution  for  an  hour,  a 
longer  time  is  useless.  (See  §  125,  1.)  Wash  the  precipitate  thor- 
oughly and  dry  at  100°  till  the  weight  is  constant.  Particles  of 
the  precipitate  which  adhere  so  firmly  to  the  glass  that  they  can- 
iiot  be  removed  mechanically  are  dissolved  in  ammonia  and  repre- 
cipitated  with  hydrochloric  acid.  Properties  of  the  precipitate, 
§  92.  Do  not  omit  to  test  a  weighed  portion  to  see  whether  it 
completely  volatilizes  on  heating.  If  a  residue  remains  it  is  to  be 
weighed  and  the  proportional  quantity  deducted  from  the  total 
weight  of  the  precipitate.  Results  accurate. 

If  the  solution  contains  any  substance  which  decomposes  hydro- 
gen sulphide,  such  as  ferric  chloride,  chromic  acid,  etc.,  the  precip- 
itate produced  in  the  cold  contains  an  admixture  of  finely  divided 
sulphur.  It  should  be  collected  in  the  same  manner  on  a  filter 
dried  at  100°,  arid  weighed,  washed  and  dried.  Extract  the 
admixed  sulphur  with  purified  carbon  disulphide  (which  should 
leave  no  residue  on  evaporation),  continuing  till  the  fluid  which 
runs  through  leaves  no  residue.  Dry  at  100°  till  the  weight  is 
constant.  From  experiments  made  in  my  laboratory  it  appears 
that  the  results  thus  obtained  are  quite  accurate,  even  when  the 
amount  of  admixed  sulphur  is  large  ;  but  the  precipitation  must 
have  been  effected  in  the  cold.  If,  on  the  contrary,  heat  is  used, 
the  sulphur  is  in  the  form  of  small  agglutinated  grains  and  cannot 
be  completely  extracted  by  cold  carbon  disulphide  on  the  filter. 
However,  it  may  be  extracted  by  removing  the  precipitate  from  the 
filter  and  repeatedly  digesting  it  with  the  disulphide  on  a  water- 
bath  (PULLER*). 

*  ZeitscJir.  f.  analyt.  Chem.,  x,  46  et  seq. 


§  127.]  ARSENOUS   AND    ARSENIC    ACIDS.  415 

Instead  of  purifying  the  arsenous  sulphide  you  may  estimate 
the  arsenic  in  the  mixture  of  the  sulphide  with  sulphur  as  follows: 
Dissolve  the  precipitate  in  strong  potassa,  and  pass  chlorine  into  the 
solution  (§  148,  II.  2,  5).  The  arsenic  and  the  sulphur  are  con- 
verted into  arsenic  and  sulphuric  acids  respectively  ;  the  former 
may  be  estimated  according  to  2,  0,  or  the  latter  according  to  §  132. 
In  the  latter  case,  deduct  the  sulphur  found  from  the  weight  of  the 
arsenical  precipitate.  There  is  no  loss  of  arsenic  in  this  process 
from  volatilization  of  the  chloride,  as  the  solution  remains  alkaline. 
The  object  may  also  be  conveniently  attained  by  the  use  of  nitric 
acid.  A  very  strong  fuming  acid,  of  86°  boiling  point,  is 
employed  ;  an  acid  of  1-42  sp.  gr.  which  boils  at  a  higher  tempera- 
ture does  not  answer  the  purpose,  as  the  separated  sulphur  would 
fuse,  and  its  oxidation  would  be  much  retarded.  The  well  dried 
precipitate  is  shaken  into  a  small  porcelain  dish,  treated  with  a  tol- 
erably large  excess  of  the  fuming  nitric  acid,  the  dish  immediately 
covered  with  a  watch-glass,  and  as  soon  as  the  turbulence  of  the 
first  action  has  somewhat  abated,  heated  on  a  water-bath  till  all  the 
sulphur  has  disappeared,  and  the  nitric  acid  has  evaporated  to  a 
small  volume.  The  filter  to  which  the  unremovable  traces  of 
arsenous  sulphide  adhere  is  treated  separately  in  the  same  manner, 
the  complete  destruction  of  the  organic  matter  being  finally  effected 
by  gently  warming  the  somewhat  dilute  solution  with  potassium 
chlorate  (BUNSEN*).  Or  the  filter  may  instead  be  extracted  with 
ammonia,  the  solution  evaporated  in  a  separate  dish,  and  the  resid- 
ual sulphide  treated  as  above.  In  the  mixed  solution  the  arsenic 
acid  is  finally  precipitated  as  ammonium  magnesium  arsenate. 
(§  127,  2,  a).  Treatment  of  the  impure  precipitate  with  ammonia, 
whereby  the  sulphide  is  dissolved,  and  the  sulphur  is  supposed  to 
remain  behind,  only  gives  approximate  results,  as  the  ammoniacal 
solution  of  arsenous  sulphide  takes  up  a  little  sulphur. 

1.  In  solutions  of  Arsenic  Acid,  or  of  a  mixture  of  the  two 
Oxides  of  Arsenic. 

Heat  the  solution  in  a  flask  (preferably  on  an  iron  plate)  to 
about  70°,  and  conduct  hydrogen  sulphide  at  the  same  time  into 
the  fluid,  so  long  as  precipitation  takes  place.  The  precipitate 
formed  is  always  a  mixture  of  sulphur  and  arsenous  sulphide, 
since  the  arsenic  acid  is  first  reduced  to  arsenous  acid  with  separa- 

*  Annal  d.  Chem.  u.  Pharm  ,  cvi,  10. 


416  DETERMINATION.  [§  127. 

tion  of  sulphur,  and  then  the  latter  is  decomposed  (H.  KOSE  *). 
Only  in  the  case  when  a  sulpho-salt  containing  pentasulphide  of 
arsenic  is  decomposed  with  an  acid,  is  the  precipitate  actually 
pentasulphide,  and  not  merely  a  mixture  of  sulphur  with  arse- 
nous  sulphide  (A.  FTJCHS  f).  To  convert  this  mixture  of  arsenous 
sulphide  and  granular  sulphur  into  pure  arsenous  sulphide 
suitable  for.  weighing,  treat  it  as  follows  :  Extract  the  washed  and 
still  moist  precipitate  on  the  filter  with  ammonia,  wash  the  resid- 
ual sulphur,  precipitate  the  solution  with  hydrochloric  acid  with- 
out heat,  filter,  dry,  extract  with  carbon  disulphide,  dry  at  100°, 
and  weigh.  Results  accurate.  The  mixture  of  arsenous  sulphide 
and  sulphur  obtained  by  hot  precipitation  may,  of  course,  also  be 
estimated  directly  or  indirectly  after  one  of  the  other  methods 
in  4,  a. 

5.    Volumetric  Methods, 
a.  Method  which  presupposes  the  presence  of  A.rsenous  Acid. 

1.  FR.  MOHE'S  method.;);  This  method  depends  upon  the 
principle  already  stated  under  Antimonous  Oxide,  §  125,  3,  a\ 
i.e.,  arsenous  acid  in  alkaline  solution  is  oxidized  by  iodine  to 
arsenic  acid  (As3O8  +  4NaOH  +  41  +  =  As2O6  +  4NaI+  2HaO). 

If,  therefore,  you  have  an  aqueous  solution  of  arsenous  acid 
or  an  alkali  arsenite,  weigh  or  measure  off  a  quantity  that  will 
contain  about  O'l  grm.  AsaO3 ;  add  to  it  20  c.  c.  of  a  saturated 
solution  of  sodium  bicarbonate  (previously  purified  by  washing 
with  water),  then  add  a  little  thin  starch  paste,  and  finally  titrate 
with  standard  iodine  solution  (§  146)  until  the  starch-iodide  reac- 
tion appears ;  4  eq.  of  'iodine  correspond  to  1  eq.  of  arsenous  acid. 
If  the  arsenous-acid  solution  is  acid,  neutralize  it  with  pure  sodium 
carbonate;  but,  on  the  other  hand,  if  alkaline,  neutralize  with 
hydrochloric  acid  before  adding  the  sodium  bicarbonate.  It  is  of 
course  understood  that  the  solution  contains  no  substances  (sul- 
phides or  thiosulphates)  that  will  act  upon  iodine.  The  results 
are  accurate.  Compare  Expt.  No.  79 ;  also  WAITZ.§ 

*  Pogg.  Annal.,  cvn,  186. 
•\  Zntschr.f.  analyt.  Chem.,  I,  189. 
}  Lehrbuch  der  Titrirmethode,  3.  Aufl.,  275. 

%Zeitsc7ir.  f.  analyt.  Chem.,  x,  1(52.  The  attempts  made  by  WAITZ  to 
convert  the  arsenic  in  arsenous  sulphide  into  alkali  arsenite  were  unsuccessful. 


[§  127.  ARSENOUS  AND  ARSENIC  ACIDS.  417 

2.  KESSLER'S    method.*      This    method    depends  upon  the 
principle  stated  under  §  125,  3,  5;  i.e.,  the  oxidation  of  arsenous 
to  arsenic  acid  is  effected  in  hydrochloric-acid  solution  by  the  use 
of  potassium  dichromate,  f  and  is  carried  out  in  exactly  a  similar 
manner.     The  results  are  reliable  only  when  care  is  taken  that 
the  hydrochloric  acid  (sp.  gr.  1*12)  constitutes  at  least  one  sixth  of 
the  volume  of  the  liquid ;   it  should  not,  however,   exceed  one 
half  the  volume,  otherwise  the  end  reaction,  due  to  the  formation 
of  iron  ferricyanide,  sets  in  more  slowly  and  loses  in  sharpness. 

If  for  any  reason  the  direct  titration  of  the  hydrochloric- 
.acid  solution  is  impracticable,  precipitate  with  hydrogen  sulphide 
(if  arsenic  acid  is  present,  at  TO0),  wash  the  precipitate,  transfer  it 
together  with  the  filter  to  a  stoppered  flask,  and  treat  it  with  an 
almost  saturated  solution  of  mercuric  chloride  in  hydrochloric  acid 
(sp.  gr.  1*12),  stopper  tightly,  digest  at  a  gentle  warmth  until 
the  precipitate  has  become  white,  dilute  with  a  measured  quantity 
of  water  (so  that  the  proportion  of  hydrochloric  acid  of  sp.  gr. 
1*12  does  not  fall  below  one  sixth),  add  solution  of  potassium 
•dichromate,  then  iron  solution,  etc.,  as  detailed  under  §  125,  3,  b. 
Results  good.  Compare  also  WAITZ.^ 

3.  BUNSEN'S  method. §    This  method  is  based  upon  the  follow- 
ing facts : 

aa.  If  potassium  dichromate  is  boiled  with  concentrated  hydro- 
chloric acid,  6  at.  chlorine  are  disengaged  for  every  2  mol.  chromic 
acid;  2CrO8+  12HC1  =  CrCl,  +  6H,O  +  6C1. 

l>b.  But  if  arsenous  acid  is  present  (not  in  excess)  there  is  not 
the  quantity  of  chlorine  disengaged  corresponding  to  the  chromic 
acid,  but  so  much  less  of  that  element  as  is  required  to  convert 
the  arsenous  into  arsenic  acid  (H3AsO,  +  2C1  -f-  HaO  =  H,AsO4 
-f-  2HC1).  Consequently  for  every  2  at.  chlorine  wanting  there 
is  to  be  reckoned  1  mol.  arsenous  acid. 

*  Pogg.  AnnaL,  xcv,  204;  cxui,  184;  cxvm,  17;  Zeitschr.f.  analyt.  Chem., 
IT,  383. 

f  Oxidation   may  also  be  effected  by  potassium   permanganate,    an   excess 
being  added,  and  this  latter  then  determined  with  iron.     The  estimation  is  inac- 
curate in  hydrochloric-acid  solution,  hence  the  permanganate  can  only  be  used  in 
sulphuric-acid  solution.     Compare  WAITZ,  ZeitscJir.f.  analyt.  Chem.,  x,  174. 
J  Zeitschr.f.  analyt.  Chem.,  x,  169. 
§  AnnaL  d.  Chem.  u.  Pharm.,  LXXXVI,  290. 


418  DETERMINATION.  [§  127. 

CG.  The  quantity  of  chlorine  is  estimated  by  determining  the 
quantity  of  iodine  liberated  by  it  from  potassium  iodide. 

These  are  the  principles  of  BUNSEN'S  method.  For  the  man- 
ner of  execution  I  refer  to  the  Estimation  of  Chromic  Acid. 

~b.  Method  which  presupposes  the  presence  of  Arsenic  Acid. 

This  method  depends  on  the  precipitation  of  the  arsenic  acid 
by  uranium  solution  and  the  recognition  of  the  end  of  the  reaction 
by  means  of  potassium  ferrocyanide.  It  is  therefore  the  same  as 
was  suggested  for  phosphoric  acid  by  LECOMTE,  and  brought  into 
use  by  NEUBAUEK,*  and  afterwards  by  PiNcus.f 

BODEKER,^;  who  first  employed  the  process  for  arsenic  acid, 
recommends  the  employment  of  a  solution  of  uranyl  nitrate,  as- 
this  is  more  permanent  than  the  hitherto  used  acetate,  which  is 
gradually  decomposed  by  the  action  of  light. 

The  uranium  solution  has  the  correct  degree  of  dilution,  if  it 
contains  about  20  grm.  of  uranium  in  1  litre.  It  should  contain 
as  little  free  acid  as  possible.  The  determination  of  its  value  may 
be  effected  with  the  aid  of  pure  sodium  arsenate  or  by  means  of 
arsenous  acid — the  latter  is  converted  into  arsenic  acid  by  boiling 
with  fuming  nitric  acid.  The  solution  is  rendered  strongly  alka- 
line with  ammonia,  and  then  distinctly  acid  with  acetic  acid.  The 
uranium  solution  is  now  run  in  from  the  burette  slowly,  the  liquid 
being  well  stirred  all  the  while,  till  a  drop  of  the  mixture  spread 
out  on  a  porcelain  plate,  gives  with  a  drop  of  potassium  ferrocya- 
nide placed  in  its  centre,  a  distinct  reddish-brown  line  where  the 
two  ilnids  meet.  The  height  of  the  fluid  in  the  burette  is  now 
read  off,  the  level  of  the  mixture  in  the  beaker  is  marked  with  a, 
strip  of  gummed  paper,  and  the  beaker  is  emptied  and  washed, 
filled  with  water  with  addition  of  about  as  much  ammonia  and 
acetic  acid  as  was  before  employed,  and  the  uranium  solution  is 
cautiously  dropped  in  from  the  burette,  till  a  drop  taken  out  of  the 
beaker  and  tested  as  above,  gives  an  equally  distinct  reaction.  The 
quantity  of  uranium  solution  used  in  this  last  experiment  is  the 
excess,  which  must  be  added  to  make  the  end-reaction  plai^  for  the 
dilution  adopted.  This  amount  is  subtracted  from  that  used  in  the 
first  experiment,  and  we  then  know  the  exact  value  of  the  uranium 
solution  with  reference  to  arsenic  acid. 


*  Arckivfur  wissenchaftHche  Heilkunde,  iv,  228. 

\Journ.f.  prakt.  Chem.,  LXXVI,  104.   $  Annal.  de  Chem.  u.  Pharm.,  cxvn,  195» 


§  127.]  AKSENOUS   AND   ARSENIC   AOIDS.  419 

In  an  actual  analysis,  the  arsenic  is  first  brought  into  the  form 
of  arsenic  acid,  a  clear  solution  is  obtained  containing  ammonium 
acetate  and  some  free  acetic  acid,*  and  the  process  is  conducted 
exactly  as  in  determining  the  value  of  the  standard  solution.  The 
experiment  to  ascertain  the  correction  must  not  be  omitted  here, 
otherwise  errors  are  sure  to  arise  from  the  different  degrees  of  dilu- 
tion of  the  arsenic  acid  solutions  used  in  the  determination  of  the 
value  of  the  standard  solution  and  in  the  actual  analyses.  The  results 
of  two  determinations  of  arsenic  given  by  BODEKEB  are  satisfactory. 
To  execute  the  method  well  requires  practice.  The  results  are  not 
exact  enough  unless  the  conditions  as  regards  amount  and  quality 
of  alkali  salts  are  nearly  similar  in  the  standardizing  of  the  uranium 
solution  and  in  its  use.  Compare  WAixz.f 

6.  Estimation  of  Arsenous  Acid  l}y  Indirect  Gravimet- 
ric Analysis. 

a.  ROSE'S  method.  Add  to  the  hydrochloric  acid  solution,  in 
the  preparation  of  which  care  must  be  taken  to  exclude  oxidizing 
substances,  a  solution  of  sodium-  or  ammonium-auric  chloride  in 
excess,  and  digest  the  mixture  for  several  days,  in  the  cold,  or,  in 
the  case  of  dilute  solutions,  at  a  gentle  warmth ;  then  weigh  the 
separated  gold  as  directed  in  §  123.  Keep  the  filtrate  to  make 
quite  sure  that  no  more  gold  will  separate.  2  at.  gold  correspond 
to  3  rnoL  arsenous  acid. 

I).  YOHI/S^:  method.  Mix  the  solution  with  a  weighed  quan- 
tity of  potassium  dichromate,  and  free  sulphuric  acid ;  estimate  the 
chromic  acid  still  present  by  the  method  given  in  §  130,  c,  and 
deduce  from  the  quantity  of  that  acid  consumed  in  the  process,  i.e., 
reduced  by  the  arsenous  acid,  the  quantity  of  the  latter,  after  the 
formula  3H, AsO3  +  2CrO3  =  3H3AsO4  +  O2O3. 

*  Alkalies,  alkali  earths,  and  zinc  oxide  may  be  present,  but  not  such  metals 
as  yield  colored  precipitates  with  potassium  ferrocyanide,  as,  for  instance, 
copper. 
•\Zeit8chr.f,  anal.  Chem.,  x,  182.          %Annal.  de  CJiem.  u.  Pharm.,  xciv,  219. 


420  DETERMINATION.  [§  128. 

Supplement  to  the  Sixth  Group. 

§128. 
8.  MOLYBDIC  ACID. 

Molybdic  acid  is  converted,  for  the  purpose  of  its  determina- 
tion, either  into  molybdenum  dioxide,  or  into  lead  molybdate,  or 
into  molybdenum  disulphide. 

a.  Molybdic  anhydride  (MoO3),  and  also  ammonium  molybdate, 
may  be  reduced  to  dioxide  by  heating  in  a  current  of  hydrogen  gas. 
This  may  be  done  either  in  a  porcelain  boat,  placed  in  a  wide  glass 
tube,  or  in  a  platinum  or  porcelain  crucible  with  perforated  cover 
(§108,  Fig.  83).  The  operation  is  continued  till  the  weight  remains 
constant.  The  temperature  must  not  exceed  a  gentle  redness, 
otherwise  the  dioxide  itself  might  lose  oxygen  and  become  partially 
converted  into  metal.  In  the  case  of  ammonium  molybdate  the 
heat  must  be  very  low  at  first  on  account  of  the  frothing.  If  you 
have  a  platinum  tube  it  is  safer  to  ignite  the  molybdic  acid  in  this 
for  2  or  3  hours  in  a  slow  current  of  hydrogen,  thus  reducing  it  to 
the  metallic  state.  When  reducing  to  dioxide  the  contents  of  the 
crucible  are  frequently  gray  below,  and  brown  above  (RAMMELS- 


b.  The  following  is  the  best  method  of  precipitating  molybdic 
acid  from  an  alkaline  solution  :  Dilute  the  solution,  if  necessary, 
neutralize  the  free  alkali  with  nitric  acid,  and  allow  the  carbonic 
acid,  which  may  be  liberated  in  the  process,  to  escape,  then  add 
neutral  mercurous  nitrate.  The  yellow  precipitate  formed  appears 
at  first  bulky,  but  after  several  hours'  standing  it  shrinks  ;  it  is 
insoluble  in  the  fluid,  which  contains  an  excess  of  mercurous 
nitrate.  Collect  on  a  filter,  and  wash  with  a  dilute  solution  of  mer- 
curous nitrate,  as  it  is  slightly  soluble  in  pure  water.  Dry,  remove 
the  precipitate  as  completely  as  practicable  from  the  filter,  and  deter- 
mine the  molybdenum  in  it  as  directed  in  a  (H.  ROSE)  ;  or  mix  the 
precipitate,  together  with  the  filter-ash,  with  a  weighed  quantity 
of  ignited  lead  oxide,  and  ignite  until  all  the  mercury  is  expelled  ; 
then  add  some  ammonium  nitrate,  ignite'  again  and  weigh.  The 
excess  obtained,  over  and  above  the  weight  of  the  lead  oxide  used, 
is  molybdenum  trioxide 


*  Pogg.  Anndl.,  cxxvii,  281;  Zeitschr.f.  analyt.  CJicm.,  v,  203. 
\Journ.f.  prakt.  Chem.,  LXVII,  472. 


§  128.]  MOLYBDIC   ACID.  421 

c.  CHATARP*  recommends  estimating  rnolybdic  acid  in  the  solu- 
tion of  its  alkali  salts  by  adding  lead  acetate  in  slight  excess  to  the 
boiling  solution  and  boiling  for  a  few  minutes.     The  precipitate 
which  is  at  first  milky  becomes  granular,  deposits  well,  and  may  be 
easily  washed  with  hot  water.     It  is  dried,  removed  from  the  filter 
as  much  as  possible,  ignited  and  weighed  as  PbMoO4.    The  method 
is  only  applicable  for  solutions  of  pure  alkali  rnolybdates. 

d.  The  precipitation  of  molybdenum  as  sulphide  is  always  a 
difficult  operation.     If  the  acid  solution   is   supersaturated  with 
hydrogen  sulphide,  warmed,  and  filtered,  the  filtrate  and  washings 
are  generally  still  colored.     They  must,  accordingly,  be  warmed, 
and  hydrogen  sulphide  again  added,  and  the  operation  must  after- 
wards, if  necessary,  be  repeated  until  the  washings  appear  almost 
colorless.    The  precipitation  succeeds  better  when  the  molybdenum 
sulphide  is  dissolved  in  a  relatively  large  excess  of  ammonium  sul- 
phide, and,  after  the  fluid  has  acquired  a  reddish-yellow  tint,  precipi- 
tated with  hydrochloric  acid.     ZENKERf  advises  then  to  boil,  until 
the  hydrogen  sulphide  is  expelled,  and  to  wash  with  hot  water,  at 
first  slightly  acidified.     To  make  quite  sure  that  all  the  molyb- 
denum is  precipitated,  treat  the  filtrate  and  washings  again  with 
hydrogen  sulphide  and  allow  to  stand  for  some  time.     The  brown 
molybdenum  sulphide  is  collected  on  a  weighed  filter,  and   the 
molybdenum  determined  in  an  aliquot  part  of  it,  by  gentle  ignition 
in  a  current  of  hydrogen  gas,  as  in  a.     The  brown  molybdenum 
sulphide  changes  in  this  process  to  the  gray  disulphide  (H.  ROSE). 

.e.  F.  PISANI;};  gives  the  following  method  for  estimating  molyb- 
dic  acid  volumetrically :  Digest  the  molybdic  acid  with  hydro- 
chloric acid  and  zinc,  dissolving  any  precipitate  which  may  form 
from  want  of  acid  and  also  the  excess  of  zinc.  The  molybdic  acid 
is  thus  reduced  to  a  molybdenum  salt  corresponding  to  molybdenum 
sesquioxide.  Convert  the  molybdenum  in  this  solution  again  into 
molybdic  acid  by  standard  potassium  permanganate.  The  brown 
color  of  the  solution  turns  first  green,  and  then  disappears.  RAM- 
MELSBERG§  confirms  the  statements  of  PISAOT. 

*  Sill  Amer.  Journ.  (3),  i,  416.  f  Journ.f.  prakt.  Chem.,  LVIII,  259. 

\  Compt.  Rend.,  LIX,  301. 

§Pogg.  Annal.,  cxxvn,  281;  Zeitschr.f.  analyt.  Chem.,  vf  203. 


422  DETERMINATION.  [§§  129,  130. 

II.  DETERMINATION  OF  ACIDS  IN  COMPOUNDS  CONTAINING 
ONLY  ONE  ACID,  FREE  OR  COMBINED;— AND  SEPARATION 
OF  ACID  FROM  BASIC  RADICALS. 

First  Group, 

FIRST     DIVISION. 

ARSENOUS  ACID— ARSENIC  ACID — CHROMIC  ACID — (Selenous 
Acid,  Sulphurous  and  Hyposulphurous  Acids,  lodic  Acid). 

§129. 
1.  ARSENOUS  AND  ARSENIC  ACIDS. 

These  have  been  already  treated  of  among. the  bases  (§  127)  on 
account  of  their  behavior  with  hydrogen  sulphide  ;  they  are  merely 
mentioned  here  to  indicate  the  place  to  which  they  properly  be- 
long. The  methods  of  separating  them  from  the  bases  will  be 
found  in  Section  Y. 

§130. 
2.  CHROMIC  Aero. 

I.  DETERMINATION. 

Chromic  acid  is  determined  either  as  chromic  oxide  or  lead 
chr  ornate.  But  it  may  be  estimated  also  from  the  quantity  of  car- 
bon dioxide  disengaged  by  its  action  upon  oxalic  acid  in  excess, 
and  also  by  volumetric  analysis.  In  employing  the  first  method 
it  must  be  borne  in  mind  that  1  mol.  chromic  oxide  corresponds  to 
2  mol.  chromic  acid. 

a.  Determination  as  Chromic  Oxide. 

a.  The  chromic  acid  is  reduced  to  the  state  of  a  chromic  salt 
and  the  amount  of  chromium  in  the  latter  determined  (§  106).  The 
reduction  is  effected  either  by  heating  the  solution  with  hydro- 
chloric acid  and  alcohol ;  or  by  mixing  hydrochloric  acid  with  the 
solution,  and  conducting  hydrogen  sulphide  into  the  mixture  ;  or 
by  adding  a  strong  solution  of  sulphurous  acid,  and  applying  a  gen- 
tle heat.  With  concentrated  solutions  the  first  method  is  gener- 
ally resorted  to,  with  dilute  solutions  one  of  the  two  latter.  With 
respect  to  the  first  method,  I  have  to  remark  that  the  alcohol  must 
be  expelled  before  the  chromium  can  be  precipitated  as  hydroxide 


§130.]  CHROMIC    ACID.  423 

by  ammonia ;  and  with  respect  to  the  second,  that  the  solution 
supersaturated  with  hydrogen  sulphide  must  be  allowed  to  stand  in 
a  moderately  warm  place,  until  the  separated  sulphur  has  com- 
pletely subsided.  The  results  are  accurate,  unless  the  weighed  pre- 
cipitate contains  silica  and  lime,  which  is  always  the  case  if  the  pre- 
cipitation is  effected  in  glass  vessels. 

ft.  The  neutral  or  slightly  acid  (nitric  acid)  solution  is  precipi- 
tated with  mercurous  nitrate,  after  long  standing  the  red  precipitate 
of  mercurous  chromate  is  filtered  off,  washed  with  a  dilute  solution 
of  mercurous  nitrate,  dried,  ignited,  and  the  residuary  chromic 
oxide  weighed  (II.  ROSE).  Results  accurate. 

b.  Determination  as  Lead  Chromate. 

The  solution  is  mixed  with  sodium  acetate  in  excess,  and  acetic 
acid  added  until  the  reaction  is  strongly  acid ;  the  solution  is  then 
precipitated  with  neutral  lead  acetate.  The  washed  precipitate  is 
either  collected  on  a  weighed  filter,  dried  in  the  water-bath,  and 
weighed;  or  it  is  gently  ignited  as  directed  §  53,  and  then 
weighed.  For  the  properties  of  the  precipitate,  see  §  93,  2.  Results 
accurate. 

c.  Determination  as  Barium  Chromate. 

Moderately  acidulate  the  alkali-chromate  solution  with  acetic 
acid,  add  a  slight  excess  of  barium  chloride,  allow  the  fine  pre- 
cipitate to  stand  12  hours,  wash  it  with  a  solution  of  ammonium 
acetate  so  far  as  possible  by  decantation,  displacing  the  last  por- 
tion of  the  solution  with  ammonium  nitrate  (or  the  chromate  may 
be  partially  reduced  on  ignition),  dry  the  precipitate,  and  ignite  it 
after  removal,  so  far  as  is  possible,  from  the  filter. 

Properties  and  composition  of  the  barium  chromate  are  given 
tinder  §  93,  2,  c  (H.  ROSE;  PEARSON  *).  The  test  analyses  given 
by  PEARSON  are  satisfactory. 

d.  Determination  ~by  means  of  Oxalic  Acid  (after  YOHL). 
When  chromic  acid  and  oxalic  acid  are  brought  together  in 

the  presence  of  water  and  excess  of  sulphuric  acid,  chromic 
sulphate  and  carbon  dioxide  are  formed,  3H2C2O4  -f-  2H2CrO4  -f- 
3H2SO4  =  6CO,  +  Cr2(SO4),  +  8H,O.  Accordingly  the  amount 
of  chromic  acid  can  be  calculated  from  the  weight  of  carbon 
dioxide  evolved.  The  process  is  the  same  as  in  the  analysis  of 
manganese  ores  (§  230).  1  part  of  chromic  acid  requires  2J 

*  Amer.  Journ.  of  Science  [2],  XLV,  298;  Zeitschr.f.  analyt.  Chlm.,  ix,  108. 


424  DETERMINATION.  [§  130. 

parts  of  sodium  oxalate.     If  it  is  intended  to  determine  potas- 
sium or  sodium  in  the  residue,  ammonium  oxalate  is  used. 

e.  Determination  by  Volumetric  Analysis, 
a.   SCHWARZ'S  method. 

The  principle  of  this  very  accurate  method  is  identical  with 
that  upon  which  PENNY'S  method  of  determining  iron  is  based 
(§  112,  2,  Z>).  The  execution  is  simple:  Acidify  the  not  too 
dilute  solution  of  the  chromate  with  sulphuric  acid,  add  in  excess 
a  measured  quantity  of  solution  of  a  ferrous  salt,  the  strength  of 
which  you  have  previously  ascertained  according  to  the  direc- 
tions of  §  112,  2,  #,  or  &,  or  the  solution  of  a  weighed  quantity 
of  ammonium-ferrous  sulphate,  free  from  ferric  salt,  and  then 
determine  in  the  manner  directed  in  §  112,  2  #,  or  5,  the  quan- 
tity of  ferrous  iron  remaining.  The  difference  shows  the  amount 
of  iron-  that  has  been  converted  by  the  chromic  acid  from  a 
ferrous  to  a  ferric  salt.  1  grm.  of  iron  corresponds  to  0*5969  of 
chromic  anhydride  (CrO8).  To  determine  the  chromic  acid  in 
lead  chromate,  the  latter  is,  after  addition  of  the  ammonium 
ferrous  sulphate,  most  thoroughly  triturated  with  hydrochloric 
acid,  water  added,  and  the  analysis  then  proceeded  with. 
/?.  BUNSEN'S  method.* 

If  a  chromate  is  boiled  with  an  excess  of  fuming  hydrochloric 
acid,  there  are  disengaged  for  every  atom  of  chromium  3  at. 
chlorine;  for  instance,  KaCr2O7  +  14HC1  =  2KC1  +  2CrCls  + 
6C1  -f-  7HaO.  If  the  escaping  gas  is  conducted  into  a  solution 
of  potassium  iodide  in  excess,  the  3  at.  chlorine  set  free  3  at. 
iodine.  The  liberated  iodine  may  next  be  determined  as  de- 
scribed in  §  146.  380 '55  of  iodine  corresponds  to  lOO'l  of 
chromic  anhydride  (CrO3). 

The  analytical  process  is  conducted  as  follows  :  Put  the  weighed 
sample  of  the  chromate  (say  0*3  to  0*4  grm.)  into  the  little  flask  d, 
Fig.  89  (blown  before  the  lamp,  and  holding  only  from  36  to  40 
c.  c.),  and  fill  the  flask  two-thirds  with  pure  fuming  hydrochloric 
acid  free  from  Cl  and  SO2),  and  add  a  compact  lump  of  magnesite 
to  keep  up  a  constant  current  of  gas  and  prevent  the  fluid  from 
receding.  Connect  the  bulbed  evolution  tube  a  with  the  neck 

*  Annal.  d.  Ghem.  u.  Pharm.,  LXXXVI,  279. 


§  130.]  CHROMIC   ACID.  425 

of  the  flask  by  means  of  a  stout  india-rubber  tube  c.  As 
shown  in  the  engraving,  a  is  a  bent  pipette,  drawn  out  at  the 
lower  end  into  an  upturned  point.  A  loss  of  chlorine  need  not  be 
apprehended  on  adding  the  hydrochloric  acid,  as  the  disengage- 


Fig.  89. 


ment  of  that  gas  begins  only  upon  the  application  of  heat.  Insert 
the  evolution  tube  into  the  neck  of  the  retort,  which  is  one-third 
filled  with  solution  of  potassium  iodide.*  This  retort  holds  about 
160  c.c.  The  neck  presents  two  small  expansions,  blown  before 
the  lamp,  and  intended,  the  lower  one,  to  receive  the  liquid  which 
is  forced  up  during  the  operation,  the  upper  one  to  serve  as  an 
additional  guard  against  spirting.  Apply  heat  now,  cautiously,  to 
the  little  flask.  After  two  or  three  minutes  ebullition  the  whole 
of  the  chlorine  has  passed  over,  and  liberated  its  equivalent  quan- 
tity of  iodine  in  the  potassium  iodide  solution.  "When  the  ebulli- 
tion is  at  an  end,  take  hold  of  the  caoutchouc  tube  c  with  the  left 
hand,  and,  whilst  steadily  holding  the  lamp  under  the  flask  with 
the  right,  lift  a  so  far  out  of  the  retort  that  the  curved  point  is  in 
the  bulb  &.  Now  remove  first  the  lamp,  then  the  flask,  dip  the 
retort  in  cold  water  to  cool  it,  and  shake  the  fluid  in  it  about  to  effect 
the  complete  solution  of  the  separated  iodine  in  the  excess  of  potas- 
sium iodide  solution.  "When  the  fluid  is  quite  cold,  transfer  it  to  a 
beaker,  rinsing  the  retort  into  the  beaker,  and  proceed  as  directed 
§  146.  The  method  gives  very  satisfactory  results.  The  apparatus 
here  recommended  differs  slightly  from  that  used  by  BUNSEN,  the 
retort  of  the  latter  having  only  one  bulb  in  the  neck,  and  the  evo- 
lution tube  110  bulb,  being  closed  instead,  at  the  lower  end,  by  a 
glass  or  caoutchouc  valve,  which  permits  the  exit  of  the  gas  from 

*  1  part  of  pure  potassium  iodide,  free  from  iodic  acid,  dissolved  in  10  parts 
of  water.  The  fluid  must  show  no  brown  tint  immediately  after  addition  of 
dilute  sulphuric  acid. 


426  DETERMINATION.  [§  130. 

the  tube,  but  opposes  the  entrance  of  the  fluid  into  it.  I  think  the 
modifications  which  I  have  made  in  BUNSEN'S  apparatus  are  calcu- 
lated to  facilitate  the  success  of  the  operation.  Instead  of  this  ap- 
paratus, that  described  in  §  112  may  also  be  very  conveniently  used. 
y.  There  need  be  only  mention  made  here  regarding  the 
method  by  RUBE,*  which  is  based  on  the  equation  2Cr()3  -\ 
6K4Fe(Ctf).+  12HCl  =  6KC1  +  2CrCli+3K.Fe,(CN)J,-f  6H3O> 
and  also  regarding  the  method  devised  by  ZuLKOwsKY,f  which  is* 
based  on  the  direct  (i.e.,  without  distillation)  estimation  of  the 
iodine  separated  by  chromic  acid,  and  which  is  carried  out  ex- 
actly as  detailed  under  §  113,  /?,  in  estimating  iron. 

II.  SEPARATION  OF    CHROMIC  ACID    FROM    THE   BASIC 
RADICALS. 
a.   Of  the  First  Group. 

a.  Reduce  the  chromic  acid  to  a  chromic  salt,  as  directed  in  I., 
and  separate  the  chromium  from  the  alkalies  as  directed  in  §  155. 

/?.  Mix  the  potassium  or  sodium  chrornate  with  about  5  parts 
of  dry  pulverized  ammonium  chloride,  and  heat  the  mixture  cau- 
tiously. The  residue  contains  the  chlorides  of  the  alkali  metals 
and  chromic  oxide,  which  may  be  separated  by  means  of  water. 

y.  Precipitate  the  chromic  acid  according  to  L,  a,  ft,  and  sep- 
arate the  mercury  and  alkali  metals  in  the  filtrate  by  §  162. 

b.  Of  the  Second  Group. 

a.  Fuse  the  compound  with  4  parts  of  sodium  and  potassium 
carbonates,  and  treat  the  fused  mass  with  hot  water,  which  dis- 
solves the  chromic  acid  in  the  form  of  an  alkali  chromate.  The 
residue  contains  the  alkali  earth  metals  in  the  form  of  carbonates ; 
but  as  they  contain  alkali,  they  cannot  be  weighed  directly.  The 
chromic  acid  in  the  solution  is  determined  as  in  I.  Strontium  and 
calcium  chromates  may  be  decomposed  by  boiling  with  potassium 
or  sodium  carbonate.  Barium  chromate  may  also  be  decomposed 
in  the  same  way,  but  the  boiling  must  be  repeated  a  second  time 
with  fresh  solution  of  alkali  carbonate  (H.  ROSE). 

*  Journ.  /.  prakt.  Chem.,  xcv,  58;   Zeitschr.f.  analyt.  Chem.,  iv,  444. 
\Journ.f.  prakt.  Chem.,  cm,  351;   Zeitschr.f.  analyt.  Chem.,  vm,  74. 


§  130.]  CHROMIC   ACID.  427 

J3.  Dissolve  in  hydrochloric  acid,  reduce  the  chromic  acid 
according  to  L,  0,  and  separate  the  chromium  from  the  alkali 
earth  metals  according  to  §  156. 

y.  Magnesium  chromate,  as  well  as  other  chromates  of  the 
alkali  earth  metals  soluble  in  water,  may  be  easily  decomposed  also, 
by  determining  the  chromic  acid  according  to  I.,  a,  /?,  or  L,  5,  and 
separating  the  magnesium,  etc.,  in  the  filtrate  from  the  excess  of 
the  salt  of  mercury  or  lead  as  directed  §  162. 

d.  Barium  strontium  and  calcium  chromates  may  also  be 
decomposed  by  the  method  described  II. ,  «,  ft.  Compare  BAHR, 
Analysis  of  barium  and  calcium  dichromates,  etc.* 

H.  ROSE  recommends  using  5  parts  of  ammonium  chloride  to 
1  part  of  the  very  finely  powdered  substance.  One  single  igni- 
tion of  the  mixture  usually  suffices  for  complete  decomposition, 
but  it  is  safer  to  repeat  the  ignition  with  ammonium  chloride,  to 
make  sure  that  the  weight  remains  constant,  before  washing  out 
the  barium  chloride  from  the  residue. 

c.   Of  the  Third  Group. 

a.  From  Aluminium. 

If  you  have  chromic  acid  to  separate  from  aluminium  in  acid 
solution,  precipitate  the  aluminium  with  ammonia  or  ammonium 
carbonate  (§  105,  #),  and  determine  the  chromic  acid  in  the  filtrate 
according  to  I.  If  the  washed  aluminium  hydroxide  has  a  yellow 
color,  treat  on  the  filter  with  ammonia,  and  wash  with  boiling 
water  ;  this  will  remove  the  last  traces  of  chromic  acid.  However, 
a  little  aluminium  hydroxide  dissolves  in  the  ammonia,  therefore 
heat  the  ammoniacal  fluid  in  a  platinum  dish  till  it  has  almost  lost 
its  alkaline  reaction,  and  collect  on  a  filter  the  flocks  of  aluminium 
hydroxide  which  separate,  and  add  them  to  the  principal  precip- 
itate. 

ft.  From  Chromium. 

aa.  Determine  in  one  portion  the  quantity  of  the  chromic  acid 
according  to  L,  d,  or  I.,  e,  a,  or  ft,  and  in  another  portion  the 
total  amount  of  the  chromium,  by  converting  it  into  sesquioxide 
by  cautious  ignition  with  ammonium  chloride,  or  by  I.,  #,  or  by 
converting  it  entirely  into  chromic  acid  by  §  106,  2. 

55.  In  many  cases  the  chromic  acid  may  be  precipitated  accord- 

*  Journ.f.  prakt.  Chem.,  LX,  60. 


428  DETERMINATION.  [§  130. 

ing  to  I.,  a,  y#,  or  L,  J.     The  chromium  and  mercury,  or  lead,  in 
the  filtrate,  are  separated  as  directed  §  162. 

cc.  The  hydrated  compounds  of  sesquioxide  of  chromium  with 
chromium  trioxide,  or  chromic  chromates,  such  as  are  obtained  by 
precipitating  a  solution  of  chromic  salt  with  potassium  chromate,. 
etc.,  may  also  be  analyzed  by  ignition  in  a  stream  of  dry  air,  in  a 
bulb  tube,  to  which  a  calcium  chloride  tube  is  attached  (Fig-  44, 
§  36).  The  loss  of  weight  represents  the  joint  amount  of  oxygen 
and  water  that  have  escaped.  If  the  increment  of  the  CaCla  tube 
is  deducted,  we  shall  have  the  oxygen.  Now  every  3  at.  oxygen 
correspond  to  2  mol.  CrO3.  The  amount  of  the  latter  being  thus 
calculated,  we  have  only  to  subtract  its  equivalent  quantity  of  ses- 
quioxide from  the  weight  of  residue  after  the  ignition,  and  the 
remainder  is  the  quantity  of  sesquioxide  originally  present.  VOGEL* 
and  also  STOEEE  and  ELLIOT^  have  employed  this  method. 

d.  Of  the  Fourth  Group. 

a.  Proceed  as  directed  in  &,  a.  Upon  treating  the  fused  mass 
with  hot  water,  oxides  of  the  basic  metals  are  left.  In  the  case  of 
manganese  the  fusion  must  be  effected  in  an  atmosphere  of  carbon 
dioxide.  Apparatus,  Fig.  83  in  §  108. 

/?.  Reduce  the  chromic  acid  as  directed  in  I.,  a,  and  separate 
the  chromium  from  the  metals  in  question,  as  directed  in  §  160. 

e.  Of  the  Fifth  and  Sixth  Groups. 

a.  Acidify  the  solution,  and  precipitate,  either  at  once  or  after 
reduction  of  the  chromic  acid  by  sulphurous  acid,  with  hydrogen 
sulphide.  The  metals  of  the  fifth  and  sixth  groups  precipitate  in 
conjunction  with  free  sulphur  (§§  115  to  127),  the  chromic  acid  is 
reduced.  Filter  and  determine  the  chromium  in  the  filtrate,  as 
directed  in  I.,  a. 

/?.  Lead  chromate  may  be  conveniently  decomposed  by  heating 
with  hydrochloric  acid  and  some  alcohol ;  the  lead  chloride  and 
chromic  chloride  formed  are  subsequently  separated  by  means  of 
alcohol  (compare  §  162).  The  alcoholic  solution  ought  always  to  be 
tested  with  sulphuric  acid ;  should  a  precipitate  of  lead  sulphate 
form,  this  must  be  filtered  off,  weighed,  and  taken  into  account. 
Compare  also  §  130,  1,  d. 

*  Journ.f.  prakt.  Chem.,  LXXVII,  484. 

f  Proceedings  of  the  American  Academy,  V,  198. 


§  131.]  SELENOUS   ACID.  429 

Supplement  to  the  First  Division. 

§131. 
1.  SELENOUS  ACID. 

From  aqueous  or  hydrochloric- acid  solutions  of  selenous  acid, 
the  selenium  is  precipitated  by  sulphurous  acid  gas,  or,  in  presence 
of  an  excess  of  acid,  by  so'dium  sulphite,  or  ammonium  sulphite. 
The  liquid  containing  the  precipitate  is  heated  to  boiling  for  J  hour, 
which  changes  the  precipitate  from  its  original  red  color  to  black, 
and  makes  it  dense  and  heavy.  The  liquid  is  tested  by  a  further 
addition  of  the  reagent  to  see  whether  any  more  selenium  will  sep- 
arate ;  the  precipitate  is  finally  collected  on  a  weighed  filter,  dried 
at  a  temperature  somewhat  below  100°,  and  weighed.  Since  H. 
ROSE*  has  shown  that  the  presence  of  hydrochloric  acid  is  an  essen- 
tial condition  to  the  complete  reduction  of  selenous  acid,  the  for- 
mer acid  must  be  added,  if  not  already  present.  To  make  quite 
sure  that  all  the  selenium  has  been  removed,  the  filtrate  is  evapo- 
rated to  a  small  volume,  with  addition  of  potassium  or  sodium  chlo- 
ride, boiled  with  strong  hydrochloric  acid,  so  as  to  reduce  any  sele- 
nic  acid  to  selenous  acid,  and  tested  once  more  with  sulphurous 
acid.  If  the  solution  contains  nitric  acid  it  must  be  evaporated 
repeatedly  with  hydrochloric  acid,  with  addition  of  sodium  or 
potassium  chloride.  If  the  latter  were  omitted  there  would  be 
considerable  loss  of  selenous  acid  (RATHKE  f). 

As  regards  the  separation  of  selenous  acid  from  basic  radicals, 
the  following  brief  directions  will  suffice : 

a.  If  the  basic  radicals  are  not  liable  to  be  altered  by  the  action 
of  sulphurous  acid  and  hydrochloric  acid,  the  selenium  may  be  at 
once  precipitated  in  the  way  just  given ;  the  filtrate,  when  evap- 
orated with  sulphuric  acid,  yields  the  base  as  sulphate. 

1).  From  basic  metals  which  are  not  thrown  down  from  acid  solu- 
tion by  hydrogen  sulphide,  the  selenous  acid  may  be  separated  by 
precipitation  with  that  reagent.  The  precipitate  (according  to 
RATHKE,  if  a  mixture  of  SeS, ,  SeaS  and  S)  contains  2  at.  sulphur  to 
1  at.  selenium.  If  it  is  dried  at  or  a  little  below  100°,  the  weight 

*  Zeitschr.  f.  analyt.  Chem.,  i,  73. 

\Journ.  f.  prakt.  CJiem.,  cviu,  249;  ZeitscJir.f.  analyt.  Chem.,  ix,  484. 

^Journ.f.  prakt.  Cliem.,  cvni,  252. 


430  DETERMINATION.  [§  131, 

of  the  selenium  may  be  accurately  ascertained.  Should,  however, 
extra  sulphur  be  mixed  with  the  precipitate,  the  latter  is  oxidized 
while  still  moist  with  hydrochloric  acid  and  potassium  chlorate,  or 
by  treatment  with  potassa  solution  with  simultaneous  heating  and 
transmission  of  chlorine.  It  is  necessary  here  to  oxidize  the  sul- 
phur completely,  as  it  may  enclose  selenium,  The  solution  now 
containing  selenic  acid  is  heated  till  it  smells  no  longer  of  chlorine, 
hydrochloric  acid  is  added,  and  the  mixture  is  reheated.  The  sele- 
nic acid  is  hereby  reduced  to  selenous  acid,  and  when  the  solution 
has  again  ceased  to  smell  of  chlorine,  the  selenium  is  precip- 
itated with  sulphurous  acid.  Instead  of  this  process  you  may  digest 
the  precipitate  of  sulphur  and  selenium  for  some  hours  with  con- 
centrated potassium  cyanide,  which  will  completely  dissolve  it,  and 
then  throw  down  the  selenium  from  the  dilute  solution  with  hydro- 
chloric acid  as  in  c  (RATHKE,  loc.  cit.). 

c.  In  many  selenites  or  selenates  the  selenium  may  also  be 
determined  by  converting  first  into  potassium  selenocyanate,  and 
precipitating  the  aqueous  solution  of  the  latter  with  hydrochloric 
acid  (QppENHEiM*).  To  this  end  the  substance  is  mixed  with  1  or 
8  times  its  quantity  of  ordinary  potassium  cyanide  (containing 
cyanic  acid),  the  mixture  is  put  into  a  long-necked  flask,  or  a  porce- 
lain crucible,  covered  with  a  layer  of  potassium  cyanide,  and  fused 
in  a  stream  of  hydrogen.  The  temperature  is  kept  so  low  that 
the  glass  or  porcelain  is  not  attacked,  and  while  cooling  care  must 
be  taken  to  exclude  atmospheric  air.  When  cold,  the  brown  mass 
is  treated  with  water,  and  the  colorless  solution  filtered,  if  neces- 
sary. The  liquid  should  be  somewhat  but  not  immoderately 
diluted.  Now  boil  some  time  (in  order  to  convert  the  small  quan- 
tity of  potassium  seienide  that  may  be  present  into  potassium  sele- 
nocyanate, by  the  excess  of  potassium  cyanide,  allow  to  cool,  super- 
saturate with  hydrochloric  acid,  and  heat  again  for  some  time.  At 
the  end  of  12  or  24  hours  all  selenium  will  have  separated,  filter, 
dry  at  100°,  and  weigh.  The  results  obtained  by  this  process  are 
accurate  (H.  Rossf ).  If  the  selenium  agglomerates  together  on 
heating,  it  may  enclose  salts.  In  such  cases,  by  way  of  control,  it 
should  be  redissolved  in  nitric  acid,  and,  after  addition  of  hydro- 
chloric acid,  precipitated  with  sulphurous  acid.  The  fluid  filtered 
from  the  selenium  precipitate  is,  as  a  rule,  free  from  selenium  ;  it 

*  Journ.f.  prakt.  Chem.,  LXXI,  280.  f  Zeitschr.  f.  anatyt.  Chem.,  I,  73. 


§131.]  SULPHUROUS   ACID.  431 

is,  however,  always  well  to  satisfy  one's  self  on  this  point  by  the 
addition  of  sulphurous  acid. 

d.  From  many  basic  radicals  selenous  acid  (and  also  selenic 
acid)  may  be  separated  by  fusing  the  compound  with  2  parts  of 
sodium  carbonate  and  one  part  of  potassium  nitrate,  extracting  the- 
fused  mass  thoroughly  by  boiling  with  water,  saturating  the  filtrate, 
if  necessary,  with  carbonic  acid,  to  free  it  from  lead  which  it  might 
contain,  then  boiling  down  wTith  hydrochloric  acid  in  excess  (to 
reduce  the  selenic  acid  and  drive  off  the  nitric  acid),  and  precipi- 
tating finally  with  sulphurous  acid. 

Selenium,  if  pure,  must  volatilize  without  residue  when  heated 
in  a  tube. 

2.  SULPHUROUS  ACID. 

To  estimate  free  sulphurous  acid  in  a  fluid  which  may  contain 
also  other  acids  (sulphuric  acid,  hydrochloric  acid,  acetic  acid),  a 
weighed  quantity  of  the  fluid  is  diluted  with  water,  absolutely  free 
from  air,*  until  the  diluted  liquid  contains  not  more  than  0-05  per 
cent,  by  weight  of  sulphurous  acid,  the  solution  is  poured  with 
stirring  into  an  excess  of  standard  solution  of  iodine,  the  free 
iodine  remaining  is  titrated  with  sodium  thiosulphate,  and  the 
iodine  used  for  the  conversion  of  sulphurous  into  sulphuric  acid  is 
thus  found.  The  reaction  is  expressed  by  the  equation,  SOa  +  2l 
-f  2H2O  =  H2SO4  +  2III.  According  to  FINKENER, f  if  the  iodine 
is  added  to  the  sulphurous  acid  the  reaction  is  not  quite  normal. 
Anyhow  this  method  of  operating  prevents  any  loss  of  sulphurous 
acid.  For  the  details,  see  §  146.  In  case  of  sulphites  soluble  in 
water  or  acids,  water  perfectly  free  from  air  is  poured  over  the 
substance,  in  sufficient  quantity  to  attain  the  degree  of  dilution 
stated  above,  sulphuric  or  hydrochloric  acid  is  added  in  excess,  and 
then  the  titration  is  effected  as  above.  The  greatest  care  must  be 
taken  in  this  method,  to  use,  for  the  purpose  of  dilution,  water 
absolutely  free  from  air. 

Sulphurous  acid  may  also  be  determined  in  the  gravimetric  way, 
by  conversion  into  sulphuric  acid,  and  precipitation  of  the  latter 
with  barium  chloride,  according  to  §  132.  This  method  is  espe- 
cially applicable  in  the  case  of  sulphites  quite  free  from  sulphuric 
acid.  The  conversion  of  the  sulphurous  into  sulphuric  acid  is 

*  Prepared  by  long-continued  boiling  and  subsequent  cooling  with  exclusion 
of  air. 

•f  Handb.  der  analyt.  Chem.  von  H.  ROSE,  6.  Aufl.  von  FINKENER,  n,  937. 


432  DETERMINATION.  [§  131. 

effected  in  the  wet  way,  best  by  pouring  the  dilute  solution  with 
stirring  into  excess  of  chlorine  or  bromine  water.  Sulphites  insolu- 
ble in  water  are  decomposed  by  boiling  with  sodium  carbonate,  and 
the  solution  of  sodium  sulphite  is  treated  as  directed.  After  driv- 
ing off  the  excess  of  chlorine  or  bromine  by  heating,  the  moderately 
acid  solution  is  precipitated  with  barium  chloride.  Sulphites  may 
be  oxidized  in  the  dry  way  by  heating  in  a  platinum  crucible,  with 
4  parts  of  a  mixture  of  equal  parts  sodium  carbonate  and  potassium 
nitrate. 

3.  THIOSULPHTJKIC  ACID. 

Thiosulphuric  acid,  in  form  of  soluble  thiosulphates,  may  be 
determined  by  means  of  iodine,  in  a  similar  way  to  sulphurous 
acid.  The  reaction  is  represented  by  the  equation,  2NaaSaO,  +  21 
=  2NaI  -f-  Na2S4O6.  The  salt  under  examination  is  dissolved  in  a 
large  amount  of  water,  starch-paste  added,  and  then  the  neutral 
solution  is  titrated  with  iodine.  That  this  method  can  give  correct 
results  only  in  cases  where  no  other  substances  acting  upon  iodine 
are  present,  need  hardly  be  mentioned.  Thiosulphuric  acid  may, 
like  sulphurous  acid,  be  converted  into  sulphuric  acid  by  means 
of  chlorine  or  bromine  water,  and  then  determined. 

4.   IODIC  ACID. 

lodic  acid  may  be  determined  by  the  following  easy  method : — 
Distil  the  free  acid  or  iodate  with  an  excess  of  pure  fuming  hydro- 
chloric acid,  in  the  apparatus  described  in  §130,  0,  ft  (chromic  acid), 
receive  the  disengaged  chlorine  in  solution  of  potassium  iodide,  and 
determine  the  separated  iodine  as  directed  in  §  130,  I,  0,  /?.  The 
decomposition  of  iodic  acid  by  hydrochloric  acid  is  represented  by 
the  equation,  H1O3  +  5HC1  =  Id  +  4C1  +  3H2O.  Since  the  4 
at.  Cl  set  free  4  at.  I,  the  amount  of  iodic  acid  or  iodic  anhydride 
can  be  calculated  from  the  weight  of  the  latter  ;  1014-8  iodine  cor- 
respond to  333-7  iodic  anhydride  (I2O5)  (BTJNSEN*).  The  following 
method  also  yields  good  results.  Mix  the  solution  with  dilute  sul- 
phuric acid,  add  potassium  iodide  in  excess,  and  determine  the 
amount  of  liberated  iodine,  after  §  146.  One  sixth  of  the  iodine 
thus  formed  is  derived  from  the  iodic  acid  (HIO9  +  5HI  =  3H2O 

+  Ie).       See  R,AMMELSBEKG.t 

*  Annul,  d.  Chem.  u.  Pharm.,  LXXXVI,  285. 

\Pogg.  Annal.,  cxxxv,  493;  Zeitschr.  f.  analyt.  Chem.,  vm,  456. 


§  131.]  NITROUS   ACID.  433 

5.  NITKOUS  ACID. 

The  nitrous  acid  in  nitrites  which  are  free  from  nitrates  may 
be  estimated  by  converting  the  nitrogen  into  ammonia  and  deter- 
mining the  latter,  or  by  determining  the  oxidizing  action  on.  ferrous 
salt.  This  method  is  conducted  exactly  as  described  under  nitric 
acid  (§  149).  When  nitric  acid  is  also  present,  nitrous  acid  may  be 
determined  very  satisfactorily  with  a  solution  of  pure  potassium 
permanganate,  provided  the  fluid  be  sufficiently  diluted  to  prevent 
the  nitrous  acid,  which  is  liberated  by  the  addition  of  a  stronger 
acid,  being  decomposed  by  water  with  formation  of  nitric  acid 
and  nitric  oxide.  For  1  part  of  nitrous  anhydride  at  least  5000 
parts  of  water  should  be  present.  The  decomposition  is  repre- 
sented by  the  following  equation  :  5HNO2  +  K9Mn,O8  +  3II2SO4 
=  5HNO3+K2SO4  +  2MnSO4  +  3H2O.  If  the  permanganate 
be  standardized  with  iron,  4  at.  iron  correspond  to  1  mol.  2s"aO,, 
since  both  of  these  require  2  at.  oxygen.  Nitrites  are  dissolved  in 
very  slightly  acidulated  water,  the  permanganate  is  added  till  the 
oxidation  of  the  nitrous  acid  is  nearly  completed,  the  solution  is 
then  made  strongly  acid,  and  finally  permanganate  is  added  to  light- 
red  coloration. 

To  determine  nitrogen  tetroxide  N,O4  in  red  fuming  nitric  acid, 
transfer  a  few  c.c.  to  about  500  c.c.  cold  pure  distilled  water  with 
stirring,  and  determine  the  nitrous  acid  produced.  1  mol.  nitrous 
anhydride  found  corresponds  to  2  mol.  nitrogen  tetroxide,  for  the 
latter — when  mixed  with  such  a  large  quantity  of  water  as  is  indi- 
cated above — is  decomposed  in  accordance  with  the  following  equa- 
tion :— jSr2O4  +  H2O  =  HNO3  +  HM)2  (Sio.  FELDHAUS*). 

Nitrous  acid  and  nitrogen  tetroxide  in  presence  of  nitric  acid 
may  also  be  estimated  by  the  reduction  of  chromic  acid.  An 
excess  of  standard  potassium  dichromate  is  added,  and  the  unde- 
composed  residue  of  chromic  acid  is  estimated  with  standard  solu- 
tion of  ferrous  salt  (Fit.  MoHBf). 

As  regards  the  estimation  of  nitrous  acid  with  lead  dioxide, 
comp.  FELDHAUS,  loo.  cit.  p.  431,  also  LANG^:  and  J.  LOWENTHAL.§ 

Regarding  the  estimation  of  nitrous  acid  in  water,  see  §  205. 

*  Zeitsclir.  f.  analyt.  Chem.,  i,  426. 

f  Lehrbuch  der  Titrirmethode,  3.  Aufl.,  236. 

\Zeitschr.f.  analyt.  Chem.,  I,  485.  §  Ib.,  in,  176. 


434  DETEEMnSTATION.  [§  132. 

Second  Division  of  the  First  Group  of  the  Acids. 
SULPHURIC  ACID  ;  (Hydrofluosilicic  Acid). 

§  132. 
SULPHURIC  ACID. 

I.  DETERMINATION. 

Sulphuric  acid  is  usually  determined  in  the  gravimetric  way  as 
barium  sulphate.  The  acid  may,  however,  be  estimated  also  by 
the  acidimetric  method  (§2 15),  and  by  certain  volumetric  methods, 
based  upon  the  insolubility  of  the  barium  sulphate  (and  lead  sul- 
phate). . 

1.  Gravimetric  Method. 

The  exact  estimation  of  sulphuric  acid  as  barium  sulphate  is  by 
no  means  so  simple  and  easy  as  it  was  formerly  supposed  to  be,  but 
requires,  on  the  contrary,  great  care  and  attention.  This  arises 
from  three  causes  (§  71,  a):  First,  the  barium  sulphate  is  found 
to  be  far  more  soluble  than  was  imagined  in  solutions  of  free  acid& 
and  of  many  salts ;  secondly,  it  is  extremely  liable  to  carry  down 
with  it  foreign  salts,  which  are  of  themselves  soluble  in  water' 
thirdly,  when  the  precipitate  has  once  separated  in  an  impure 
state,  it  is  often  very  difficult  to  purify  it  completely. 

The  solution  should  contain  but  little  free  hydrochloric  acid, 
and  no  nitric  or  chloric  acid.  If  either  of  the  two  last  are  present, 
evaporate  repeatedly,  on  the  water-bath  with  pure  hydrochloric 
acid.  Dilute  considerably,  heat  nearly  to  boiling,  add  barium  chlo- 
ride in  moderate  excess,  and  allow  to  settle  for  a  long  time  at  a 
gentle  heat.  Decant  the  clear  fluid  through  a  filter,  treat  the  pre- 
cipitate with  boiling  water,  allow  to  settle,  decant  again,  and  so  on, 
till  the  washings  are  free  from  chlorine.  Finally  transfer  the  pre- 
cipitate to  the  filter,  dry  and  treat  according  to  §  53,  using  only  a 
moderate  red  heat. 

After  the  precipitate  has  been  weighed  it  is  well  to  warm  it  for 
some  time  with  dilute  hydrochloric  acid  on  the  water-bath.  Then 
pour  off  the  hydrochloric  acid  through  a  small  filter,  wash  the  pre- 
cipitate by  decantation  with  boiling  water  without  removing  it  to 
the  filter,  evaporate  the  filtrate  and  washings  nearly  to  dryness  in 
a  platinum  or  porcelain  dish,  add  water,  collect  the  minute  amount 


§  132.]  SULPHURIC    ACID.  435 

of  barium  sulphate  here  left  undissolved  upon  the  small  filter, 
wash,  dry,  incinerate,  add  the  ash  to  the  bulk  of  the  precipitate, 
ignite  again,  and  weigh.  If  the  precipitate  has  lost  weight,  this 
shows  that  it  at  first  contained  foreign  salts. 

This  method  of  purification  sometimes  fails  when  the  precipi- 
tate contains  ferric  oxide  or  platinum  (GLAUS*),  and  it  invariably 
fails  when  the  solution  contained  any  notable  quantity  of  nitric 
acid.f  In  such  cases  there  is  only  one  resource,  namely,  to  fuse 
with  about  four  parts  of  sodium  carbonate,  warm  with  water,  filter, 
wash  with  boiling  water,  acidify  the  filtrate  slightly  with  hydro- 
chloric acid,  and  determine  the  sulphuric  acid  again. 

The  results  are  thoroughly  satisfactory  if  these  directions  are 
attended  to ;  if  not,  the  result  may  be  two  or  three  per  cent,  too 
high  or  too  low. 

2.   Volumetric  Methods. 

a.  After  CARL  MOHB.^  We  require  a  normal  solution  of 
barium  chloride,  containing  122-166  grm.  of  the  pure  crystallized 
salt  in  1  litre,  and  also  normal  nitric  or  hydrochloric  acid  and 
normal  soda  (§215).  Add  to  the  fluid  to  be  examined  for 
sulphuric  acid — which,  should  it  contain  much  free  acid,  is  previ- 
ously to  be  nearly  neutralized  with  pure  sodium  carbonate — a  meas- 
ured quantity  of  barium  chloride  solution,  best  a  round  number 
of  cubic  centimetres,  in  more  than  sufficient  proportion  to  precipi- 
tate the  sulphuric  acid,  but  not  in  too  great  excess.  Digest  the 
mixture  for  some  time  in  a  warm  place,  then  precipitate,  without 
previous  filtration,  the  excess  of  barium  chloride  with  ammonium 
carbonate  and  a  little  ammonia,  filter  off  the  barium  sulphate  and 
carbonate,  wash  until  the  water  running  off  acts  no  longer  upon 
red  litmus  paper,  and  then  determine  the  barium  carbonate  by  the 
alkalimetric  method  given  in  §  223.  Deduct  the  c.c.  of  normal 
acid  used  from  the  c.c.  of  barium  chloride,  and  the  remainder  will 
be  the  c.c.  of  barium  chloride  corresponding  to  the  sulphuric  acid 
present.  The  results  of  this  method  are  quite  satisfactory,  if  the 
solution  does  not  contain  too  much  free  acid ;  but  in  presence  of  a 
large  excess  of  free  acid,  the  action  of  the  salt  of  ammonia  will 
retain  barium  carbonate  in  solution,  which,  of  course,  will  make 

*  JaJiresber.  von  KOPP  und  WILL,  1861,  323,  note. 

f  Compare  my  paper  in  Zeitschr.  f.  analyt.  Chem.,  ix,  52. 

\  Ann.  der  Chem.  u.  Pharm.,  xc,  165. 


436  DETERMINATION.  [§  132 

the  amount  of  sulphuric  acid  appear  higher  than  is  really  the 
case.  It  need  hardly  be  mentioned  that  this  method  is  altogether 
inapplicable  in  presence  of  phosphoric  acid,  oxalic  acid,  or  any 
other  acid  precipitating  barium  salt  from  neutral  solutions,  and  that 
no  basic  radicals  except  the  alkalies  may  be  present. 

J.  After  AD.  CLEMM.*  In  order  to  render  C.  MOHK'S  method 
more  expeditious,  and  hence  better  adapted  for  the  use  of  manu- 
facturers, CLEMM  has  modified  it.  In  it,  also,  the  absence  is 
required  of  all  other  acids  which  yield  insoluble  barium  salts ;  all 
bases  except  the  alkalies  must  also  be  absent.  In  addition  to  the 
standard  solutions  mentioned  under  «,  there  is  also  required  a 
normal  solution  of  pure  sodium  carbonate  (53 '45  grm.  anhydrous 
salt  contained  in  1  litre).  Add  a  little  litmus  tincture  to  the 
solution  contained  in  a  measuring  flask,  and  if  necessary  exactly 
neutralize  with  carbonate-free  KaOH  solution  or  hydrochloric  acid. 
Add  now  a  measured  excess  of  barium-chloride  solution  to  pre- 
cipitate all  the  sulphuric  acid  present,  and  then  add  a  volume  of 
normal  sodium -carbonate  solution  equal  to  that  of  the  barium- 
chloride  solution  used,  nil  with  water  to  the  mark,  shake,  filter, 
and  in  an  aliquot  portion  of  the  filtrate  (about  one  half)  determine 
the  sodium  carbonate  according  to  §  220.  The  acid  required  to 
neutralize  the  residual  sodium  carbonate  is,  of  course,  the  equiva- 
lent of  the  sulphuric  acid  present :  K2SO4  +  2BaCl2  =  BaSO4  + 
2KC1  +  BaCl2 ;  and  BaSO4  +  KC1  +  BaCla  +  2]STa2CO3  =  BaSO4 
+  KC1  +  BaCOs  +  2NaCl  +  Na2CO3.  In  dilute  solution  the  slight 
excess  of  sodium  carbonate  has  no  action  on.  the  barium  sulphate, 
hence  no  error  will  arise  on  this  score.  The  results  are  suffi- 
ciently accurate  for  technical  purposes. 

c.  After  E.  BoHLio.f  This  method,  which  is  also  adapted 
for  technical  purposes,  depends  upon  the  fact  that  the  sulphates 
of  alkalies  are  completely  decomposed  by  precipitated  barium  car- 
bonate in  the  presence  of  an  excess  of  carbonic  acid  and  at  100°, 
barium  sulphate  and  an  alkali  bicarbonate  being  formed ;  the 
heating  prevents  the  solution  of  any  notable  quantity  of  barium 
carbonate  because  of  the  presence  of  free  carbonic  acid.  The 
alkali  which  has  combined  with  the  carbonic  acid  corresponds 


*  Zeitschr  f.  analyt.  Chem.  ix,  122. 
-ft.,  ix,  310. 


§  132.]  SULPIIUKIC   ACID.  437 

to  the  sulphuric  acid  originally  present  as  sulphate.     Regarding 
the  details  see  the  original  paper. 

d.  After  R.  WILDENSTEIN  (first  method  *).  The  principle  of 
this  method  depends  upon  precipitating  the  sulphuric  with  barium 
chloride  and  estimating  the  excess  of  barium  chloride,  using 
potassium  chromate.  If  the  solution  is  neutral,  the  chromate  is 
added  directly ;  if  acid,  after  previous  addition  of  ammonia  free 
from  carbonate  in  slight  excess.  There  are  required:  1.  Barium- 
chloride  solution,  1  c.  c.  of  which  should  correspond  to  0*02  grm. 
sulphuric  anhydride,  SO,,  and  prepared  by  dissolving  61-03 
grm.  of  pure  crystallized  barium  chloride,  BaCla  -(-  2H,O,  to 
make  one  litre.  2.  Potassium-chromate  solution,  of  which 
2  c.  c.  should  precipitate  1  c.  c.  of  the  barium-chloride  solu- 
tion. It  is  prepared  by  dissolving  18 '3853  grm.  potassium  bi- 
chromate in  some  water,  adding  ammonia  until  the  reddish- 
yellow  color  has  given  place  to  a  pale-yellow,  and  then  diluting 
to  measure  1  litre.  The  solution  should  be  neutral. 

The  two  solutions  must  first  be  tested,  to  see  whether  they 
correspond  properly.  For  this  purpose  dilute  10  c.  c.  of  the 
barium-chloride  solution  with  about  50  c.  c.  of  water,  boil,  and 
add  20 '4  c.  c.  of  the  potassium-chromate  solution.  The  precipi- 
tate rapidly  subsides,  and  the  supernatant  liquid  must  be  yellow- 
ish. On  now  adding  barium-chloride  solution  by  drops,  exactly 
0'2  c.  c.  of  the  solution  must  be  required  to  effect  complete 
decomposition — in  all,  therefore,  10 '2  c.  c.  To  carry  out  the 
sulphuric-acid  determination,  dissolve  the  substance  in  about 
50  c.  c.  of  water,  heat  to  boiling  in  a  200  c.  c.  flask,  and  run  in 
barium-chloride  solution  until  perfectly  certain  that  all  the  sul- 
phuric acid  has  been  precipitated,  yet  avoiding  too  great  an  excess. 
Boil  then  for  one-half  to  one  minute,  and  if  acid  neutralize 
with  ammonia  free  from  carbonate,  then  add  to  the  hot  liquid, 
whether  turbid  or  not,  potassium-chromate  solution  in  quantities 
of  0*5  c.  c.  The  liquid  rapidly  clears  up  on  gently  agitating,  so 
that  the  appearance  of  the  yellow  color,  when  the  chromate  begins 
to  be  present  in  excess,  may  be  readily  observed.  When  this  point 
arrives,  add  barium -chloride  solution  slowly  drop  by  drop  until 

*  Zeitschr.f.  analyL  Chem.,  i,  323. 


438  DETERMINATION.  [§  132. 

complete  discolorization  is  just  effected,  for  which  purpose  a  few 
drops,  and  at  most  0*4  c.  c.,  are  required.  Half  the  number  of 
c.  c.  of  potassium-chromate  solution  used  is  deducted  from  the 
entire  number  of  c.  c.  of  barium -chloride  solution  used,  and  from 
the  difference  calculate  the  sulphuric  acid.  Results  good. 

In  applying  this  method  to  the  sulphates  of  magnesium,  zinc, 
or  cadmium,  dissolve  the  sulphate   in  ammonia  with  the  addition 
of  ammonium  chloride,   heat  with   a   little    calcium  chloride  in 
order  to  remove  any  carbonate  that   may  be   present,  then  add 
barium  chloride,  and  finally  the  potassium  chromate  (FLEISCHER  *). 
e.    After  R,  WILDENSTEIN  (second  method  f). 
Of  all  the  methods  for  the  volumetric  estimation  of  sulphuric 
acid,  the  simplest,  and  that  which  is  capable  of  the  most  general 
application,  is  to  drop  in  to  the  solution  containing  excess  of  hydro- 
chloric acid,  standard  barium-chloride  solution,  till 
the  exact  point  is  reached  when  no  more  precipita- 
tion takes  place.    This  point  is  difficult  to  hit,  and 
~h  hence  the  method  has  only  found  a  very  limited  use. 
WILDENSTEIN  has  given  this  method  a  practical 
form,   which  renders   it    possible   to  complete    an 
analysis  in  about  half  an  hour,  and  at  the  same  time 
to  obtain    satisfactory    results.       He  employs    the 
apparatus,  Fig.   90.      A  is  a  bottle  of  white  glass, 
the  bottom  of  which  has  been  removed;  it  holds 
900  to  950  c.  c.     £  is  a  strong  funnel-tube  with 
bell-shaped  funnel,  and  bent  as  shown,  provided  below  with  a  piece 
of  india-rubber  tubing,  a  screw  compression-cock,  and  a  small  piece 
of  tubing  not  drawn  out.     The  length  from  c  to  d  is  about  7J— 8, 
from  d  to  e  about  12,  cm.     The  opening  of  the  funnel-tube  /, 
which  should  have  a  diameter  of  2 -5  to  3  cm.,  is  covered  as  fol- 
lows: Take  a  piece  of  fine  new  calico  or  muslin  free  from  sul- 
phuric acid  and  about  6  cm.   square,  lay    on  it   two. pieces   of 
Swedish  paper  of  the  same  size  and  then  another  piece  of  stuff 
like  the  first,  now  bind  these  all  together  over  the  opening/1,  care- 
fully and  without  injuring  the  paper,  by  means  of  a  strong  linen 

*  Journ.  f.  prakt.  Chem.,  N.  F.  v,  318.  Here  also  a  modification  is  given  by 
which  the  excess  of  ammonium  chromate  maybe  detected  in  colored  liquids,  but 
unfortunately  the  process  is  far  less  simple. 

\Zeitschr.f.  analyt.  Chem.,  i,  432. 


§  132.]  SULPHURIC   ACID.  439 

thread  which  has  been  drawn  a  few  times  over  wax,  and  cut  it  off 
even  all  round.  We  have  now  a  small  siphon -filter,  which 
enables  us  to  filter  off  a  portion  of  fluid  contained  in  A,  and 
turbid  from  barium  sulphate,  clear  and  with  comparative  rapidity. 

On  gradually  adding  barium  chloride  to  the  dilute  acid  solution 
of  a  sulphate  a  point  occurs  which  may  be  compared  with  the  neutral 
point  in  precipitating  silver  with  sodium  chloride  (see  §115,  5,  b) ; 
i.e.,  there  is  a  certain  moment  when  a  portion  filtered  off  will  give 
a  turbidity  both  with  sulphuric  acid  and  barium  chloride  after  the 
lapse  of  a  few  minutes.  On  this  account  we  must  either  proceed 
on  the  principle  recommended  for  the  estimation  of  silver,  i.e.,  dis- 
regarding the  quantity  of  barium  chloride  in  the  solution,  to  stand- 
ardize it  by  adding  it  to  a  known  amount  of  sulphate,  till  a  pre- 
cipitate ceases  to  be  formed  ;  or  else  we  must — and  WILDENSTEIN 
recommends  this  latter  course — consider  as  the  end-point  of  the 
reaction  the  point  at  which  barium  chloride  ceases  to  produce  a 
distinctly  visible  precipitation  in  the  clear  filtrate  after  a  lapse  of 
two  minutes. 

The  barium  chloride  solution  is  prepared  so  that  1  c.c.  corre- 
sponds to  0-02  sulphuric  anhydride, by  making  a  solution  contain- 
ing the  requisite  calculated  and  carefully  weighed  amount  of  the 
pure  salt  per  litre. — A  solution  of  sulphuric  acid  containing  0'02 
grm .  SO3  per  c.  c.  may  also  be  required.  The  process  is  as  follows : 

First  prepare  the  solution  of  the  sulphate  to  be  analyzed  (using 
;about  3  or  4  grin.),  then  fill  A.  with  hot  water,  open  the  cock  with 
the  screw  or  by  the  aid  of  a  glass  rod,  and  wait  till  the  syphon  B 
is  quite  full  of  water.  If  the  water  runs  down  the  tube  c  e  with- 
out filling  it  entirely,  close  and  open  the  cock  a  few  times,  and  this 
inconvenience  will  be  removed.  (It  is  not  allowable  to  suck  at  0, 
or  to  fill  the  syphon  with  the  wash-bottle  at  <?,  as  either  proceeding 
would  inevitably  lead  to  injuring  the  filter.)  Now  close  the  cock 
and  pour  out  the  hot  water,  replace  it  by  400  c.c.  of  boiling  water, 
add  the  ready-prepared  solution  of  the  sulphate,  and  a  small  quan- 
tity of  hydrochloric  acid,  if  necessary,  and  run  in  the  barium  chlo- 
ride solution,  at  first  in  rather  large  portions,  at  last  in  \  c.c. 
Before  each  fresh  addition  of  barium  chloride  open  the  cock  and 
allow  rather  more  liquid  to  flow  into  a  beaker  than  corresponds  to 
.the  contents  of  the  syphon.  This  quantity  should  be  previously 
ascertained,  and  a  mark  indicating  it  made  on  the  beaker.  Now 


440  DETERMINATION.  [§  132, 

close  the  cock  and  pour  the  filtrate  without  loss  back  into  A.  (As 
the  beaker  is  used  over  and  over  again  for  the  same  purpose,  it 
need  not  be  rinsed  out.)  Now  run  some  of  the  fluid  into  a  test- 
tube,  so  as  to  one  third  fill  it,  add  to  the  clear  fluid  2  drops  of 
barium  chloride  from  the  burette  and  shake.  If  a  precipitate  or 
turbidity  is  produced,  return  the  portion  to  the  main  quantity.  The 
experiment  is  finished  when  the  last  portion  tested  shows  after  the 
lapse  of  exactly  two  minutes  no  distinctly  visible  turbidity.  The 
drops  of  barium  chloride  used  for  the  last  testing  are  of  course  not 
reckoned.  The  slight  error  involved  from  the  fact  that  the  small 
quantity  of  fluid  in  the  syphon  is  finally  unacted  on,  is  too  small 
to  be  noticed.  During  the  experiment  the  filter  must  not  be 
injured  by  the  stirring.  In  case  the  end  reaction  has  been  over- 
stepped, add  1  c.c.  of  dilute  sulphuric  acid  (equivalent  to  the  barium 
chloride)  to  A,  and  endeavor  to  hit  it  again.  Here  1  c.c.  w^ill 
have  to  be  subtracted  from  the  c.c.  of  barium  chloride  used. 

The  results  obtained  by  WILDENSTEIN  are  of  sufficient  accuracy 
for  technical  purposes.  Some  experiments  made  in  my  own  labo- 
ratory were  also  quite  satisfactory. 

f.  The  methods  of  LEVOL,*  J^APPENHEiMjf  SCHWARZ,:);  etc., 
depending  on  the  precipitation  of  the  sulphuric  acid  with  stand- 
ard lead  solution,  are  only  of  limited  application,  because  chlo- 
rides, hydrochloric  acid,  and  ammonium  salts  cause  disturbances 
in  the  reactions. 

II.  SEPARATION  or  SULPHURIC  ACID  FROM  THE  B^sic 
RADICALS. 

a.  In  Sulphates  which  are  soluble  in  Water  or  Hydrochloric 
Acid. 

The  solution  should  be  free  from  nitric  acid.  Precipitate  the 
sulphuric  acid  according  to  I.  by  barium  chloride  (or  barium  ace- 
tate). The  filtrate  contains  the  excess  of  barium  chloride,  together 
with  the  chlorides  of  the  metals  present ;  separate  barium  from  the 
latter  by  methods  given  in  the  fifth  section.  The  fluid  obtained  by 
treating  the  ignited  barium  sulphate  with  hydrochloric  acid,  evap- 

*  Bulletin  de  la  Societe  d'Encourag.,  Avril,  1853;   Journ.  /.  prakt.  Chem., 
LX,  384. 

fMoHK's  Lehrbuch  der  Tilrirmethode,  3.  Aufl.,  411. 
\Zeitschr.f.  analyt,  Chem.,  n,  392. 


§  132.]  SULPHUKIC    ACID.  441 

orating  and  filtering  from  the  small  amount  of  barium  sulphate, 
must  be  added  to  the  first  solution  before  separating  barium 
from  it. 

If  the  barium  sulphate,  after  treatment  with  hydrochloric 
acid,  still  contains  foreign  bases,  dissolve  the  barium  sulphate  in 
sulphuric  acid  with  heat,  pour  the  solution  carefully  into  cold 
water,  and  filter  off  the  precipitated  barium  sulphate.  The  foreign 
bases  will  remain  in  solution. 

b.  In  Sulphates  which  are  insoluble  or  difficultly  soluble  in 
Water  or  in  Hydrochloric  Acid. 

a.  From  barium,  strontium  and  calcium :  Fuse  the  finely  pul- 
verized substance  in  a  platinum  crucible,  with  5  parts  of  mixed 
sodium  and  potassium  carbonates.  Put  the  crucible,  with  its  con- 
tents, into  a  beaker,  or  into  a  platinum  or  porcelain  dish,  pour 
water  over  it,  and  apply  heat  until  the  alkali  sulphates  and  carbon- 
ates are  completely  dissolved  ;  filter  the  hot  solution  from  the  resid- 
uary alkali-earth  carbonates,  wash  the  latter  thoroughly  with  water,- 
to  which  a  little  ammonia  and  ammonium  carbonate  has  been  added, 
and  determine  according  to  §§  101  to  103.  If  the  precipitates  have 
been  well  washed,  it  is  perfectly  admissible  to  ignite  and  weigh  at 
once.  Precipitate  the  sulphuric  acid  from  the  filtrate,  as  in  I., 
after  acidifying  with  hydrochloric  acid.  Finely  pulverized  calcium 
and  strontium  sulphates  may  be  completely  decomposed  also  by 
boiling  with  a  solution  of  potassium  carbonate.* 

This  process  will  also  answer  for  barium  sulphate,  but  is  far 
more  difficult,  and  effective  only  on  repeatedly  boiling  the  pre- 
cipitate with  excess  of  alkali- carbonate  solution,  after  decanting 
the  liquid  (H.  KosEf).  KosEf). 

ft.  From  lead :  The  simplest  way  of  effecting  the  decomposi- 
tion of  lead  sulphate  consists  in  digesting  it,  at  the  common  tem- 
perature, with  a  solution  of  hydrogen-sodium  or  hydrogen-potas- 
sium carbonate,  filtering,  washing  the  precipitate,  determining  the 
sulphuric  acid  in  the  filtrate  as  in  I.,  dissolving  the  precipitate, 
which  contains  alkali,  in  nitric  or  acetic  acid,  and  determining  the 
lead  in  the  solution,  by  one  of  the  methods  given  in  §  162. 

Presence  of  strontium  and  calcium  necessitates  no  alteration  in 
this  method ;  but  if  barium  also  is  present,  and  it  is  accordingly 

*  Sodium  carbonate  does  not  answer  so  well. 
•\Journ.f.  prakt.  CJiem.,  LXIV,  382;  LXV,  316. 


442  DETERMINATION.  [§  133. 

necessary  to  ignite*  the  mixture  witli  alkali  carbonates,  a  small 
portion  of  lead  always  remains  in  solution  in  the  alkaline  fluid  ;  this 
must  be  precipitated  by  passing  through  it  carbon  dioxide  before 
filtering. 

y.  From,  mercury  in  mercurous  sulphate :  Mercurous  sulphate 
is  best  dissolved  by  warming  with  dilute  hydrochloric  acid  with 
addition  of  potassium  chlorate  or  bromine,  and  the  solution  is 
treated  according  to  a.  If  the  salt  is  boiled  with  solution  of  potas- 
sium carbonate,  the  mercurous  carbonate  first  formed  is  decom- 
posed, and  the  residue  contains  metallic  mercury  and  mercuric 
oxide ;  a  small  part  of  the  latter  passes  into  the  filtrate. 

III.     ESTIMATION    OF    FKEE   SULPHURIC  ACID  IN  THE 
PRESENCE  OF  SULPHATES. 

We  have  occasionally  to  estimate  the  free  acid  in  presence  of 
sulphates,  as,  for  instance,  in  vinegar,  wine,  etc.  According  to  A. 
GiRABJ>t  the  following  is  the  only  direct  method  which  can  be 
relied  on :  Evaporate  on  the  water-bath  to  dryness  and  exhaust  the 
residue  with  absolute  alcohol ;  determine  the  combined  acid  in  the 
residue,  and  the  free  acid  in  the  alcoholic  extract,  after  mixing  with 
water  and  evaporating  off  the  alcohol.  It  has  been  said  that  the 
object  may  be  obtained  by  the  use  of  barium  carbonate,  which  is 
supposed  to  throw  down  the  free  acid  only,  but  this  is  erroneous, 
since  alkali  sulphates  in  aqueous  solution  are  partially  decomposed 
at  the  ordinary  temperature  by  barium  carbonate.  In  some  cases 
the  amount  of  free  sulphuric  acid  present  may  be  calculated  after 
having  determined  the  total  amount  of  basic  and  acid  radicals 
present.  When  no  other  free  acid  is  present,  free  sulphuric  acid 
may  be  determined  by  the  acidimetric  process. 

Supplement  to  the  Second  Division. 

§133. 
HYDROFLUOSILICTC  ACID. 

If  you  have  hydrofluosilicic  acid  in  solution,  add  solution  of 
potassium  chloride,  then  a  volume  of  strong  alcohol  equal  to  the 
fluid  present,  collect  the  precipitated  potassium  silicofluoride  on  a 

*  This  is  best  done  in  a  porcelain  crucible. 

\Gompt.  Rend.,  LXXXVIII,  515;  Zeitschr.f.  analyt.  Chem.,  iv,  219. 


§  133.]  HYDROFLUOSILICIC   ACID.  443 

weighed  filter,  and  wash  with  a  mixture  of  equal  volumes  of  alco- 
hol and  water.  Dry  the  washed  precipitate  at  100°,  and  weigh. 
Mix  the  alcoholic  filtrate  with  hydrochloric  acid,  evaporate  to  dry- 
ness,  and  treat  the  residue  with  hydrochloric  acid  and  water.  If 
this  leaves  an  undissolved  residue  of  silicic  acid,  this  is  a  sign  that 
die  examined  acid  contained  an  excess  of  silicic  acid  ;  the  weight 
of  the  residue  shows  the  amount  of  excess.  Potassium  silicofluor- 
ido  dried  at  100°  has  the  formula  (KF)2  Si F4 ;  for  its  properties, 
scr  §  68.  Instead  of  weighing  it,  it  may  be  estimated  volumetric- 
ally  according  to  §  97,  5.  The  analysis  of  metallic  silicofluorides  is 
best  effected  by  heating  in  platinum  vessels,  with  concentrated  sul- 
phuric acid ;  silicon  fluoride  and  hydrofluoric  acid  volatilize,  the  basic 
metals  are  left  behind  in  the  form  of  sulphates,  and  may,  in  many 
cases,  after  volatilization  of  the  excess  of  sulphuric  acid,  be  weighed 
.as  such.  If  the  metallic  silicofluorides  to  be  analyzed  contain  water, 
the  latter  cannot  be  estimated  by  mere  ignition,  since  silicon  fluoride 
would  escape  with  the  water.  H.  ROSE  recommends  the  following 
method :  Mix  them  most  intimately  with  0  parts  of  recently  ignited 
lead  oxide,  cover  the  mixture  in  a  small  retort,  with  a  layer  of 
pure  lead  oxide,  weigh  the  retort,  heat  cautiously  until  the  contents 
begin  to  fuse  together,  remove  the  aqueous  vapor  still  remaining 
in  the  vessel  by  suction,  and  weigh  the  retort  again  when  cold. 
The  diminution  of  weight  shows  the  quantity  of  water  expelled. 
Do  not  neglect  testing  the  drops  of  th&  escaping  water  with 
litmus  paper;  the  result  is  ^^urate  only  if  they  have  no  acid 
reaction. 

F.  STOLE  A*  proposes  the  following  process,  at  least  for  com- 
pounds soluble  in  water :  Put  into  a  crucible  double  as  much  mag- 
nesia as  is  necessary  to  decompose  the  silicofluoride  to  be  analyzed, 
ignite  it  as  strongly  as  possible,  allow  to  cool,  and  weigh.  Add  water 
to  form  a  thick  paste,  and  then  the  weighed  silicofluoride ;  if  the 
amount  of  water  present  is  not  enough  to  dissolve  the  compound, 
add  some  more,  mix  with  a  platinum  wire  which  must  afterwards 
be  wiped  off  clean,  dry,  ignite,  and  weigh.  The  increase  in  weight 
shows  the  amount  of  anhydrous  silicofluoride,*provided  no  oxide  is 
present  which  takes  up  oxygen. 

*  ZeitscJir.  f.  analyt.  Chem.,  vn,  93. 


444  DETEBMINATION.  [§  134. 

Third  Division  of  the  First  Group  of  the  Adds. 

PHOSPHORIC     ACID BORIC  ACID — OXALIC    ACID HYDROFLUORIC 

ACID. 

§134 
1.  PHOSPHORIC  ACID. 

I.  DETERMINATION. 

Orthophosphoric  acid  may  be  determined  in  a  great  variety  of 
ways.  The  forms  in  which  this  determination  may  be  effected 
have  been  given  already  in  §  93,  4.  The  most  appropriate  forms 
for  the  purpose,  however,  are  magnesium  pyrophosphate  and  ura- 
nyl  pyrophosphate.  The  determination  as  magnesium  pyrophos- 
phate is  frequently  preceded  by  precipitation  in  another  way, 
especially  as  ammonium  phospho-molybdate,  occasionally  as  stannic 
phosphate  or  mercurous  phosphate.  The  other  forms  in  which 
phosphoric  acid  may  be  determined  give  also,  in  part,  very  good 
results,  but  admit  only  of  a  more  limited  application.  "With 
respect  to  volumetric  methods,  those  which  depend  upon  the  use 
of  standard  solution  of  uranium  are  the  best. 

With  regard  to  meta-  and  pyrophosphoric  acids,  I  have  simply 
to  remark  here  that  these  acids  cannot  be  determined  by  any  of  the 
methods  given  below.  The  best  way  to  effect  their  determination 
is  to  convert  them  into  orthophosporic  acid,  as  follows  : 

a.  In  the  dry  way.  By  protracted  fusion  with  from  4  to  6 
parts  of  mixed  sodium  and  potassium  carbonates.  This  method  is, 
however,  applicable  only  in  the  case  of  alkali  meta-  and  pyrophos- 
phates,  and  of  those  metallic  mata-  or  pyrophosphates  which  are 
completely  decomposed  by  fusion  with  alkali  carbonates  ;  it  fails, 
accordingly,  for  instance,  with  the  salts  of  the  alkali-earth  metals, 
magnesium  excepted. 

ft.  In  the  wet  way.  The  salt  is  heated  for  some  time  with  a 
strong  acid,  best  with  concentrated  sulphuric  acid  (WEBER*).  This 
method  leads  only  to^the  attainment  of  approximate  results,  in  the 
case  of  all  salts  whose  basic  radicals  form  soluble  salts  of  the  acid 
added,  since  in  these  cases  the  meta-  or  pyrophosphoric  acid  is 
never  completely  liberated  ;  but  the  desired  result  may  be  fully 
attained  by  the  use  of  any  acid  which  forms  insoluble  salts  compounds 
the  basic  radicals  present.  Respecting  the  partial  conversion 
*  Pogg.  Annal.,  LXXIII,  137. 


§  134.]  PHOSPHORIC    ACID.  445 

in  the  former  case,  I  have  found  that  it  approaches  the  nearer  to 
completeness  the  greater  the  quantity  of  free  acid  added,*  and  that 
the  ebullition  must  be  long  continued.  Compare  Expt.  "No.  32. 

BUNCE'S  statement,  f  that  phosphoric  acid  volatilizes  when  a 
phosphate  is  evaporated  to  dryness  with  hydrochloric  or  nitric  acid 
and  the  residue  heated  a  little,  is  quite  erroneous  (compare  my 
paper  on  the  subject,  in  A.nnal.  der  Chem.  und  Pharm.,  LXXXVI, 
216).  But,  on  the  other  hand,  it  must  be  borne  in  mind  that 
orthophosphoric  acid  under  these  circumstances  changes,  not  indeed 
ut  100°,  but  at  a  temperature  still  below  150°,  to  pyrophosphoric 
acid ;  thus,  for  instance,  upon  evaporating  common  hydrogen 
sodium  phosphate  with  hydrochloric  acid  in  excess,  and  drying 
the  residue  at  150°,  we  obtain  2NaCl  +  NaaH3P2O, . 

a.  Determination  as  Lead  Phosphate. 

Proceed  as  with  arsenic  acid,  §  127,  1,  a — i.e.,  evaporate  with 
a  weighed  quantity  of  oxide  of  lead,  and  ignite.  This  method  pre- 
supposes that  no  other  acid  is  present  in  the  aqueous  or  nitric  acid 
solution ;  it  has  this  great  advantage,  that  it  gives  correct  results, 
no  matter  whether  ortho-,  meta-,  or  pyrophosphoric  acid  is  present. 

1).  Determination  as  Magnesium  Pyrophosphate. 

OL.  Direct  determination.  Suitable  in  all  cases  in  which  it  is 
quite  certain  that  the  acid  present  is  orthophosphoric,  either  free 
or  combined  as  an  alkali  phosphate. 

The  solution  should  be  neutral,  or  only  moderately  ammoniacal. 
Add  ammonium  chloride,  and  then  the  usual  magnesia  mixture 
(§  62,  6),  or  still  better,  a  mixture  of  magnesium  chloride,  am- 
monium chloride,  and  ammonia,;}:  in  sufficient  but  not  too  ex- 
cessive quantity  (see  §  62,  6);  10  c.c.  of  the  mixture  will  pre- 
cipitate 0*24  grm.  PaO6.  The  precipitate  being  under  these 
conditions  somewhat  slowly  formed,  appears  distinctly  crystalline. 

*  There  are,  however,  other  considerations  which  forbid  going  too  far  in  this 
respect. 

t  Sillim.  Journ.,  May,  1851,  405. 

|  This  mixture  deserves  the  preference  because  it  can  be  employed  with 
greater  certainty  than  the  sulphate  solution  (as  when  the  latter  is  not  quite  cor- 
rectly used  the  precipitate  is  apt  to  contain  some  basic  magnesium  sulphate) 
and  gives  accurate  results.  It  is  made  as  follows :  Dissolve  83  grm.  crystal- 
lized magnesium  sulphate  in  boiling  water,  add  5  c.c.  hydrochloric  acid,  then 
an  aqueous  solution  of  82  grm.  crystallized  barium  chloride,  boil,  decant, 
filter,  and  test  with  sulphuric  acid  (no  precipitate  should  form).  Mix  the  fil- 
trate and  washings,  concentrate,  cool,  transfer  to  a  litre  flask,  add  165  grm. 
pure  ammonium  chloride,  260  c.c.  ammonia,  and  water  to  1  litre.  Set  aside 
for  a  few  days  and  filter  if  necessary.  This  solution  contains  the  same  quan- 
tity of  magnesia  as  the  mixture  given  under  §  62,  6, 


446  DETERMINATION.  [§  134, 

After  some  time  add  ammonia  gradually  to  the  amount  of  one- 
third  of  the  fluid.  Allow  to  stand  12  hours  in  a  well-covered 
vessel  in  the  cold,  filter,  test  the  filtrate  with  magnesia  mixture 
and  ammonia,  and  wash  the  precipitate  with  ammonia  diluted  with 
3  volumes  of  water  till  the  washings,  when  acidified  with  nitric 
acid  and  tested  with  silver  nitrate,  are  no  longer  rendered  turbid; 
proceed  according  to  §  104,  2.  The  precipitate  is  not  abso- 
lutely insoluble  in  ammoniated  water,  therefore  it  is  Well  to 
wasli  by  suction,  as  this  reduces  the  necessary  amount  of  wash- 
water  to  a  minimum.  The  results  are  accurate  (Expt.  JSTo.  80 ; 
also  KISSEL*).  If  there  is  reason  to  suspect  the  purity  of 
the  precipitate,  dissolve  it  in  hydrochloric  acid  and  throw 
down  again  with  ammonia,  adding  some  magnesia  mixture.  If  the 
magnesia  mixture  is  omitted,  the  solution,  being  free  from  magnesia, 
will  dissolve  some  of  the  precipitate.  Compare  KISSEL,  loc.  cit. 
For  properties  of  the  precipitate  and  residue,  see  §  74.  If  the 
solution  contains  pyrophosphoric  acid,  the  precipitate  is  floccnlent 
and  dissolves  to  a  notable  degree  in  ammoniated  water  (WEBER). 

/?.  Indirect  determination,  with  previous  precipitation  as  ammo- 
nium  pJiospliomolybdate,  after  SoNNENSCHEHsr.f 

Applicable  in  all  cases  in  which  the  phosphoric  acid  present  is 
orthophosphoric,  even  in  presence  of  salts  of  the  alkali-earth  metals, 
aluminium,  ferric  iron,  &c.  Tartaric  acid,  however,  and  similarly 
acting  organic  substances  must  be  absent.  No  considerable  quan- 
tity of  free  hydrochloric  acid  may  be  present.  Large  quantities  of 
ammonium  chloride,  and  of  metallic  chlorides  generally,  also  of 
certain  ammonium  salts,  especially  the  oxalate  and  citrate  (KONIG)  J, 
are  to  be  avoided.  Ammonium  nitrate  assists  the  precipitation  and 
neutralizes  the  injurious  action  of  very  large  quantities  of  nitrates 
and  sulphates  (E.  RICHTEKS)§.  The  molybdenum  solution  described 
"  Qual.  Anal.,"  §  52,  is  employed  as  the  precipitant.  It  con- 
tains 5  per  cent,  of  molybdic  acid.  The  fiuid  to  be  examined 
for  phosphoric  acid  should  be  concentrated ;  it  may  contain 
free  nitric  or  free  sulphuric  acid.  Transfer  to  a  beaker 
and  add  a  considerable  quantity  of  the  molybdenum  solu- 
tion. About  40  parts  molybdic  acid  must  be  added  for 
every  1  part  phosphoric  anhydride,  therefore  80  c.c.  of  the 
molybdic  solution  for  0*1  grm.  Stir  without  touching  the 
sides,  and  keep  covered  12  hours  at  about  40°;  then  remove 
a  portion  of  the  clear  supernatant  fluid  with  a  pipette,  mix 

*Zeitschr.f.  analyt.  Chem.,  vni,  170.       -\Journ.f.  prakt.  Chem.,  LIU,  343. 
IZeitschr.f,  analyt,  Chem.,  x,  305.  %lb.  x,  469. 


134.]  PHOSPHORIC   ACID.  447 

it  with  an  equal  volume  of  molybdenum  solution,  and  allow  it  to 
stand  some  time  at  40°.  If  a  further  precipitation  takes  place, 
return  the  portion  to  the  main  quantity,  add  more  molybdenum 
solution,  allow  to  stand  again  12  hours,  and  test  again.  "When 
complete  precipitation  has  been  effected  pour  the  fluid  off  through 
a  small  filter  and  wash  the  precipitate  entirely  by  decantation, 
using  a  mixture  of  100  parts  molybdate  solution,  20  parts  nitric 
acid  of  1*2  sp.  gr.,  and  80  parts  water.*  The  washing  must  be 
thorough,  and  the  last  runnings  must  not  be  precipitated  by  excess 
of  ammonia,  even  if  lime,  iron,  &c.,  was  present  in  the  solution. 
Now  dissolve  the  precipitate  in  the  least  quantity  of  ammonia, 
pour  the  fluid  through  the  small  filter,  when  the  minute  amount  of 
precipitate  thereon  will  be  dissolved,  wash  the  filter  with  ammonia 
diluted  with  three  volumes  of  water,  mix  the  filtrate  and  washings, 
and  add  hydrochloric  acid  carefully  till  the  precipitate  produced, 
instead  of  redissolving  instantly,  takes  a  little  time  to  disappear; 
finally  throw  down  with  magnesia  mixture  (compare  a).  If  the 
ammonia  leaves  a  small  amount  of  the  precipitate  undissolved, 
treat  the  residue  with  nitric  acid  and  test  the  filtrate  with  molybdic 
solution  in  order  to  save  any  phosphoric  acid.  Results  accurate.f 
As  this  method  requires  so  large  a  quantity  of  molybdic  acid,  it 
is  usually  resorted  to  only  in  cases  where  methods  £>,  a,  and  c  are 
inapplicable ;  and  the  phosphoric  acid  in  the  quantity  of  substance 
taken  is  not  allowed  to  exceed  0*3  grm.  Arsenic  acid  and  silicic 
acid,;):  if  present,  must  first  be  removed.  Of  all  the  methods  for 
determining  phosphoric  acid  which  are  admissible  in  the  presence 
of  ferric  and  aluminium  salts,  this  is  the  best  in  my  opinion,  espe- 
cially for  the  estimation  of  small  quantities  of  the  acid  in  presence 
of  large  quantities  of  these  salts. 

*  According  to  E.  RICHTERS  (Zeitschr.  f.  anatyt.  Chem.,  x,  471)  you  may  also 
wash  with  a  solution  of  ammonium  nitrate  containing  15  grm.  in  100  c.c.  slightly 
acidified  with  nitric  acid  and  containing  a  few  per  cents  of  molybdic-acid 
solution. 

f  Zeitschr.  f.  analyt.  Chem.,  m,  446,  and  vi,  403. 

\  Silicic  acid  may  also  be  thrown  down,  in  form  of  a  yellow  precipitate,  by 
acid  solution  of  ammonium  molybdate,  especially  in  presence  of  much  ammo- 
nium chloride  (W.  KNOP,  Chem.  CentralbL,  1857,  691).  Mr.  GRUNDMANN,  who 
repeated  KNOP'S  experiments  in  my  laboratory,  obtained  the  same  results.  The 
precipitate  dissolves  in  ammonia.  If  the  solution,  after  addition  of  some  ammo- 
nium chloride,  is  allowed  to  stand  for  some  time,  the  silicic  acid  separates,  and 
the  phosphoric  acid  may  then  be  precipitated  from  the  filtrate  with  magnesia 
mixture;  it  is,  however,  always  the  safer  way  to  remove  silicic  acid  first. 


448  DETERMINATION.  [§  134 

y.  Indirect  determination,  with  previous  precipitation  as  mer- 
curous  phosphate,  after  II.  ROSE.* 

Applicable  for  the  separation  of  phosphoric  acid  (also  of  pyro- 
and  metaphosphoric  acid)  from  all  basic  radicals,  except  aluminium. 
Comp.  §  135,  k. 

Dissolve  the  phosphate  in  neither  too  large  nor  too  small  a 
quantity  of  nitric  acid,  in  a  porcelain  dish,  add  pure  metallic  mer- 
cury in  sufficient  quantity  to  leave  a  portion,  even  though  only  a 
small  one,  undissolved  by  the  free  acid.  Evaporate  on  the  water- 
bath  to  dryness.  If  the  warm  mass  still  evolves  an  odor  of  nitric 
acid,  moisten  it  with  water,  and  heat  again  on  the  water-bath,  until 
it  smells  no  longer  of  nitric  acid.  Add  now  hot  water,  pass  through 
a  small  filter,  and  wash  until  the  washings  leave  no  longer  a  fixed 
residue  upon  platinum.  Dry  the  filter,  which,  besides  mercurous 
phosphate,  contains  also  basic  mercurous  nitrate  and  free  mercury, 
mix  its  contents,  in  a  platinum  crucible,  with  mixed  sodium  and 
potassium  carbonates  in  excess,  roll  the  filter  into  the  shape  of  a 
ball,  place  it  in  a  hollow  made  in  the  mixture,  and  cover  the  whole 
with  a  layer  of  the  mixed  carbonates.  Expose  the  crucible,  under 
a  chimney  with  good  draught,  for  about  half  an  hour  to  a  moderate 
heat,  so  that  it  does  not  get  red-hot.  At  this  temperature,  the 
mercurous  nitrate  and  the  metallic  mercury  volatilize.  Heat  now 
over  the  lamp  to  bright  redness,  and  treat  the  residue  with  hot 
water,  which  will  dissolve  it  completely,  if  no  ferric  oxide  be 
present,  and  if  no  oxide  of  platinum  has  been  formed.  The  latter 
may  occur  on  account  of  too  rapid  heating,  which  might  produce 
sodium  nitrate,  which  would  act  upon  the  platinum.  Supersatu- 
rate the  clear  (if  necessary,  filtered)  solution  with  hydrochloric 
acid,  add  ammonia  and  magnesia  mixture,  and  proceed  as  in  or.f 

#.  Indirect  determination,  with  previous  precipitation  as 
stannic  phosphate. 

aa.  After  W.  KEISSIG.^:     Dissolve  the  substance,  which  must 

*  Fogg.  Annal.,"LXXVi,  218. 

f  If  ferric  oxide  is  present  to  any  extent,  it  may  easily  retain  some  phos- 
phoric acid.  (Compare  §  135,  g,  a.)  ROSE'S  modification,  to  be  employed 
when  alumina  is  present,  is  given  in  §  135,  K,  y. 

%Annal.  d.  Chem.  u.  Pharm.,  xcvm,  339,  The  method  is  a  serviceable 
modification  of  REYNOSO'S  process  (Journ.  f.  prakt.  Chem.,  LIV,  261),  which, 
though  free  from  errors  in  principle,  yet  presents  certain  difficulties,  particularly 
the  fact  that  the  slight  impurities  in  the  tin  are  a  source  of  considerable  error, 
since  at  least  8  times  as  much  tin  must  be  taken  as  the  quantity  of  phosphoric 
acid  present.  These  statements  of  Reissig  fully  coincide  with  my  own  investi- 
gations- 


§  134.]  PHOSPHORIC    ACID.  449 

l>e  free  from  chlorides,  in  concentrated  nitric  acid,  add  at  least 
eight  times  as  much  of  the  purest  tin  (in  the  form  of  foil  or 
grains)  as  there  is  phosphoric  acid  present,  and  warm  for  5  to  6 
hours,  until  the  precipitate  has  completely  subsided.  (If  alumina 
or  ferric  oxide  is  present,  some  portion  of  these  is  carried 
down  in  the  precipitate.  GIKARD.)  "Wash  by  decantation  and 
filtration,  rinse  into  a  platinum  dish,  and  digest  with  a  small 
quantity  of  very  concentrated  potassa  solution.  These  form  both 
potassium  in  etas  tan  n  ate  and  phosphate,  which,  on  adding  hot 
water,  yield  a  clear  solution,  dissolving  with  great  readiness  if  not 
too  much  potassa  has  been  used.  Any  traces  of  precipitate  re- 
maining adhering  to  the  filter  are  similarly  dissolved.  The  total 
alkaline  fluid  is  transferred  to  a  tared  litre  flask,  diluted  to  weigh 
900  grin,  and  saturated  with  hydrogen  sulphide;  some  ammonium 
pentasulphide  and  acetic  acid  are  then  added  until  the  tin  sulphide 
is  precipitated  and  the  liquid  is  weakly  acid.  Now  add  water  to 
make  the  contents  weigh  1000  grm.  again  (or  some  other  round 
number),  shake,  let  stand  12  to  16  hours,  filter  off  the  superna- 
tant clear  liquid  into  a  porcelain  dish,  and  again  weigh  the  flask, 
which  now  contains  the  rest  of  the  fluid  and  the  tin  sulphide. 
The  difference  between  the  two  weights  gives  the  weight  of  the 
filtrate  in  which  the  phosphoric  acid  is  to  be  determined.  The 
proportion  which  this  weight  bears  to  the  total  quantity  of  fluid, 
i.e.,  1000  grm.  minus  the  weight  of  the  tin  sulphide  (which  may 
be  calculated  with  sufficient  accuracy  from  the  quantity  of  tin 
originally  taken,  or  which  may  be  directly  estimated),  is  readily 
found. 

Evaporate  to  a  small  volume  the  filtrate  mixed  with  the  wash- 
ings of  the  filter,  and  in  this  determine  the  phosphoric  acid  accord- 
ing to  5,  OL.  The  above  method  of  separating  the  phosphoric-acid 
liquid  from  the  tin  sulphide  must  be  resorted  to,  because  in 
filtering  off  and  washing  the  sulphide  small  quantities  of  tin  are 
dissolved  out,  no  matter  whether  pure  water  or  hydrogen-sulphide 
water  is  used.  Results  accurate. 

55.  After  GIRARD.*  In  order  to  render  the  method,  origi- 
nally based  on  the  separation  of  phosphoric  acid  as  stannic  plms- 

*  Compt.  Rend.,  LIV,  468;    Zeitschr.f.  analyt.  Chem.,  i,  366. 


450  DETERMINATION.  [§  134. 

phate,  applicable  also  in  the  presence  of  alumina  and  ferric  oxide, 
GIRARD  employs  the  following  method  :  Dissolve  the  substance  in 
highly  concentrated  nitric  acid,  remove  all  chlorine  either  by  pre- 
cipitation with  silver  nitrate  or  by  repeated  evaporation  with 
nitric  acid,  add  8  times  as  much  tinfoil  as  there  is  phosphoric 
acid  present,  and  warm  the  mixture  5  or  6  hours,  until 
the  precipitate  has  completely  subsided,  leaving  the  super- 
natant fluid  clear.  Wash  with  hot  water  by  decantation 
and  finally  by  filtration.  The  precipitate  consists  of  meta- 
stannic  acid  and  stannic  phosphate,  together  with  a  little  ferric 
and  aluminium  phosphates.  Heat  it  either  at  first  with  a  small 
quantity  of  aqua  regia,  and  then  with  ammonia  and  ammonium 
sulphide,  or  immediately  with  ammonium  sulphide  in  excess.  The 
latter  process  is  recommended  by  O.  BABER,*  on  the  ground  that 
the  former  leaves  a  little  phosphoric  acid  in  the  precipitate.  The 
whole  is  digested  about  two  hours  and  then  filtered ;  the  precipi- 
tate, consisting  of  ferrous  sulphide  and  aluminium  hydroxide,  is 
washed  with  warm  ammonium  sulphide,  then  with  water  contain- 
ing a  little  ammonium  sulphide  dissolved  in  nitric  acid,  and  the 
solution  thus  formed  mixed  with  the  filtrate  from  the  tin  precipi- 
tate which  contains  the  principal  quantity  of  the  basic  metals. 
From  the  ammonium-sulphide  filtrate,  which  contains  stannic  sul- 
phide and  ammonium  phosphate,  the  phosphoric  acid  is  at  once 
precipitated  by  magnesia  mixture.  I  may  add  that  GIRARD  con- 
siders 4  to  5  parts  tin  sufficient  for  1  part  P2O6.  The  results 
afforded  by  his  test  analyses  are  unexceptionable.  According  to 
jANovsKY,f  at  least  6  parts  of  tin  must  be  used.  The  tin  should 
be  free  from  arsenic.  If  the  tin  contains  arsenic,  direct  precipi- 
tation with  magnesia  mixture  will  give,  besides  ammonium -mag- 
nesium phosphate,  some  ammonium-magnesium  arsenate  also, 
and  consequently  the  results  would  be  too  high.  In  such  a  case 
the  ammonium-sulphide  solution  is  best  treated  as  in  aa. 

e.  Indirect  determination  after  previous  precipitation  as  bis- 
muthic  phosphate, 

This  method  was  proposed  by  CHANCEL  J   and  modified  by 

*  Zeitechr.f.  die  gesammten  Naturwissensch.,  1864,  293. 
f  ZeiUchr,  f.  analyt.  Chem.,  xi,  157. 

j  Compt.  Rend.,  L,  416;  Chem.  CentralbL,  1860,  212  ;  Compt.  Rend.,  LI,  882  ; 
Chem.  CentralbL,  1861,  221. 


PHOSPHORIC  ACID.  451 

BIRNBAUM  and  CHOJNAKI.*  It  is  not  applicable  in  the  pres- 
ence of  sulphuric  or  hydrochloric  acid,  nor  can  it  lay  claim 
to  rapidity  or  accuracy.  f 

c.   Determination  as  Uranyl  Pyrophosphate. 


After  LECONTE,  A.  AKENDT,  and  "W.  KNOP.^  (Very  suitable 
in  presence  of  alkali  and  alkali-earth  metals,  but  not  in  pres- 
ence of  any  notable  amount  of  aluminium  ;  in  presence  of  ferric 
iron,  the  method  can  be  applied  only  with  certain  modifications.)  § 
Where  it  is  possible,  prepare  an  acetic-acid  solution  of  the  com- 
pound. If  you  have  a  nitric-  or  hydrochloric-acid  solution,  re- 
move the  greater  portion  of  the  free  acid  by  evaporation,  add 
ammonia  until  red  litmus  paper  dipped  into  it  turns  very  dis- 
tinctly blue,  and  then  redissolve  the  preciptate  formed  in  acetic 
acid.  If  mineral  acids  were  present,  add  also  some  ammonium 
acetate  ;  this  addition  is  beneficial  under  any  circumstances.  Mix 
the  fluid  now  with  solution  of  uranyl  acetate  and  heat  the  mix- 
ture to  boiling,  which  will  cause  the  phosphoric  acid  to  separate 
in  form  of  pale  greenish-yellow  ammonium  uranyl  phosphate. 

Wash  the  precipitate,  first  by  decantation,  boiling  up  each 
time,  then  by  filtration  ;  the  operation  may  be  materially  facili- 
tated by  adding  a  few  per  cents  of  ammonium  nitrate  to  the 
water.  Dry  the  precipitate  and  ignite  as  directed  in  §  53.  It  is 
advisable  to  evaporate  small  quantities  of  nitric  acid  on  the  ignited 
precipitate  repeatedly  and  to  reignite.  The  residue  must  have 
the  color  of  the  yolk  of  an  egg.  For  the  properties  of  the  pre- 
cipitate and-  residue,  see  §  93,  4,  e.  Should  it  be  necessary  to- 
dissolve  the  ignited  residue  again,  for  the  purpose  of  reprecipitat- 
ing  it,  this  can  be  done  only  after  fusing  it  with  a  large  excess  of 


*  Zeilsckr.f.  analyt,  Chem.,  ix,  203. 

f  Compare  HOLZBERGER,  Archiv.  der  Pharm. ,  (2),  CXVT,  37  ;  BABER, 
Zeitschr.  f.  d.  ges.  Naturwiss.,  1864,  293  ;  GIRARD,  Compt.  rend.,  LIV,  468; 
FRESENIUS,  NEUBAUER,  and  LUCK,  Zeitschr.  f.  analyt.  Chem.,  x,  135 ;  ADRI- 
AANSZ,  ib.,  x,  473. 

\  LECONTE  was  the  first  to  recommend  the  method  of  precipitating  phos- 
phoric acid  from  acetic-acid  solutions  by  means  of  a  salt  of  uranium  (Jahresb.  von 
LIEBIG  und  KOPP,    fUr   1853,  642);  A.   ARENDT  and  W.  KNOP  have  subse- 
quently subjected  it  to  a  careful  and  searching  examination  (Chem.  Centralbl. 
1850,  769,  803  ;  and  1857,  177). 

§  Chem.  Centralbl.,  1857,  182. 


DETERMINATION.  [§  131 

mixed  sodium  and  potassium  carbonates,  and  thereby  converting 
the  pyrophosphoric  into  orthophosphoric  aci(L .  Results  accurate. 
Compare  the  test  analyses  given  by  the  authors,  Expt.  !N"o.  81, 
and  KISSEL'S  experiments.* 

d.  Determination  as  Basic  Ferric  Phosphate. 

a.  Mix  the  acid  fluid  containing  the  phosphoric  acid  with  an 
excess  of  solution  of  ferric  chloride  of  known  strength,  add,  if 
necessary,  sufficient  ammonia  to  neutralize  the  greater  portion  of 
the  free  acid,  mix  with  ammonium  acetate  in  not  too  large  ex- 
cess, and  boil.  If  the  quantity  of  solution  of  ferric  chloride 
added  was  sufficient,  the  precipitate  must  be  brownish-red.  This 
precipitate  consists  of  basic-ferric  phosphate  and  basic-ferric  ace- 
tate, and  contains  the  whole  of  the  phosphoric  acid  and  of  the 
ferric  iron.  Filter  off  boiling,  wash  with  boiling  water  mixed  with 
some  ammonium  acetate,  and  dry  carefully.  [Detach  the  greater 
part  of  the  precipitate  from  the  filter,  incinerate  the  filter,  trans- 
fer to  the  crucible  the  main  part  of  the  precipitate,  moisten  with 
strong  nitric  acid,  dry,  moisten  again  with  nitric  acid,  and  dry 
and  ignite  in  a  platinum  crucible  with  access  of  air  (§  53).  With- 
out these  precautions  reduction  of  ferric  oxide  to  magnetic  oxide 
is  liable  to  occur.]  If  the  weight  of  the  residue  has  been  in- 
creased by  this  operation,  which  is  not  the  case,  however,  as  a 
rule,  the  procedure  must  be  repeated  until  the  weight  is  constant. 
Deduct  from  the  weight  of  the  residue  that  ferric  oxide  produced 
from  the  solution  added;  the  difference  is  the  P2OB. 

[This  modification  of  SCHULZE'S  method  was  first  recommended 
by  A.  MuLLER;f  it  has  been  adopted  also  by  WAY  and  OGSTON, 
in  their  analyses  of  ashes.  ^  MULLER'S  improvement  consists  in 
the  use  of  a  solution  of  ferric  chloride  of  known  strength,  whereby 
the  determination  of  iron  in  the  residue  (as  given  in  §  113,  3)  is 
dispensed  with.] 

/3.  J.  WEEREN'S  method,  suitable  for  the  estimation  of  the  phos- 
phoric acid  in  phosphates  of  the  alkali  and  alkali-earth  metals.  § 


*  Zeitschr  f.  analyt.  Chem.,  vm,  167. 

f  Journ.  f.  prakt.  Chem.,  XLVII,  341. 

\  Journal  of  the  Royal  Agricultural  Society,  viu,  part  I. 

%  Journ.  f.  prakt.  Chem.,  LXVII,  8. 


3  134.]  PHOSPHORIC  ACID.  453 

Mix  the  nitric  acid  solution  of  the  phosphate  under  examination, 
which  must  contain  no  other  strong  acid,  with  a  solution  of  ferric 
nitrate,  of  known  strength,  in  sufficient  proportion  to  insure  the 
formation  of  a  basic  salt  (3  or  4  parts  of  iron  should  be  present  for 
1  part  P2O6)  ;  evaporate  to  dryness,  heat  the  residue  to  160°,  until 
no  more  nitric  acid  fumes  escape,  treat  with  hot  water  containing 
ammonium  nitrate  until  all  nitrates  of  the  alkali  and  alkali-earth 
metals  are  removed,  collect  the  yellow-ochreous  precipitate  on  a 
filter,  dry,  ignite  (see  §  53),  weigh,  and  deduct  from  the  weight 
the  quantity  of  iron  added  reckoned  as  ferric  oxide.  LATSCHINOW* 
recommends  heating  the  residue  to  200°,  warming  with  water  and 
a  few  drops  of  sulphuric  acid,  adding  ammonia  and  then  treating 
with  hot  solution  of  ammonium  nitrate.  He  says  that  the  phos- 
phoric acid  is  thus  more  completely  separated,  and  the  precipitate 
may  be  more  readily  filtered  off. 

e.  Determination    as  Normal    Magnesium   Phosphate    Mg3 


(Fu.  SCHULZE'S  method,  suitable  more  particularly  to  effect  the 
separation  of  phosphoric  acid  from  the  alkalies,  f) 

Mix  the  solution  of  the  alkali  phosphate,  which  contains  ammo- 
nium chloride,  with  a  weighed  excess  of  pure  magnesium  oxide, 
evaporate  to  dryness,  ignite  the  residue  until  the  ammonium  chlo- 
ride is  expelled,  and  separate  the  magnesium,  which  is  still  present 
in  form  of  magnesium  chloride,  by  means  of  mercuric  oxide  (§104. 
3,  5).  Treat  the  ignited  residue  with  water,  filter  the  solution  of 
the  chlorides  of  the  alkali  metals,  wash  the  precipitate,  dry,  ignite, 
and  weigh.  The  excess  of  weight  over  that  of  the  magnesium 
oxide  used  shows  the  quantity  of  the  P2O6.  Results  satisfactory. 

f.  SCHLOSING'S  method  J  does  not  appear  to  offer  any  advan- 
tages. The  phosphate  is  mixed  with  silica  and  ignited  in  carbon 
monoxide,  the  expelled  phosphorus  being  taken  up  by  copper  or  by 
silver  nitrate. 

g.  Determination  by  Volumetric  Analysis  (  With  Uraniutn 
Solution). 

This  method  was  recommended  originally  by  LECONTE.§     It 


*Zeitschr.f.  analyt.  Chem.,  vn,  213. 
\Journ.f.  prakt.  Cliem.,  LXIII,  440. 
%Zeitschr.f.  analyt.  Chem.,  iv,  118  and  vn,  473. 
§  Jahresber.  von  LIEBIG  u.  KOPP,  ftir  1853,  642. 


454  DETERMINATION.  [§  Ic54. 

was  improved  and  described  in  detail  by  NEUBAUER,*  and  was 
afterwards  recommended  by  PINCTTS,  f  and  subsequently  by 
BODEKER.^:  The  principle  of  the  method  is  as  follows;  Uranyl 
acetate  precipitates  from  solutions  rendered  acid  by  acetic  acid, 
hydrogen  uranyl  phosphate,  or — in  the  presence  of  considerable 
quantities  of  ammonium  salts — ammonium  uranyl  phosphate.  The 
proportion  between  the  uranium  and  the  phosphoric  acid  is  the 
same  in  both  compounds.  Both  compounds  when  freshly  precipi- 
(tated  and  suspended  in  water  are  left  unchanged  by  potassium 
ferrocyanide ;  uranyl  acetate,  on  the  other  hand,  is  indicated  by 
this  reagent  with  great  delicacy  by  the  formation  of  an  insoluble 
reddish-brown  precipitate. 

According  to  NEUBAUER§  the  following  solutions  are  employed : 

a.  A  solution  of  phosphoric  acid  of  'known  strength.  Pre- 
pared by  dissolving  10-13  grm.  pure,  crystallized,  unehMoresced, 
powdered,  and  pressed  hydrogen  sodium  phosphate  in  water  to  1 
litre.  50  c.c.  contain  0*1  grm.  PQOB.  It  is  well  to  control  this  solu- 
tion by  evaporating  50  c.c.  in  a  weighed  platinum  dish  to  dry  ness, 
igniting  strongly,  and  weighing.  The  weight  should  be  0'1881 
grm. 

I.  An  acid  solution  of  sodium  acetate.  Prepared  by  dissolv- 
ing 100  grm.  sodium  acetate  in  900  c.  c.  water  and  adding  acetic 
acid  of  1*04  sp.  gr.  to  1  litre. 

c.  A  solution  of  uranyl  acetate  (or  nitrate)  (§  63,  3).  This 
is  standardized  against  the  hydrogen  sodium  phosphate  solution. 
1  c.  c  indicates  0-005  grm.  P2O5.  The  solution  is  made  at  first  a 
little  stronger  than  necessary,  so  that  it  may  contain  in  the  litre,  say, 
32-5  grm.  UO,(C2H3O2)2+2H4O  or  34 grm.  UO2(C2H3O2),+3H2O 
(corresponding  to  22  grm.  UOaO);  its  value  is  then  determined, 
and  it  is  diluted  accordingly.  To  determine  its  value  proceed  as 
follows  :  Transfer  50  c.c.  of  the  a  solution  to  a  beaker,  add  5  c.c. 
of  the  1)  solution,  and  heat  in  a  water-bath  to  90 — 100°.  Now  run 
in  uranium  solution,  at  first  a  large  quantity,  at  last  in  i  c.c. ,  testing 
after  each  addition  whether  the  precipitation  is  finished  or  not. 
For  this  purpose  spread  out  one  or  two  drops  of  the  mixture  on  a 
^white  porcelain  surface  and  introduce  into  the  middle,  by  means  of  a 
thin  glass  rod,  a  small  drop  of  freshly  prepared  potassium  ferrocya- 
.nide  solution  or  a  little  of  the  powdered  salt.  As  soon  as  a  trace  of 

-*Archiv.  fur  wissemchaftliche  Heilkunde,  IV,  228. 

•\Journ.f.  prakt.  Chem.,  LXXVI,  104.       J  Anal,  de  Ghem.  etPharm.,  cxvn,  195. 

|§His  Anleitung  zur  Harnanaly se,  6.  Aufl.,  171. 


§  134  ]  PHOSPHORIC   ACID.  455 

-excess  of  uranyl  acetate  is  present,  a  reddisli-brown  spot  forms  in 
the  drop,  which,  surrounded  as  it  is  by  the  colorless  or  almost 
colorless  fluid,  may  be  very  distinctly  perceived.  When  the  final 
reaction  has  just  appeared,  heat  a  few  minutes  in  the  water-bath 
and  repeat  the  testing  on  the  porcelain.  If  now  the  reaction  is 
.still  plain  the  experiment  is  concluded.  If  the  uranium  solution 
had  been  exactly  of  the  required  strength,  20  c.c.  would  have  been 
used ;  but  it  is  actually  too  concentrated,  hence  less  than  20  c.c. 
must  have  been  used.  Suppose  it  was  18  c.c.,  then  the  solution 
will  be  right,  if  for  every  18  c.c.  we  add  2  c.c.  of  water.  If  in  this 
first  experiment  we  find  that  the  solution  is  much  too  strong,  the 
solution  is  diluted  with  somewhat  less  water  than  is  properly  speak- 
ing required,  another  experiment  is  made,  and  it  is  then  diluted 
exactly. 

The  actual  analysis  must  be  made  under  as  nearly  as  possible 
similar  circumstances  to  those  under  which  the  standardizing:  of  the 

o 

uranium  solution  was  performed,  especially  as  regards  the  sodium 
acetate.  This  salt  retards  the  precipitation  of  uranium  by  potas' 
sium  ferrocyanide,  hence  the  test-drop  on  the  porcelain  plate 
becomes  darker  and  darker.  The  analyst  should  accustom  himself 
to  observing  the  first  appearance  of  the  slightest  brownish  colora- 
tion in  the  middle  of  the  drop,  and  should  take  this  as  the  end- 
reaction.  It  need  hardly  be  added  that  the  same  person  must 
make  the  analysis  who  has  standardized  the  solution  (NEUBAUER). 

The  method  is  applicable  to  free  phosphoric  acid,  alkali  phos- 
phates, and  magnesium  phosphate,  also  in  the  presence  of  small 
quantities  of  the  phosphates  of  other  alkali-earth  metals,  but  can- 
not be  employed  in  presence  of  ferric  and  aluminium  salts.  Dis- 
solve the  substance  in  water  or  the  least  possible  quantity  of  acetic 
acid,  add  5  c.c.  of  the  b  solution,  dilute  to  50  c.c.,  and  proceed  with 
the  addition  of  uranium  as  above.  The  results  are  very  satisfac- 
tory. Compare  KISSEL'S  experiments.*  If  the  above  process  is 
followed  in  the  presence  of  much  calcium,  for  instance  with  a  solu- 
tion of  calcium  phosphate  in  dilute  acetic  acid,  the  results  are 
almost  always  too  low,  as  little  calcium  phosphate  is  precipitated 
along  with  uranyl  phosphate.  [The  best  means  of  obviating  that 
•error  is,  according  to  ABESSEK,  JANI,  and  MARCKER,f  to  standardize 
the  uranium  solution  under  the  same  conditions  as  nearly  as  possible 

*  Zcitschr.  f.  analyt.  Chem.,  vm,  167.  f  Ib.,  xn,  262. 


456  DETERMINATION.  [§  134, 

as  exist  when  the  solution  is  used  for  the  actual  determination  of 
phosphoric  acid.  It  must  therefore  be  standardized  with  calcium 
phosphate.  Prepare  a  solution  of  suitable  strength  by  dissolving- 
pure  Ca3(PO4)2  in  the  smallest  possible  quantity  of  nitric  acid  and 
diluting  to  the  desired  volume.  Determine  accurately  the  amount 
of  Ca3(PO4)2  in  this  solution  by  evaporating  to  dryness  in  a  plati- 
num vessel  50  c.  c.,  moistening  the  residue  with  ammonia  and 
igniting.  The  residual  somewhat  hygroscopic  calcium  phosphate 
is  quickly  weighed  in  the  covered  platinum  vessel.] 

According  to  R.  FRESENITJS,  NEUBAUER,  and  LUCK,*  the  dif- 
ficulty may  also  be  easily  avoided  by  adding  the  phosphate  solu- 
tion to  the  uranium  solution  until  the  ferrocyanide  ceases  to  give 
a  reaction.  The  standardization  of  the  uranium  solution  should 
in  this  case  be  conducted  as  follows :  To  25  c.  c.  of  the 
uranium  solution  in  a  beaker  add  5  c.  c.  of  the  sodium- 
acetate  solution  and  3  c.  c.  of  acetic  acid  (sp.  gr.  1*04),  heat 
on  a  water-bath  and  run  in  from  a  burette  sodium-phosphate 
solution  until  a  drop  brought  into  contact  with  a  few  frag- 
ments of  potassium  ferrocyanide  on  a  porcelain  plate  just 
ceases  to  react.  After  every  addition  of  phosphate  the  beaker 
must  be  replaced  in  the  hot  water,  and  a  few  minutes  must  be 
allowed  to  elapse  before  again  testing;  the  sodium-phosphate 
solution  may  further  be  added  freely  so  long  as  the  solution 
remains  yellowish.  In  carrying  out  the  analysis,  take  care  that 
the  solution  of  calcium  acetate  contains  no  considerable  excess  of 
free  acetic  acid,  that  its  concentration  does  not  differ  very  greatly 
from  that  of  the  sodium-phosphate  solution,  and  that  its  total 
volume  is  known  before  any  part  of  it  is  introduced  into  the 
burette. 

Regarding  the  volumetric  methods  proposed  by  FLEISCHER  f 
(alumina  method)  and  SCHWARZ  £  (lead  method),  see  the  sources 
given.  The  latter  method,  although  based  on  correct  principles 
and  sufficiently  exact  for  neutral  liquids,  is  nevertheless  of  very 
limited  application,  since  the  presence  of  acetic  acid  seriously 
impairs  its  accuracy.  See  FK.  MOHR.§ 

*  ZeitscJir.  f.  analyt.  Chem.,  x,  147. 

}lb.,  iv,  19,  and  vi,  28. 

J  Ding.  Polyt.  Journ.,  CLXIX,  289  ;  Zeitschr.  f.  analyt.  Chern.,  u,  392. 

§J.b.,  n,  256. 


f?135.]  PHOSPHORIC   AOID.  457 

II.   SEPARATION  OF  PHOSPHORIC  ACID  FROM  THE  BASIC  RADICALS. 

§  135. 

a.  from  the  Alkalies  (see  also  d,  &,  and  Z). 

a.  Add  ammonium  chloride  or  hydrochloric  acid,  then  lead 
acetate,  exactly,  till  no  more  precipitate  is  produced,  and  lastly 
some  pure  lead  carbonate  (prepared  by  precipitating  lead  acetate 
with  ammonium  carbonate,  BABER  *),  allow  to  digest  for  some 
time,  filter  off  the  precipitate  consisting  of  lead  phosphate,  chlo- 
ride, and  carbonate,  wash,  precipitate  from  the  filtrate  the  slight 
excess  of  lead  by  hydrogen  sulphide,  filter,  and  evaporate  with 
hydrochloric  acid  (in  the  case  of  lithium,  sulphuric  acid).  If  the 
phosphoric  acid  is  to  be  estimated  in  the  same  portion,  proceed 
with  the  first  precipitate  (after  washing,  to  remove  the  larger 
quantity  of  chloride)  according  to  §  135,  b. 

/3.  (Only  applicable  in  the  case  of  fixed  alkalies.)  Separate 
the  phosphoric  acid  as  ferric  phosphate,  according  to  one  of  the 
methods  given  in  §  134,  d.  Or,  if  you  do  not  wish  to  determine  the 
phosphoric  acid  it  is  very  convenient  to  acidify  with  hydrochloric 
acid,  add  ferric  chloride,  dilute  rather  considerably,  add  ammonia 
till  the  fluid  is  neutral,  and  boil ;  all  the  phosphoric  acid  will  then 
separate  with  ferric  oxychloride  as  ferric  phosphate.  This  modi- 
fication is  recommended  when  the  phosphoric  acid  is  to  be  pre- 
cipitated, but  not  estimated.  The  separation  of  phosphoric  acid 
may  also  be  effected  as  magnesium  phosphate  (§  134,  e).  The 
alkalies  are  contained  in  the  filtrate  as  nitrates  or  chlorides. 

1).   From  Barium,  Strontium,  Calcium,  and  Lead. 

The  compound  under  examination  is  dissolved  in  hydrochloric 
or  nitric  acid  and  the  solution  precipitated  with  sulphuric  acid  in 
slight  excess.  In  the  separation  of  phosphoric  acid  from  strontium, 
calcium,  and  lead,  alcohol  is  added  with  the  sulphuric  acid.  The 
phosphoric  acid  in  the  filtrate  is  determined  according  to  §  134,  J, 
a,  after  removal  of  the  alcohol  by  evaporation.  The  determination 
of  the  phosphoric  acid  is  effected  most  accurately  by  saturating  the 
fluid  with  sodium  carbonate,  evaporating  to  dryness,  and  fusing  the 
residue  with  sodium  and  potassium  carbonates.  The  fused  mass  is 
then  dissolved  in  water,  and  the  further  process  conducted  as  in 
§  134,  &,  a. 

*Zeitschr.f.  die  ges.  Naturwiss.,  1864,  298;  Zeitschr.f.  analyt.  Chem.,  iv,  120- 


458  DETERMINATION.  [§  135. 

c.  From  Magnesium  (see  also  d,  h,  &,  Z). 

Add  ferric  chloride  in  sufficient  excess,  dilute,  add  excess  of 
barium  carbonate,  allow  to  remain  for  several  hours  with  frequent 
stirring,  filter  and  separate  magnesium  and  barium  in  the  filtrate 
after  §  154. 

d.  From  the  whole  of  the  Alkali-earth  Metals  and  fixed  Alka- 
lies (comp.  A,  &,  Z). 

a.  Dissolve  in  the  least  possible  quantity  of  nitric  acid,  add  a 
little  ammonium  chloride,  precipitate  exactly  with  lead  acetate,  add 
a  little  lead  carbonate  (precipitated),  digest,  filter,  precipitate  the 
excess  of  lead  rapidly  from  the  filtrate  by  hydrogen  sulphide,  filter 
and  determine  the  basic  metals  in  the  filtrate.  Results  good. 

(3.  Dissolve  in  water,  and — in  case  of  phosphates  of  the  alkali- 
earth  metals — the  least  possible  nitric  acid,  add  neutral  silver  nitrate 
and  then  silver  carbonate,  till  the  fluid  reacts  neutral.  All  phos- 
phoric acid  now  separates  as  Ag3PO4.  Warming  is  unnecessary. 
Filter,  wash  the  precipitate,  dissolve  it  in  dilute  nitric  acid,  precipi- 
tate the  silver  with  hydrochloric  acid,  and  determine  the  phosphoric 
acid  in  the  filtrate  according  to  §  134,  5,  a.  The  filtrate  from  the 
silver  phosphate  is  freed  from  silver  by  hydrochloric  acid,  and  the 
basic  metals  are  then  determined  according  to  the  methods  already 
given  (G.  CHANCEL*).  A  good  and  convenient  method  unless  the 
proportion  of  alkali  is  very  large.  (If  the  substance  contains  alu- 
minium or  ferric  iron,  they  are  completely  precipitated  by  the 
silver  carbonate,  and  are  found  with  the  silver  phosphate.) 

y.  Separate  the  phosphoric  acid  as  uranyl  phosphate  (§  134,  c\ 
and  the  excess  of  uranium  from  the  alkali-earth  metals,  &c.,  in  the 
filtrate,  according  to  §§  160  and  161,  Supplement.  Results  good. 

d.  Separate  the  phosphoric  acid  according  to  §134,  d,  a  or  ft. 
The  alkali-earth  metals  are  obtained  in  solution  in  the  first  case, 
as  chlorides,    together  with  alkali  acetate  and  chloride;  in  the 
second  case  as  nitrates.     Results  good. 

e.  From  Aluminium. 

The  best  method  of  separating  phosphoric  acid  from  aluminium 
is  that  depending  on  precipitation  by  ammonium  inolybdate 
(§  135,  Z).  The  separation  of  the  acid  as  stannic  phosphate  (A,  a) 
is  also  satisfactory. 

*  Compt.  rend.,  XLIX,  997;  Journ.  /.  prakt.  Chem.,  LXXIX,  222. 


§  135.]  PHOSPHORIC   ACID.  459 

The  older  methods  are  scarcely  ever  used  now ;  hence  the  two 
most  in  use  formerly  will  be  but  briefly  described. 

a.  OTTO'S  method.  This  depends  on  the  precipitation  of 
phosphoric  acid  with  magnesia  mixture  from  the  solution  to 
which  tartaric  acid  and  ammonia  are  added.  It  is  difficult  to 
obtain  a  precipitate  free  from  alumina,  even  after  repeated  pre- 
cipitation ;  on  the  other  hand,  a  certain  quantity  of  phosphoric 
acid  remains  in  solution.  Compare  ILYREN,*  F.  KNAPp,f  and 
K.  PRIBRAM.;); 

ft.  BERZELIUS'S  method.  Mix  the  finely  powdered  substance 
with  about  IL^  parts  of  pure  silicic  acid  (the  artificially  prepared 
is  the  best)  and  6  parts  sodium  carbonate,  and  expose  to  a  strong 
red  heat  for  half  an  hour  in  a  platinum  crucible.  Treat  the 
mass  with  water,  add  ammonium  bicarbonate  in  excess,  digest  for 
some  time,  filter,  and  wash.  Aluminium-sodium  silicate  remains 
in  the  filter,  while  the  filtrate  contains  sodium  phosphate,  sodium 
bicarbonate,  and  ammonium  carbonate.  (Had  the  solution  been 
filtered  before  adding  the  ammonium  bicarbonate,  some  of  the 
aluminium  compound  would  have  gone  into  solution.)  Phosphoric 
acid  is  determined  in  the  solution  according  to  §  1 34,  J,  a ;  the 
alumina  is  separated  frpm  the  residue  and  determined  according 
to  §  140.  The  method  is  tedious  and  troublesome,  as- the  precipi- 
tate is  washed  only  with  difficulty ;  the  results  are  accurate,  how- 
ever. Compare  SCHWEITZER.  § 

[Of  several  other  methods  which  have  been  used,  the  follow- 
ing (by  WACKENRODER  and  FRESENIUS)  is  one  of  the  easiest  to 
carry  out:  Precipitate  the  not  too  acid  solution  with  ammonia, 
taking  care  not  to  use  a  great  excess  of  that  reagent,  and  add 
barium  chloride  so  long  as  a  precipitate  continues  to  form.  Digest 
for  some  time  and  then  filter.  The  precipitate  contains  the 
whole  of  the  aluminium  and  the  whole  of  the  phosphoric  acid,  the 
latter  combined  partly  with  aluminium,  partly  with  barium. 
Filter  it  off,  wash  it  a  little,  and  dissolve  in  the  least  possible 
quantity  of  hydrochloric  acid.  Warm,  saturate  the  solution  with 
barium  carbonate,  add  pure  solution  of  potassa  in  excess,  apply 

*  Journ.  de  Pharm.,  xxi,  28. 

•\Zeitnchr.f.  analyt.  Chem.,  iv,  151. 

\  Vierieljahresschr.  f.  prakt.  Pharm.,  xv,  184 

§Zeitschr.f.  analyt.  Chem.,  ix,  89. 


460  DETERMINATION.  [§  135. 

heat,  precipitate  the  barium  which  the  solution  may  contain  with 
sodium  carbonate,  and  filter.  You  have  now  the  whole  of  the 
aluminium  in  the  solution,  the  whole  of  the  phosphoric  acid  in  the 
precipitate.  Acidify  the  solution  with  hydrochloric  acid,  boil 
with  some  potassium  chlorate,  and  precipitate  as  directed  in  §  105. 
Dissolve  the  precipitate  in  hydrochloric  acid,  precipitate  the 
barium  with  dilute  sulphuric  acid,  filter,  and  determine  the  phos- 
phoric acid  in  the  filtrate  by  precipitation  with  solution  of  magne- 
sium in  the  manner  described  in  §  134,  J,  a.  HERMANN  has 
applied  a  perfectly  similar  method  in  his  analysis  of  (impure) 
gibbsite.] 

f.  From  Chromium  (see  also  h,  &,  I). 

Fuse  with  sodium  carbonate  and  nitrate  and  separate  the 
chromic  acid  and  phosphoric  acid  in  the  manner  described  in  §  166. 

g.  From  the  Metals  of  the  Fourth  Group  (see  also  A,  &,  Z). 

a.  The  method  so  often  used  of  fusing  with  sodium  carbonate 
does  not  give  accurate  results  on  account  of  the  constant  presence 
of  some  phosphoric  acid  in  the  washed  residue.  Compare  W. 
SCHWEIKERT  *  and  G.  SCHWEITZER. f  The  former  has  studied  the 
separation  of  zinc  from  phosphoric  acid  by  this  method ;  the  latter 
the  separation  of  iron. 

ft.  Dissolve  in  hydrochloric  acid,  add  tartaric  acid,  ammo- 
nium chloride,  and  ammonia,  and  finally,  in  a  flask  which  is  to  be 
closed  afterwards,  ammonium  sulphide,  put  the  flask  in  a  moder- 
ately warm  place,  allowing  the  mixture  to  deposit  until  the  fluid 
appears  of  a  yellow  color,  without  the  least  tint  of  green ;  filter, 
and  determine  the  metals  as  directed  in  §§  108  to  114.  The 
phosphoric  acid  is  found  from  the  loss  cr  determined  according 
to  §  134,  5,  a.  The  magnesia  mixture  may  immediately  be  added, 
to  the  filtrate,  which  contains  ammonium  sulphide.  The  washed 
precipitate  is  redissolved  in  just  sufficient  hydrochloric  acid,  and 
the  solution  reprecipitated  by  ammonia  with  addition  of  magnesia 
mixture.  This  method  is  not  well  adapted  for  nickelous  phos- 
phate. 

A.  From  Metals  of  the  Second,  Third,  and  Fourth  Groups. 

a.   More  especially  from  the  second  group,  aluminium,  manga- 

*  Annal.  d.  Chem.  u.  Pharm.,  CXLV,  57;  Zeitschr.f.  andlyt.  Chem.,  vn,  246. 
f  Zeitschr.f.  anatyt.  Chem.,  ix,  84. 


§  135.]  PHOSPHORIC    ACID.  461 

nose,  nickel,  cobalt,  zinc ;  and  also  from  ferric  iron,  if  the  quan- 
tity of  the  latter  is  not  too  considerable. 

The  phosphoric  acid  is  precipitated  as  stannic  phosphate,  ac- 
cording to  §  134,  5,  tf,  aa.  The  filtrate  contains  the  bases  free 
from  any  foreign  body  requiring  removal,  which,  of  course, 
greatly  facilitates  their  estimation.*  REISSIG  obtained  very  good 
results  by  this  method.  If  the  precipitation  of  the  phosphoric 
acid  is  to  be  effected  in  the  presence  of  iron  and  aluminium  by 
tin,  GIRARD'S  method  should  be  used  (§  134,  &,  tf,  bb). 

/?.  From  ferric  iron,  aluminium,  alkali  earths,  and  all  other 
bases  not  precipitated  by  barium  carbonate  (H.  ROSE).  Evaporate 
off  as  much  as  possible  of-the  free  acid  from  the  hydrochloric-acid 
solution,  neutralize  it  partially  with  sodium  carbonate,  add  excess 
of  barium  carbonate,  digest  for  several  days  in  the  cold,  filter,  and 
wash  with  cold  water.  The  precipitate  contains  all  the  phos- 
phoric acid  combined  with  iron,  aluminium,  and  barium,  as  well 
as  the  excess  of  barium  carbonate.  The  filtrate  contains  the  re- 
maining bases.  Dissolve  the  precipitate  in  the  least  possible 
quantity  of  dilute  hydrochloric  acid,  cautiously  precipitate  the 
barium  with  sulphuric  acid,  filter,  saturate  filtrate  with  sodium 
carbonate  and  evaporate  it,  together  with  the  precipitate  in  it,  to 
dryness,  add  to  the  residue  an  equal  quantity  of  pure  silicic  acid 
and  six  times  its  quantity  of  sodium  carbonate,  and  heat  in  large 
platinum  crucible,  first  lightly  and  gradually  very  strongly.  Con- 
duct the  remaining  operations  just  as  detailed  under  §  135,  0,  ft. 

y.  From   much   ferric  iron  in  the  presence  of  alkali  earths 

(FRESENIUs).f 

While  the  method  detailed  under  §  134,  d,  is  applicable  in 
this  case,  it  is  nevertheless  exceedingly  tedious  where  a  small 
quantity  of  phosphoric  acid  is  to  be  separated  from  a  very  large 
quantity  of  ferric  iron.  A  better  process  is  as  follows :  Heat  the 
hydrochloric-acid  solution  to  boiling,  remove  the  heat,  and  add  a 
solution  of  sodium  sulphite  until  sodium  carbonate  causes  an 
almost  white  precipitate ;  then  boil  the  mixture  until  the  odor 
of  sulphurous  acid  has  disappeared,  nearly  neutralize  any  free 
acid  witli  sodium  carbonate,  add  a  few  drops  chlorine  water,  and 

*  If  the  nitric  acid  is  not  concentrated,  a  little  stannous  nitrate  is  formed, 
which  dissolves  and  must  afterwards  be  precipitated  from  the  acid  fluid  by 
hydrogen  sulphide.  BABER,  Zeitschr.  f.  d.  ges.  Naturwiss. ,  1864,  324. 

\  Journ.  /.  prakt.  Chem. ,  XLV,  258. 


462  DETERMINATION.  [§  135. 

finally  add  an  excess  of  sodium  acetate.  The  smallest  quantities 
of  phosphoric  acid  are  thus  immediately  precipitated  as  ferric  phos- 
phate. (Arsenic  and  silicic  acids  give  a  similar  precipitate,  hence 
if  present  they  must  be  previously  removed. )  Chlorine  water  is 
now  added  drop  by  drop  until  the  liquid  is  reddish,  then  boil 
until  the  precipitate  has  subsided  well,  filter  while  hot,  and  wash 
with  hot  water  containing  a  little  ammonium  acetate.  The  pre- 
cipitate now  contains  all  the  phosphoric  acid,  together  with  a 
small  quantity  of  iron,  while  the  filtrate  contains  the  great  bulk 
of  the  iron  and  all  the  alkali  earths.  The  precipitate  is  treated 
according  to  §  135  l\  i.e.,  the  phosphoric  acid  is  precipitated 
from  its  nitric-acid  solution  as  ammonium  phosphomolybdate,  and 
the  iron  and  aluminium  from  the  filtrate  by  means  of  ammonium 
sulphide  in  excess.  If  the  precipitate  is  free  from  aluminium,  it 
may  be  ignited,  weighed,  and  the  iron  in  it  determined  volumet- 
rically  (§  113),  the  difference  giving  the  phosphoric  acid.  This 
method  may  be  variously  modified  ;  thus  the  reduction  of  the  iron 
solution  may  be  effected  by  hydrogen  sulphide,  and  the  excess  of 
this  removed  by  carbonic-acid  gas.  Again,  the  precipitation  of 
the  ferric  phosphate  may  be  effected  by  digestion  with  calcium 
carbonate  (free  from  phosphate)  in  moderate  excess.  The  pre- 
cipitation of  ferric  phosphate  by  means  of  ammonium  sesquicar- 
bonate  must  be  effected  below  21°,  otherwise  some  phosphoric 
acid  will  remain  in  the  filtrate  (SPILLER*). 

i.  From  the  Metals  of  the  Fifth  and  Sixth  Groups. 

Dissolve  in  hydrochloric  or  nitric  acid,  precipitate  with  hydro- 
gen sulphide,  filter,  determine  the  bases  by  the  methods  given  in 
§§  115  to  127,  and  the  phosphoric  acid  in  the  filtrate  by  the 
method  described  in  §  134,  J,  tx.  From  silver  the  phosphoric  acid 
is  separated  in  a  more  simple  way  still,  by  adding  hydrochloric 
acid  to  the  nitric-acid  solution ;  from  lead  it  is  separated  most 
readily  according  to  5. 

k.  From  all  Basic  Metals,  except  Mercury  (li.  ROSE). 

The  phosphoric  acid  is  separated  as  mercurous  phosphate  by 
ROSE'S  method  (§  134,  5,  y). 

a.  If  the  substance  is  free  from  iron  and  aluminium,  the 
filtrate  from  the  mercurous  phosphate  contains  all  the  metals  as 
nitrates,  together  with  much  mercurous  nitrate,  and  occasionally 

*  Journ.  Chem.  Soc.  Ser.,  2,  TV,  148;  Zeitschr.f.  analyt.  Chem.,  v,  224. 


§  135.]  PHOSPHORIC  ACip.  463 

also  some  mercuric  salt.  The  former  is  removed  by  the  addition 
of  hydrochloric  acid.  The  precipitated  mercurous  chloride  is  free 
from  other  metals :  if  large  in  quantity,  it  should  be  separated  by 
filtering ;  if  slight,  filtering  may  be  omitted.  Add  next  ammonia 
to  slight  alkaline  reaction  (with  previous  addition  of  ammonium 
chloride  if  magnesium  is  present).  Filter  rapidly  from  the  mer- 
cury compound  which  will  be  precipitated  so  as  to  avoid  forma- 
tion of  calcium  carbonate  by  contact  with  air.  The  filtrate  contains 
the  basic  radicals  from  which  phosphoric  acid  has  been  separated. 
The  mercury  compound  which  has  been  separated  by  ammonia  is 
dried  and  ignited  (under  a  chimney  with  good  draught).  Should 
a  residue  remain,  this  must  be  examined.  If  it  consists  of  phos- 
phates of  the  alkali-earth  metals,  the  treatment  with  mercury  and 
nitric  acid  must  be  repeated;  if,  on  the  contrary,  it  consists  of 
magnesium  oxide  or  of  carbonates  of  the  alkali-earth  metals,  it  is 
dissolved  in  hydrochloric  acid,  and  the  solution  added  to  the  fluid 
containing  the  chief  portion  of  the  basic  metals,  which  may  then 
be  separated  and  determined  in  the  usual  manner.  The  following 
method  is  often  advantageously  resorted  to  instead  of  the  one 
described :  The  nitrate  from  the  mercurous  phosphate  is  evaporated 
to  dryness,  in  a  platinum  dish,  and  the  residue  ignited,  in  a  plati^ 
num  crucible,  under  a  chimney  with  good  draught.  If  alkali 
nitrates  are  present,  some  ammonium  carbonate  must  be  added 
from  time  to  time  during  the  process  of  ignition,  to  guard  against 
injury  to  the  crucible  from  the  formation  of  caustic  alkali.  The 
ignited  residue  is  treated,  according  to  circumstances,  first  with 
water  and  then  with  nitric  acid,  or  at  once  with  nitric  acid. 

ft.  If  the  substance  contains  iron  l)ut  not  aluminium,  the 
greater  part  of  the  iron  is  left  undissolved  with  the  mercurous 

o  Jr 

phosphate.  The  dissolved  part  is  separatd  from  the  other  bases  by 
the  methods  given  in  Section  Y. ;  the  iron  in  the  undissolved  part 
is  obtained,  after  ignition  of  the  residue  with  sodium  carbonate 
and  treating  the  ignited  mass  with  water,  as  ferric  oxide  contain- 
ing alkali  (and  generally  also  some  phosphoric  acid).  This  is  dis- 
solved in  hydrochloric  acid,  and  precipitated  with  ammonia. 

y.  If  the  xnbxtain-<'  ronldiii^  aluminium,  the  process  just  given 
cannot  be  used,  as  aluminium  phosphate  is  not  decomposed  by 
fusion  with  alkali  carbonates,  while  aluminium  nitrate,  like  ferric 
nitrate,  is  decomposed  by  simple  evaporation.  In  this  case  proceed 
as  follows ;  Dissolve  the  substance  in  the  least  quantity  of  nitric 


464  "DETEKMINATION.  [§  135. 

acid,  precipitate  hot  with  mercurous  nitrate,  add  a  little  mercuric 
nitrate,  and  then  pure  potash  or  soda,  till  a  permanent  red  precipi- 
tate appears.  The  precipitate  contains  no  aluminium,  and  is  to  be 
treated  according  to  a  or  /3  (II.  ROSE,  E.  E.  MUNKOE*). 

I.  From  all  Bases  without  exception. 

Apply  SONNEJSTSCHEIN'S  method  (§  134,  &,  /?),  and  in  the  filtrate 
from  the  ammonium  phospho-molybdate  separate  the  bases  from 
the  molybdic  acid.  As  molybdic  acid  comports  itself  with  hydro- 
gen sulphide  and  ammonium  sulphide  like  a  metal  of  the  sixth 
group,  it  is  best  to  precipitate  metals  of  the  sixth  and  also  of  the 
fifth  group  from  acid  solution  with  hydrogen  sulphide,  before  pro- 
ceeding to  precipitate  the  phosphoric  acid  with  molybdic  acid ;  the 
latter  will  then  have  to  be  separated  only  from  the  metals  of  the 
first  four  groups.  This  is  done  in  the  following  manner  :  Mix  the 
acid  fluid,  in  a  flask,  with  ammonia  till  it-  acquires  an  alkaline 
reaction,  add  ammonium  sulphide  in  sufficient  excess,  close  the 
mouth  of  the  flask,  and  digest  the  mixture.  As  soon  as  the  solution 
appears  of  a  reddish-yellow  color,  without  the  least  tint  of  green, 
filter  off  the  fluid,  which  contains  molybdenum  and  ammonium 
sulphide,  wash  the  residue  with  water  mixed  with  some  ammonium 
sulphide,  and  separate  the  remaining  metallic  sulphides  and  hydrox- 
ides of  the  fourth  and  third  groups  by  the  methods  which  will  be 
found  in  Section  Y.  Mix  the  filtrate  cautiously  with  hydrochloric 
acid  in  moderate  excess,  remove  the  molybdenum  sulphide  accord- 
ing to  §  128,  d,  and  determine  the  metals  of  the  first  and  second 
groups  in  the  filtrate. 

This  method  of  separating  the  phosphoric  acid  from  basic  radi- 
cals is  highly  to  be  recommended ;  especially  in  cases  where  a 
small  quantity  of  phosphoric  acid  has  to  be  determined  in  presence 
of  a  very  large  quantity  of  ferric  and  aluminium  salts,  as,  for  exam- 
ple, in  iron  ores,  soils,  &c.  As  arsenic  acid  and  silicic  acid  give, 
with  molybdic  acid  and  ammonia,  similar  yellow  precipitates,  it  is 
necessary,  if  these  acids  are  present,  to  remove  them  first. 

As  the  separation  of  the  basic  metals  from  the  large  excess  of 
molybdic  acid  used  is  somewhat  tedious,  the  best  way  is  to  arrange 
matters  so  that  this  process  may  be  altogether  dispensed  with. 
Supposing,  for  instance,  you  have  a  fluid  containing  ferric  iron, 
aluminium,  and  phosphoric  acid,  estimate,  in  one  portion,  by  cau- 
tious precipitation  with  ammonia,  the  total  amount  of  the  three 
*  Amer.  Journ.  of  Sci.  and  Arts,  May,  1871;  Zeitschr.  /.  analyt.  Chem,,  x,  467 


§130.]  BORIC    ACID   AND   BORIC    ANHYDRIDE.  465 

bodies;  in  another  portion  the  phosphoric  acid,  by  means  of 
molybdic  acid;  and  in  a  third,  the  iron,  in  the  volumetric  way. 
The  aluminium  can  then  be  calculated  by  difference.  Attention 
has  already  been  called  (§  135,  A,  y)  to  a  method,  often  very  con- 
venient, which  consists  in  precipitating  the  phosphoric  acid  together 
with  a  small  quantity  of  the  iron  and  then  determining  in  this 
precipitate  the  acid  and  iron,  as  well  as  the  aluminium  carried 
down.  In  this  method  the  molybdic  acid  need  be  separated  only 
from  the  small  quantity  of  iron  and  aluminium,  and  not  from  the 
other  bases,  thus  greatly  simplifying  the  process. 

§136. 

BORIC  ACID  (H3BO3)  AND  BORIC  ANHYDRIDE  (B2O3). 
I.   Determination. 

Boric  acid  is  estimated  either  indirectly  or  in  the  form  of  potas- 
sium borofiuoride. 

\.  The  determination  of  the  boric  acid  in  an  aqueous  or  alco- 
holic solution  cannot  be  effected  by  simply  evaporating  the  fluid 
and  weighing  the  residue,  as  a  notable  portion  of  the  acid  volatil- 
izes and  is  carried  off  with  the  aqueous  or  alcoholic  vapor.  This 
is  the  case  also  when  the  solution  is  evaporated  with  lead  oxide  in 
excess. 

a.  Mix  the  solution  of  the  boric  acid  with  a  weighed  quantity 
of  perfectly  anhydrous  pure  sodium  carbonate,  in  amount  about  1  \ 
times  the  supposed  quantity  of  B203  present.  Evaporate  the  mix- 
ture to  dryness,  heat  the  residue  to  fusion,  and  weigh.  The  residue 
contains  a  known  amount  of  Na2O,  and  unknown  quantities  of  COa 
and  B2O3  combined  as  sodium  borate  and  carbonate.  Determine 
the  COa  by  one  of  the  methods  given  in  §  139,  and  find  the  BaO3 
from  the  difference  (II.  ROSE). 

1}.  In  the  method  a,  if  between  1  and  2  mol.  sodium  carbonate 
(NaaCO3)  are  used  to  1  mol.  B2O3 — and  this  can  easily  be  done  if 
one  knows  approximately  the  amount  of  the  latter  present — all  the 
carbonic  acid  io  expelled  by  the  boric  acid.  Hence  we  have  only 
to  deduct  the  NaaO  from  the  residue  to  find  the  B2O3.  As  the 
tumultuous  escape  of  carbonic  acid  may  lead  to  loss,  it  is  well,  after 
1  inving  thoroughly  dried  the  residual  saline  mass,  to  project  it  in 
small  portions  cautiously  into  the  red-hot  crucible.  Results  good 
(F.  G.  SCHAFFGOTSCH).* 

c.  When  the  amount  of  acid  is  quite  unknown,  and  an  estima- 
tion of  carbonic  acid  in  the  residue  is  objected  to,  you  may  proceed 
*  Pogg.  Ann.,  cvn,  427. 


466  DETERMINATION.  [§  136. 

thus :  Evaporate  the  solution  of  the  acid  with  addition  of  a  weighed 
quantity  of  anhydrous  neutral  borax  (sodium  metaborate  NaBOJ 
free  from  carbonic  acid  to  dryness,  and  heat  the  residue  to  redness 
with  great  caution  (on  account  of  the  intumescence)  till  the  weight 
is  constant.  The  amount  of  neutral  borax  must  be  so  adjusted 
that  it  may  not  be  entirely  converted  into  common  borax  (2NaBO2 
B2O3)  (II.  KOSE). 

d.  If  a  solution  contains,  besides  boric  acid,  only  alkalies  or 
magnesium,  the  acid  may  be  determined,  according  to  C.  MARIG- 
NAC,*  in  the  following  manner :  Neutralize  the  solution  with 
hydrochloric  acid,  add  double  magnesium  and  ammonium  chloride 
in  sufficient  quantity  to  give  at  least  2  parts  of  MgO  to  l.part  of 
B2OS,  then  add  ammonia  and  evaporate  to  drynces.  If  on  adding 
the  ammonia  a  precipitate  is  formed  which  does  not  redissolve 
readily  on  warming,  add  more  ammonium  chloride.  The  evapora- 
tion is  conducted,  at  least  towards  the  end,  in  a  platinum  dish,  a 
few  drops  of  ammonia  being  added  from  time  to  time.  Ignite  the 
dry  mass,  treat  with  boiling  water,  collect  the  insoluble  precipitate 
(consisting  of  magnesium  borate  mixed  with  excess  of  magnesium 
oxide)  on  a  filter,  and  wash  with  boiling  water  till  the  wrashings 
remain  clear  with  nitrate  of  silver.  The  filtrate  and  washings  are 
mixed  with  ammonia,  evaporated  to  dryness,  ignited,  and  washed 
with  boiling  water  as  before. 

The  two  insoluble  residues  are  ignited  together  in  the  platinum 
dish  before  used,  as  strongly  as  possible,  and  for  a  sufficiently  long 
time,  in  order  to  decompose  the  slight  traces  of  magnesium  chlo- 
ride that  might  still  be  present.  After  weighing  determine  the 
magnesium  oxide,  and  find  the  boric  acid  from  the  difference. 
The  determination  of  the  magnesium  may  be  made  by  dissolving 
the  residue  in  hydrochloric  acid  and  precipitating  as  ammonium 
magnesium  phosphate,  or  more  quickly,  and  almost  as  accurately, 
by  dissolving  in  a  known  quantity  of  standard  sulphuric  acid  at  a 
boiling  temperature  and  determining  the  excess  of  acid  with  stand- 
ard soda  (comp.  Alkalimetry). 

Should  a  little  platinum  remain  behind  on  dissolving  the  resi- 
due, it  must  be  weighed  and  subtracted  from  the  weight  of  the 
whole  (unless  the  dish  was  weighed  first).  Results  satisfactory. 
MARIGNAC  obtained  in  two  experiments  0-276  instead  of  O28. 

2.  If  boric  acid  is  to  be  determined  as  potassium  borqftuoride, 
alkalies  only  (preferably  only  potassa)  maybe  present.    The  process- 
*  Zeitschr.  f.  analyt.  Chem.,  I,  405. 


§  136.]  BOKIC   ACID   AND   BOEIC   ANHYDKIDE.  467 

is  conducted  as  follows :  Mix  the  fluid  with  pure  solution  of  potassa, 
adding  for  each  mol.  boric  acid  supposed  to  be  present,  at  least  1 
mol.  potassa ;  add  pure  hydrofluoric  acid  (free  from  silicic  acid)  in 
excess,  and  evaporate,  in  a  platinum  dish,  on  the  water-bath,  to 
dry  ness.  The  fumes  from  the  evaporating  fluid  should  redden 
litmus  paper,  otherwise  there  is  a  deficiency  of  hydrofluoric  acid. 
The  residue  consists  now  of  KF,BF3  and  KF,HF.  Treat  the  dry 
saline  mass,  at  the  common  temperature,  with  a  solution  of  1  part 
of  potassium  acetate  in  4  parts  of  water,  let  it  stand  a  few  hours, 
with  stirring,  then  decant  the  fluid  portion  on  to  a  weighed  filter, 
and  wash  the  precipitate  repeatedly  in  the  same  way,  finally  on  the 
filter,  with  solution  of  potassium  acetate,  until  the  last  rinsings  are 
no  longer  precipitated  by  calcium  chloride.  By  this  course  of  pro- 
ceeding, the  hydrogen  potassium  fluoride  is  removed,  without  a 
.particle  of  the  potassium  borofluoride  being  dissolved.  To  remove 
the  potassium  acetate,  wash  the  precipitate  now  with  84  per  cent, 
alcohol,  dry  at  100°,  and  weigh.  As  potassium  chloride,  nitrate, 
and  phosphate,  sodium  salts,  and  even,  though  with  some  difficulty, 
potassium  sulphate,  dissolve  in  solution  of  potassium  acetate,  the 
presence  of  these  salts  does  not  interfere  with  the  estimation  of  the 
boric  acid ;  however,  sodium  salts  must  not  be  present  in  consider- 
able proportion,  as  sodium  fluoride  dissolves  with  very  great  diffi- 
culty. The  results  obtained  by  this  method  are  satisfactory.  STEO- 
MEYEK'S  experiments  gave  from  97'5  to  10O7  instead  of  100. 
When  the  amount  of  alkali  salt  to  be  removed  is  very  large,  the 
saline  mass  left  on  evaporation  should  be  warmed  with  the  solution 
of  potassium  acetate,  allowed  to  stand  12  hours  in  the  cold  and 
then  filtered.  In  this  way  the  quantity  of  potassium  acetate 
required  will  be  much  reduced.  For  the  composition  and  proper- 
ties of  potassium  borofluoride,  see  §  93,  5.  As  the  salt  is  very 
likely  to  contain  potassium  silicofluoride  it  is  indispensable  to  test 
it  for  that  substance  ;  this  is  done  by  placing  a  small  sample  of  it 
on  moist  blue  litmus  paper,  and  putting  another  sample  into  cold 
concentrated  sulphuric  acid.  If  the  blue  paper  turns  red,  and 
effervescence  ensues  in  the  sulphuric  acid,  the  salt  is  impure,  and 
contains  potassium  silicofluoride.  To  remove  this  impurity,  dis- 
solve the  remainder  of  the  salt,  after  weighing  it,  in  boiling  water, 
add  ammonia,  and  evaporate,  redissolve  in  boiling  water,  add 
ammonia,  <fcc.,  repeating  the  same  operation  at  least  six  times. 
Finally,  after  warming  once  more  with  ammonia,  filter  off  the 


468  DETEBMINATION.  [§  136. 

silicic  acid,  evaporate  to  dryness,  and  treat  again  with,  solution  of 
potassium  acetate  and  alcohol  (A.  STROMEYER).*  I  was  obliged  to 
modify  STROMEYER'S  method  for  effecting  the  separation  of  the 
silicic  acid,  the  results  of  my  experiments  having  convinced  me 
that  treating  the  salt  only  once  with  ammonia,  as  recommended  by 
that  chemist,  is  not  sufficient  to  effect  the  object  in  view. 

II.  Separation  of  Boric  Acid  from  the  Basic  Radicals. 

a.  From  the  Alkalies. 

Dissolve  a  weighed  quantity  of  the  borate  in  water,  add  an 
excess  of  hydrochloric  acid,  and  evaporate  the  solution  on  the 
water-bath.  Towards  the  end  of  the  operation  add  a  few  more 
drops  of  hydrochloric  acid,  and  keep  the  residue  on  the  water-bath, 
until  no  more  hydrochloric  acid  vapors  escape.  Determine  now 
the  chlorine  in  the  residue  (§  141),  calculate  from  this  the  alkali, 
and  you  will  find  the  boric  acid  from  the  difference. 

E.  SCHWEIZER,  wTith  whom  this  method  originated,  states  that 
it  gave  him  very  satisfactory  results  in  the  analysis  of  borax.  It 
will  answer  also  for  the  estimation  of  the  basic  metals  in  the  case 
of  some  other  borates.  It  is  self-evident  that  the  boric  acid  may  be 
estimated,  in  another  portion  of  the  salt,  by  L,  1,  <?,  or  2.  If  you 
have  to  estimate  boric  acid  in  presence  of  large  proportions  of 
alkali  salts,  make  the  fluid  alkaline  with  potassa,  evaporate  to  dry- 
ness,  extract  the  residue  with  alcohol  and  some  hydrochloric  acid, 
udd  solution  of  potassa  to  strongly  alkaline  reaction,  distil  off  the 
alcohol,  and  then  proceed  as  in  I.,  1,  c,  or  2  (AuG.  STROMEYER,  loc. 
cit.). 

LuNGEf  determined  the  soda  in  boronatrocalcite  alkalimetri- 
cally,  by  dissolving  the  mineral  in  normal  nitric  acid  (§  215)  and 
titrating  back  with  normal  soda,  till  the  tint  of  the  litmus  added 
becomes  violet. 

1).  From  Calcium. 

Dissolve  in  hydrochloric  acid  in  the  heat,  avoiding  too  large  an 
excess,  neutralize  with  ammonia  and  precipitate  with  ammonium 
oxalate  (.LUNGE,  loc.  cit.). 

c.  From,  almost  all  other  Bases  except  Alkalies. 

The  compounds  are  decomposed  by  boiling  or  fusing  with 
potassium  carbonate  or  hydroxide  ;  the  precipitated  base  is  filtered 
off,  and  the  boric  acid  determined  in  the  filtrate,  according  to  L,  1, 
*  Annal.  d.  Chem.  u.  Pharm.,  c,  82.  t^-»  cxxxvm,  53. 


§  136. J  BORIC   ACID  AND  BORIC   ANHYDRIDE.  469 

d,  or  2.  If  magnesium  was  present,  a  little  of  this  is  very  likely 
to  get  into  the  filtrate,  and — if  process  L,  2,  is  employed — upon 
neutralizing  with  hydrofluoric  acid,  this  separates  an  insoluble 
magnesium  fluoride,  which  may  either  be  filtered  off  at  once,  or 
removed  subsequently,  by  treating  the  potassium  borofluoride  with 
boiling  water,  in  which  that  salt  is  soluble,  and  the  magnesium 
fluoride  insoluble. 

d.  From  the  Metallic  Oxides  of  the  Fourth,  Fifth,  and  Sixth 
Groups. 

The  metallic  oxides  are  precipitated  by  hydrogen  sulphide,  or, 
as  the  case  may  be,  ammonium  sulphide,*  and  determined  by  the 
appropriate  methods.  The  quantity  of  boric  acid  may  often  be 
inferred  from  the  loss.  If  it  has  to  be  estimated  in  the  direct  way, 
the  filtrate,  after  addition  of  solution  of  potassa  and  some  potassium 
nitrate,  is  evaporated  to  dry  ness,  the  residue  ignited,  and  the  boric 
acid  estimated  by  I.,  1,  d,  or  2.  In  cases  where  the  metal  has  been 
precipitated  by  hydrogen  sulphide  from  acid  or  neutral  solutions, 
the  boric  acid  may  also  be  determined  in  the  filtrate — in  the  absence 
of  other  acids — by  I.,  1,  a  or  b  or  c,  after  the  complete  removal  of 
the  hydrogen  sulphide  by  transmitting  carbon  dioxide  through  the 
fluid. 

e.  From  the  whole  of  the  Fixed  Basic  Radicals. 

A  portion  of  the  very  finely  pulverized  substance  is  weighed, 
put  into  a  capacious  platinum  dish,  and  digested  with  a  sufficient 
quantity  of  hydrofluoric  acid  (which  leaves  no  residue  when  evapo- 
rated in  a  platinum  dish) ;  pure  concentrated  sulphuric  acid  is  then 
gradually  added,  drop  by  drop,  and  the  mixture  heated,  gently  at 
first,  then  more  strongly,  until  the  excess  of  the  sulphuric  acid  is 
completely  expelled.  In  this  operation  the  boric  acid  goes  off  in 
the  form  of  fluoride  of  boron  (B2O3  +  6HF  =  2BF8  +  3HaO). 
The  basic  metals  contained  in  the  residue  in  the  form  of  sulphates 
are  determined  by  the  appropriate  methods,  and  the  quantity  of 
the  boric  acid  is  found  by  difference.  It  is  of  course  taken  for 
granted  that  the  substance  is  decomposable  by  sulphuric  acid. 

*  Boric  acid  cannot  be  separated  completely  from  aluminium  by  precipitation 
of  the  hydrochloric  acid  solution  with  ammonium  sulphide  or  with  ammonium 
carbonate  (WoHLBR)-  Ann.  d.  Chem.  u.  Pharm.,  CXLI,  268;  Zeitsc/ir.  f.  analyt. 
Chem.,  vi,  225. 


470  DETERMINATION.  [§  137. 


3.  OXALIC  ACID. 

I.  Determination. 

Oxalic  acid  is  either  precipitated  as  calcium  oxalate,  and  esti- 
mated  after  determination  of  the  calcium  in  the  latter  as  oxide, 
carbonate,  or  sulphate  •  or  the  amount  contained  in  a  compound 
is  inferred  from  the  quantity  of  solution  of  potassium  permanga- 
nate required  to  effect  its  conversion  into  carbonic  acid  ;  or  from 
the  quantity  of  gold  which  it  reduces  ;  or  from  the  amount  of  car- 
bonic acid  which  it  affords  by  oxidization. 

a.  Determination,  as  Calcium  Carbonate,  <&c. 

Precipitate  with  solution  of  calcium  acetate,  added  in  moderate 
excess,  and  treat  the  precipitated  calcium  oxalate  as  directed  in 
§  103.  If  this  method  is  to  yield  accurate  results,  the  solution 
must  be  neutral  or  slightly  acid  with  acetic  acid  ;  it  must  not  con- 
tain salts  of  aluminium,  chromium,  or  of  the  heavy  metals,  more 
especially  cupric  or  ferric  salts  ;  therefore,  where  these  conditions 
do  not  exist,  they  must  h'rst  be  supplied. 

b.  Determination  by  means  of  Solution  of  Potassium  Perman- 
ganate. 

Standardize  the  solution  of  potassium  permanganate,  as  directed 
§  112,  2,  a,  cc,  by  means  of  oxalic  acid  ;  then  dissolve  the  substance 
in  about  150  c.c.  water,  or  acid  and  water  (sulphuric  acid  is  the 
best  acid  to  use)  ;  add,  if  necessary,  a  further  quantity  of  sulphuric 
acid  (about  6  or  8  c.c.  strong  sulphuric  acid  should  be  present),  heat 
to  about  60°,  and  then  run  in  the  permanganate,  with  constant 
stirring,  until  the  fluid  just  shows  a  red  tint.  Knowing  the  quan- 
tity of  oxalic  acid  which  100  c.c.  of  the  standard  permanganate 
will  oxidize,  a  simple  calculation  will  give  the  quantity  of  oxalic 
acid  corresponding  to  the  c.c.  of  permanganate  used  in  the  experi- 
ment. The  results  are  very  accurate. 

c.  Determination  from  the  reduced  Gold  (H.  EOSE). 

a.  In  compounds  soluble  in  water.  Add  to  the  solution  of  the 
oxalic  acid  or  the  oxalate  a  solution  of  sodium  auric  chloride,  or 
ammonium  auric  chloride,  and  digest  for  some  time  at  a  tempera- 
ture near  ebullition,  with  exclusion  of  direct  sunlight.  Collect  the 
precipitated  gold  on  a  filter,  wash,  dry,  ignite,  and  weigh.  2  at. 


§  137.]  OXALIC   ACID.  471 

An.   (197-2  X  2  =  394-4  correspond  to  3  mol.  C2O3  (72  X  3  = 
216). 

/?.  In  compounds  insoluble  in  water.  Dissolve  in  the  least 
possible  amount  of  hydrochloric  acid,  dilute  with  a  very  large 
quantity  of  water,  in  a  capacious  flask,  cleaned  previously  with 
solution  of  soda ;  add  solution  of  gold  in  excess,  boil  the  mixture 
some  time,  let  the  gold  subside,  taking  care  to  exclude  sunlight, 
and  proceed  as  in  a. 

d.  Determination  as  Carbonic  Acid. 

This  may  be  effected  either, 

a.  By  the  method  of  organic  analysis ;  or 

fi.  By  mixing  the  oxalic  acid  or  oxalate  with  finely  pulverized 
manganese  dioxide  in  excess,  arid  adding  sulphuric  acid  to  the  mix- 
ture, in  an  apparatus  so  constructed  that  the  disengaged  CO2  passes 
-off  perfectly  dry.  The  theory  of  this  method  may  be  illustrated 
by  the  following  equation:  H2C2O4  + MnO2 +  H2SO4  =  MnSO4 
-f  2H2O  +  2CO,.  Each  eq.  of  oxalic  acid  hence  affords  2  eq. 
of  carbon  dioxide.  For  the  apparatus  and  process,  I  refer  to  the 
chapter  on  the  examination  of  manganese  ores,  in  the  Special  Part 
of  this  work.  Here  I  may  remark  that  free  oxalic  acid  must  first 
be  prepared  for  the  process  by  slight  supersaturation  with  alkali 
free  from  carbonic  acid,  and  also  that  9  parts  of  oxalic  anhydride 
(CaO8)  require  theoretically  11  parts  of  (pure)  manganese  dioxide. 
Since  an  excess  of  the  latter  substance  does  not  interfere  with  the 
accuracy  of  the  results,  it  is  easy  to  find  the  amount  to  be  added. 
The  manganese  dioxide  need  not  be  pure,  but  it  must  contain  no 
carbonate.  This  method  is  expeditious,  and  gives  very  accurate 
results,  if  the  process  is  conducted  in  an  apparatus  sufficiently  light 
to  admit  of  the  use  of  a  delicate  balance.  Instead  of  manganese 
dioxide,  potassium  chromate  may  be  used  (compare  §  130,  1,  d), 
and  instead  of  estimating  the  carbonic  acid  by  loss  it  may  be  col- 
lected in  a  weighed  soda-lime  tube  and  the  increased  weight 
noted  (§  139,  0);  the  latter  method  is  always  to  be  preferred  in 
the  case  of  small  quantities. 

II.   Separation  of  Oxalic  Acid  from  the  Basic  Radicals. 

The  most  convenient  way  of  analyzing  oxalates  is,  in  all  cases, 

to  determine  in  one  portion  the  acid,  by  one  of  the  methods  given 

in  I.,  in  another  portion  the  basic  radical,  particularly  as  the  latter 

object  may  be  generally  effected  by  simple  ignition  in  the  air, 


472  DETERMINATION.  [§  138. 

which  reduces  the  salt  either  to  the  metallic  state  (e.g.,  silver  oxa- 
late),  or  to  pure  oxide  (e.g.,  lead  oxalate),  or  to  carbonate  (e.g., 
the  oxalates  of  the  alkalies  and  alkali-earth  metals).  In  some 
cases,  as  when  bases  are  present  which  are  reduced  by  carbonic 
oxide,  or  where  the  resulting  carbonate  gives  up  its  carbonic 
either  not  at  all  or  with  difficulty  on  ignition,  the  estimation  of 
the  base  is  more  easily  effected  by  simple  fusion  with  borax  glass 
(compare  §  139,  II,  c).  The  increase  in  weight  of  the  platinum 
crucible  containing  the  borax  glass  after  fusion  corresponds  to  the 
weight  of  the  base  present.  The  loss  of  weight  corresponds  to 
the  oxalic  acid,  or  the  oxalic  acid  and  water,  as  the  case  may  be. 

If  the  acid  and  basic  radical  have  to  be  determined  in  one  and 
the  same  portion  of  the  oxalate,  the  following  methods  may  be 
resorted  to : 

a.  The  oxalic  acid  is  determined  by  I.,  <?,  and  the  gold  separated 
from  the  basic  metals  in  the  filtrate  by  the  methods  given  in  Sec- 
tion Y. 

I.  In  many  soluble  salts  the  oxalic  acid  may  be  determined  by 
the  method  I.,  a ;  separating  the  basic  metals  afterwards  from  the 
excess  of  the  calcium  salt  by  the  methods  given  in  Section  Y. 

c.  Many  oxalates  of  metals  which  are  completely  precipitated 
as  carbonates  or  oxides  by  excess  of  sodium  or  potassium  carbonate, 
may  be  decomposed  by  boiling  with  excess  of  these  reagents, 
metallic  oxide  or  carbonate  being  formed  on  the  one,  and  alkali 
oxalate  on  the  other  side. 

d.  All  oxalates  of  the  metals  of  the  fourth,  fifth,  and  sixth 
groups  may  be  decomposed  with  hydrogen  sulphide  or  ammonium 
sulphide. 

§138. 
4.  HYDROFLUORIC  ACID. 

I.  DETERMINATION. 

Free  hydrofluoric  acid  in  aqueous  solution*  is  determined  either 
with  standard  alkali  or  as  calcium  fluoride  (§  215).  In  the  latter 
case  sodium  carbonate  is  added  in  moderate  excess,  then  the  solu- 

*  In  analyzing  fluorides  you  must  always  avoid  bringing  acid  solutions  in 
contact  with  glass  or  porcelain.  If  platinum  or  silver  dishes  of  sufficient  size 
are  not  at  hand  you  may  sometimes  use  gutta-percha  vessels  or  glass  vessels 
coated  with  wax  or  paraffin. 


§  138.]  HYDROFLUORIC   ACID.  473 

tion  being  boiled,  calcium  chloride  is  added  as  long  as  a  precipitate 
continues  to  form  ;  when  the  precipitate,  which  consists  of  calcium 
fluoride  and  carbonate,  has  subsided,  it  is  washed,  first  by  decanta- 
tion,  afterwards  on  the  filter,  and  dried  ;  when  dry,  it  is  ignited  in 
a  platinum  crucible  (§  53) ;  water  is  then  poured  over  it  in  a  plati- 
num or  porcelain  dish,  acetic  acid  added  in  slight  excess,  the  mix- 
ture evaporated  to  dryness  on  the  water-bath,  and  heated  on  the 
latter  until  all  odor  of  acetic  acid  disappears.  The  residue,  which 
consists  of  calcium  fluoride  and  acetate,  is  heated  with  water,  the 
calcium  fluoride  filtered  off,  washed,  dried,  ignited  (§  53),  and 
weighed.  As  a  control  of  the  purity  of  the  calcium  fluoride,  it  is 
well  to  convert  it  after  weighing  into  sulphate.  If  the  precipitate 
of  calcium  fluoride  and  carbonate  were  treated  with  acetic  acid, 
without  previous  ignition,  the  washing  of  the  fluoride  would  prove 
a  difficult  operation.  Presence  of  nitric  or  hydrochloric  acid  in  the 
aqueous  solution  of  the  hydrofluoric  acid  does  not  interfere  with 
the  process  (H.  ROSE). 

II.  SEPARATION  OF  FLUORINE  FROM  THE  METALS. 
1.  Fluorides  Soluble  in  Water. 

If  the  solutions  have  an  acid  reaction,  sodium  carbonate  is 
added  in  excess.  If  there  is  an  odor  of  ammonia  now,  heat  till  the 
latter  is  expelled.  If  the  sodium  carbonate  produces  no  precipitate, 
the  fluorine  is  determined  by  the  method  given  in  I.,  and  the 
metals  in  the  filtrate  are  separated  from  calcium  and  sodium  by 
the  methods  given  in  Section  Y.  But  if  the  sodium  carbonate  pro- 
duces a  precipitate,  the  mixture  is  heated  to  boiling,  then  filtered, 
and  the  fluorine  determined  in  the  filtrate  by  the  method  given  ?n 
I. ;  the  metals  are  in  the  precipitate,  which  must,  however,  first  be 
tested,  to  make  sure  that  it  contains  no  fluorine.  Neutral  solutions 
are  mixed  with  a  sufficient  quantity  of  calcium  chloride,  and  the 
mixture  heated  to  boiling  in  a  platinum  dish  or,  but  less  appropri- 
ately, in  a  porcelain  dish  ;  the  precipitate  of  calcium  fluoride  is 
allowed  to  subside,  thoroughly  washed  with  hot  water  by  decanta- 
tion,  transferred  to  the  filter,  dried,  ignited,  and  weighed.  The 
basic  metals  in  the  filtrate  are  then  separated  from  the  excess  of  the 
calcium  salt  by  the  usual  methods.  That  the  basic  metals  may  be 
determined  also  in  separate  portions  by  the  methods  given  in  2  a, 
need  hardly  be  stated. 


474  DETERMINATION.  [§  188. 

2.  Insoluble  Fluorides. 

a.  Decomposition  by  Sulphuric  Acid  (Indirect  Estimation  of. 
the  Fluorine], 

a.  Anhydrous  Compounds. 

The  finely  pulverized  and  weighed  substance  is  heated  for  some 
time  with  pure  concentrated  sulphuric  acid,  an'd  finally  ignited  until 
the  free  sulphuric  acid  is  completely  expelled.  In  the  presence 
of  alkalies,  ammonium  carbonate  must  be  added  during  the  igni- 
tion. The  residuary  sulphate  is  weighed,  and  the  metal  contained 
in  it  calculated  ;  the  fluorine  is  estimated  by  loss.  In  cases  where 
we  have  to  deal  with  a  metal  whose  sulphate  gives  off  part  of  the 
sulphuric  acid  upon  ignition,  or  where  the  residue  contains  several 
metals,  it  is  necessary  to  subject  the  residue  to  analysis  before  this 
calculation  can  be  made.  In  the  case  of  many  compounds,  for 
instance  of  aluminium  fluoride  (which  after  ignition  requires  pro- 
longed heating  with  sulphuric  acid  for  its  decomposition),  long 
continued  strong  ignition  does  not  leave  the  sulphate,  but  the  oxide 
in  a  pure  state.  Topaz  (a  silicate  of  aluminium  in  isomorphous 
mixture  with  aluminium  silicofluoride)  is  not  decomposed  by  boil- 
ing sulphuric  acid,  but  it  is  decomposed  by  fusion  with  potassium 
disulphate. 

fi.  Ilydrated  Fluorides. 

A  sample  of  the  substance  is  heated  in  a  tube. 

aa.  The  Water  expelled  does  not  redden  Litmus  Paper.  The 
water  is  determined  by  ignition ;  the  fluorine  and  metal  as  directed 
in  <7,  a. 

bb.  The  Water  expelled  has  an  acid  reaction.  The  substance  is 
treated  with  sulphuric  acid  as  directed  in  #,  a,  to  determine  the 
metal,  on  the  one  hand,  and  the  water  -|-  fluorine  on  the  other. 
Another  weighed  portion  is  then  mixed,  in  a  small  retort,  with  about 
6  parts  of  recently  ignited  lead  oxide  ;  the  mixture  is  covered  with 
a  layer  of  lead  oxide,  the  retort  weighed,  and  the  water  expelled 
by  the  application  of  heat,  increased  gradually  to  redness.  No 
hydrofluoric  acid  escapes  in  this  process.  The  weight  of  the  expelled 
water  is  inferred  from  the  loss.  The  first  operation  having  given 
us  the  water  -|-  fluorine,  and  the  second  the  water  alone,  the  dif- 
ference is  consequently  the  fluorine. 

I).  Decomposition  by  Fusion  with  Alkali  Carbonates. 

Many  insoluble  fluorides,  aluminium  fluoride  for  instance,  may 


§138.]  HYDROFLUORIC    ACID.  475 

be  completely  decomposed  by  fusion  with  alkali  carbonate  alone ; 
others,  such  as  calcium  fluoride,  require  the  addition  of  silicic  acid. 
In -the  first  case  the  fluorine  is  estimated  in  the  aqueous  solution  of 
the  fusion  according  to  I.,  in  the  latter  according  to  §  166,  5.  The 
temperature  must  not  be  too  high,  or  some  alkali  fluoride  may  be 
lost. 

3.  Fluorides  completely  Decomposable  by  Sulphuric 
Acid. 

As  might  be  inferred  from  2,  almost  all  fluorides  are  decom- 
posed by  heating  with  sulphuric  acid  with  evolution  of  hydroflu- 
oric acid.  If  silica  or  silicate  is  added  to  the  fluoride  in  sufficient 
quantity,  silicon  fluoride  and  water  escape  instead  of  hydrofluoric 
acid :  SiO,  +  4HF  =  SiF4  +  2H.O. 

On  this  reaction  two  methods  of  determining  fluorine  have 
been  based.  In  the  first,  which  I  published  some  years  ago,*  the 
fluoride  of  silicon  is  determined  by  increase  of  weight  of  absorp- 
tion tubes ;  this  I  believe  to  be  in  many  cases  the  only  method 
which  is  applicable,  and  when  carefully  carried  out  yields  the 
most  accurate  results.  In  the  second  method  the  silicon  fluoride 
is  determined  by  the  loss  in  weight  of  the  evolution  apparatus 
or  of  glass. 

a.  Estimation,  by  Absorption  of  the  'evolved  Fluoride  Silicon. 

The  method  as  here  given  is  the  result  of  a  long  series  of  experi- 
ments ;  the  conditions  laid  down  must  be  most  carefully  attended 
to.  The  fluoride  must  be  in  the  finest  powder.  As  silicic  acid  we 
use  finely  powdered  quartz,  which  has  been  ignited  in  the  air  to 
destroy  any  organic  admixture.  The  sulphuric  acid  should  have  a 
sp.  gr.  of  1  •  848,  it  must  be  colorless  and  free  from  oxides  of  nitro- 
gen and  sulphurous  acid.  The  gasometer  must  be  filled  with  clean 
aij-,  and  not  with  air  from  the  laboratory,  for  any  dust  of  organic 
matter,  traces  of  coal  gas,  &c.,  would  interfere  with  the  accuracy  of 
the  result.  The  apparatus  required  is  shown  Fig.  91.  A  contains 
atmospheric  air,  1)  is  half  filled  with  sulphuric  acid,  c  contains  soda- 
lime  with  plugs  of  cotton,  d  pieces  of  glass  moistened  with  sul- 
phuric acid.  The  air  is  thus  freed  from  carbonic  acid  and  suspended 
matter  and  dried  by  sulphuric  acid,  e  is  the  decomposing  flask ; 
it  has  a  capacity  of  about  250  c.c.  /is  half  filled  with  sulphuric 
acid  ;  its  cork,  which  should  not  fit  air-tight,  bears  a  thermometer 

*  ZeitscUr.f.  tuialyt.  Chem.,  v,  190. 


476 


DETERMINATION. 


[§  138. 


whose  bulb  dips  into  the  acid,    e  and/*  should  be  so  placed  on  the 
iron  plate  that  the  temperature  in  both  may  be  equal,     g  is  empty. 


h  contains  fused  calcium  chloride  in  the  first  limb,  and  pumice 
impregnated  with  anhydrous  cupric  sulphate  in  the  second.  These 
U-tubes  serve  to  retain  the  small  amount  of  sulphuric  acid  and  the 


§  138.]  HYDROFLUORIC   ACID.  477 

hydrochloric  acid  which  may  accompany  it.  The  calcium  chloride 
and  the  cnpric  sulphate  must  both  be  anhydrous,  or  they  will 
decompose  and  retain  silicon  fluoride.  ?',  &,  and  I  are  the  weighed 
absorption  tubes;  they  are  10  or  12  cm.  high,  and  about  12  mm. 
wide,  i  contains  in  the  first  limb  pumice  moistened  with  water 
between  plugs  of  cottonrin  the  bend  and  half  of  the  second  limb  soda- 
lime,  in  the  upper  half  of  the  second  limb  fused  calcium  chloride 
between  plugs  of  cotton .  The  tube  after  being  charged  weighs  about 
40  or  50  grm.  ~k  completes  the  absorption ;  it  is  filled  half  with 
soda-lime  and  half  with  fused  calcium  chloride.  I  takes  up  again 
the  small  amount  of  water  carried  away  from  i  and  k ;  the  bend  is 
filled  with  pieces  of  glass  moistened  with  sulphuric  acid.  These 
absorption  tubes  retain  the  silicon  fluoride,  the  carbonic  acid  which 
may  be  possibly  evolved  from  the  soda-lime  by  hydrofluosilicic  acid, 
and  the  aqueous  vapor ;  and  the  air  escapes  through  the  unweighed 
guard  tube  in  into  the  atmosphere.  The  latter  contains  in  the  first 
limb  calcium  chloride,  in  the  second  soda-lime.  The  flexible  con- 
nections should  not  be  long,  and  should  be  washed  and  dried 
before  use. 

When  the  apparatus  has  been  tested  and  found  air-tight,  place 
the  weighed  and  very  finely  divided  substance  in  e.  The  substance 
should  be  free  from  carbonic  acid  (§  166,  8),  and  the  quantity  taken 
should  give  not  less  than  O'l  grm.  silicon  fluoride  if  possible.  Add 
for  every  part  of  fluoride  supposed  to  be  present  10  or  15  parts  of 
finely  powdered  quartz  (previously  strongly  ignited  in  the  air,  and 
then  40  or  50  c.c.  pure  concentrated  sulphuric  acid.  Connect  0,  on 
the  one  hand,  with  d,  and,  on  the  other,  with  <?,  and  pass  a  moderate 
current  of  air,  which  should  enter  the  fluid  in  the  decomposing  flask 
from  the  bottom.  Heat  the  iron  plate,  shake  e  frequently  and  raise 
the  temperature  very  gradually,  till  the  thermometer  in  f  indicates 
150°  to  160°.  The  commencement  of  the  decomposition  shows 
itself  not  only  by  the  appearance  of  bubbles  of  gas  in  the  fluid, 
more  particularly  at  the  edge,  but  also  by  the  separation  of  hydrated 
silica  in  i.  The  bubbles  of  gas  will  disappear  on  shaking  the  fluid  ; 
as  soon  as  they  cease  to  form  again  remove  the  lamp ;  the  time 
usually  occupied  in  the  decomposition  is  one  hour  for  small  quantities 
of  fluoride (0-1  grm. ), two  or  three  hours  for  large  quantities  (1  grm.).. 
After  a  while  shut  off  the  current  of  air,  remove  the  weighed  tubes 
i,  &,  and  Z,  and  during  the  weighing  of  these  connect  h  with  m  by 
means  of  a  glass  tube.  After  weighing  replace  i,  k,  and  £,  heat 


478  DETERMINATION.  [§  138. 

again  to  150°  or  160°,  and  pass  the  air  again  for  half  an  hour  or  an 
hour,  weighing  «',  A:,  and  /  again.  If  any  alteration  of  weight  has 
occurred,  the  process  must  be  continued. 

The  increase  in  weight  of  the  absorption  tubes  after  deducting 
O'OOl  grm.  for  every  hour  during  which  the  air  has  been  passing 
(i.e.,  for  every  6  litres  of  air)  represents  the  amount  of  silicon 
fluoride.  The  small  correction  is  necessary  because  air,  even  when 
it  comes  in  contact  only  with  short  washed  pieces  of  india-rubber, 
always  gives  traces  of  sulphurous  and  carbonic  acid  when  passed 
through  hot  concentrated  sulphuric  acid.  The  results  thus  obtained 
are  very  satisfactory,  and  differ  from  the  truth  at  the  most  by  a 
few  milligrammes. 

b.  Other  methods  of  Estimating  the  Silicon  Fluoride  expelled. 

d.  Method  of  WOHLER.  Only  applicable  when  the  substance 
is  readily  decomposed  by  sulphuric  acid,  and  the  amount  of  fluorine 
is  large.  Transfer  the  very  finely  divided  substance,  if  necessary, 
intimately  mixed  with  10  or  15  parts  of  ignited  quartz  powder,  to  a 
small  flask,  add  pure  sulphuric  acid,  close  quickly  with  a  cork  fitted 
with  a  small  tube  filled  with  fused  calcium  chloride  (or  better  still, 
half  with  fused  calcium  chloride  and  half  with  anhydrous  cupric 
sulphate  on  pumice),  weigh  the  whole  apparatus  as  quickly  as  pos- 
sible, warm  it  till  no  more  fumes  of  silicon  fluoride  escape,  remove 
the  last  particles  of  gas  in  the  apparatus  by  aj*  air  pump,  allow  to 
cool,  and  weigh.  The  loss  of  weight  indicates  the  amount  of  silicon 
fluoride. 

ft.  Regarding  the  methods  of  F.  v.  KOBELL  *  and  ZALESKY  t 
depending  on.  the  loss  of  weight  of  Bohemian  glass  of  known 
composition,  I  would  remark  that  the  accuracy  of  the  results  said 
to  be  afforded  by  them  still  require  direct  proof. 

y.  [S.  L.  PENFIELD  J  determines  the  amount  of  expelled  silicon 
fluoride  by  an  indirect  volumetric  method  ;  viz. :  by  passing  it  into 
a  solution  of  potassium  chloride,  and  titrating  the  hydrochloric  acid 
which  is  set  free  with  standard  ammonia  solution.  3SiF4  -|-  2 
H20  =  2  H2F2SiF4  +  Si02  and  H2F2SiF4  +  2KC1  =  (KF)2SiF4  +  2 
HC1.  Two  mol.  HC1  thus  liberated  correspond  to  six  at.  F. 

The  process  of  decomposing  the  fluorine  compound  is  conducted 

*  Journ.  f.  prakt.  Chem.,  xcnt  385 ; ,  Zeitschr.  /.  analyt.  Chem.,  v,  204 
t  Zeitschr.  f.  analyt.  Chem.,  v,  905. 
American  Chem.  Journ.,  i,  27. 


§  139.]  CARBONIC    ACID.  479 

as  in  a,  and  the  same  apparatus  may  be  used  except  that  the  four 
last  U-tubes  *',  &,  /,  m,  are  replaced  by  two  larger  U-tnbes  for  hold- 
ing the  solution  of  potassium  chloride. 

The  aqueous  solution  of  KC1  is  mixed  with  an  equal  volume  of 
alcohol  to  effect  complete  precipitation  of  the  hydrofluosilicic  acid. 
The  titration  may  be  either  effected  directly  in  U-tubes  (the  second 
of  which  will  contain  but  a  very  small  quantity  of  acid)  or  after 
transferring  to  a  beaker  and  rinsing  the  tubes  with  alcohol  and 
water.  Care  must  be  taken  to  loosen  and  break  up  the  silicic  acid 
and  to  have  at  least  half  of  the  final  volume  at  the  end  of  the  titra- 
tion consist  of  alcohol.  Results  given  by  the  author  (loc.  cit.)  very 
satisfactory.] 


Fourth  Division  of  the  First  Group  of  the  Acids. 

CAKBONIC    ACID— SILICIC    ACID. 

§  139. 

1.   CARBONIC  ACID. 
I.  Determination. 

a.  In  a  mixture  of  Gases. 

After  thoroughly  drying  the  gases  with  a  ball  of  calcium 
chloride,  or  saturating  with  moisture  (§  16),  measure  them  accu- 
rately in  a  graduated  tube  over  mercury,  insert  a  ball  of  potassium 
hydroxide,*  cast  on  a  platinum  wire  in  a  pistol-bullet  mould, .take 
care  that  the  end  of  the  platinum  wire  remains  under  the  surface 
of  the  mercury,  leave  in  the  tube  for  24  hours,  or  until  the  vol- 
ume of  the  gas  ceases  to  show  further  diminution,  withdraw  the 
ball,  and  measure  the  gas  remaining ;  reinsert  the  same  or  a  fresh 
ball  of  potassa,  and  repeat  till  no  further  absorption  takes  place. 
The  carbonic-acid  gas  is  inferred  from  the  difference,  provided 
the  gaseous  mixture  contained  no  other  gas  liable  to  absorption  by 
potassa  (compare  §§  12—16  and  §  198).  In  very  accurate  analyses 
you  must  bear  in  mind  that  carbonic  acid  does  not  exactly  follow 
the  law  of  MAKIOTTE  (§  198,  fi). 

*  The  ordinary  hydroxide  is  not  adapted  for  the  purpose.     It  should  be 
fused  with  a  quarter  of  its  weight  of  water  in  a  platinum  crucible. 


480  DETERMINATION.  [§139. 

If  the  amount  of  carbonic  acid  is  very  small,  this  process  does 
not  yield  sufficiently  accurate  results.  In  such  cases  one  of  the 
methods  recommended  in  "  The  Analysis  of  Atmospheric  Air  " 
should  be  employed.  Several  kinds  of  special  apparatus  are  in 
use  for  the  estimation  of  carbonic  acid  in  coal  gas,  and  for  the 
purposes  of  sugar  works.  I  may  mention  those  proposed  by 
F.  RUDOKFF  *  and  LEHMANN  and  H.  WAIILERT  f  for  the  first  pur- 
pose, and  by  C.  SCHEIBLER  ;£  and  C.  STAMMER§  for  the  second. 
Besides  these  volumetric  methods  the  gravimetric  processes  given 
by  myself  for  the  analysis  of  gaseous  mixtures  ||  may  often  be 
used  with  great  advantage.  These  will  be  given  in  the  Special 
Part. 

I.   In  Aqueous  Solution. 

a.  WITH  CALCIUM  HYDROXIDE. 

Into  a  flask,  holding  about  300  c.  c.  put  2*5  to  3  grm.  cal- 
cium hydroxide,  perfectly  free  from  carbonate. T  If  a  lime  pure 
enough  for  the  purpose  cannot  be  obtained,  determine  the  car- 
bonic acid  in  it  according  to  §  139,  II,  e\  weigh  the  quantity 
taken,  and  later  deduct  the  carbonic  acid  in  the  weighed  quan- 
tity from  the  result.  Provide  the  flask  with  a  good  india-rubber 
stopper,  tare  or  weigh  exactly,  add  the  carbonic-acid  water  with 
gentle  agitation  till  the  flask  is  two-thirds  or  three-quarters  full, 
and  close  at  once. 

In  adding  the  carbonic-acid  water  every  care  must  of  course 
be  taken  to  guard  against  loss  of  carbonic  acid.  If  the  water 
flows  from  a  pipe,  it  is  allowed  simply  to  run  in.  If  it  is  in  a  jug 
or  bottle,  cool  it  to  4°,  and  transfer  the  quantity  required  with  a 
siphon.**  If  the  water  is  in  a  basin  or  well,  provide  the  flask 
with  an  apparatus,  as  shown  in  Fig.  92,  and  consisting  of  a  stop- 
per in  which  two  glass  tubes  are  inserted,  one  a  few  inches  long, 
pushed  down  only  to  the  lower  surface  of  the  stopper,  the  other 

*  Pogg.  Annal. ,  cxxv,  71.  f  Zeitschr.  f.  analyt.  Chem. ,  vii,  58. 

\Dinglerspolyt.Journ.,  CLXXXIII,  306;  Zeitschr.  f.  analyt.  Chem.,  vi,  261. 

§  Dingier' s polyt.  Journ.,  en,  368;  Zeitschr.  f.  analyt.  Chem.,  xi,  231. 

||  Zeitschr.  f.  analyt.  Chem. ,  in,  343. 

If  This  is  prepared  by  slaking  freshly  burnt  lime  with  water  in  such  a  man 
ner  that  the  hydrate  obtained  appears  dry  and  pulverulent.     It  is  preserved  in 
small  bottles,  the  corks  or  stoppers  of  which  are  covered  with  sealing-wax. 

**  If  the  water  is  poured  directly  from  the  jug  into  the  flask,  carbonic-acid 
gas  is  very  likely  to  get  into  the  latter  as  well  as  the  water. 


139.] 


CARBONIC    ACID. 


481 


Fig.  92. 


extending  through  the  stopper  a  short  distance  into  the  flask,  but 

only  to  the  i^per  surface  of  the  stopper.      Sink  the  flask  into  the 

water  until   the   upper  end  of  the 

tube  ab  is  below  the  surface,  and 

water  will  enter  one  tube  and  air 

escape  through   cd.     Water  which 

is  not  very  rich  in  free  carbonic  acid 

may  be  removed  from  the  basin  or 

well  by  a  plunging-siphon. 

'Now  weigh  the  flask  with  its 
stopper  again  and  you  will  find  the 
quantity  of  water  taken.  No  way 
of  measuring  the  water  is  so  accurate 
in  retaining  all  the  carbonic  acid 
and  in  giving  the  quantity  of  water 
taken. 

If  there  is  much  interval  between 
the  mixing  of  the  water  and  the 

lime  and  the  estimation  of  the  carbonic  acid  in  the  precipitate, 
the  calcium  carbonate,  which  is  at  first  amorphous,  passes  sponta- 
neously into  the  crystalline  condition ;  but  if  the  carbonic  acid  is 
to  be  determined  soon  after  the  mixing,  heat  for  some  time  on 
the  water-bath,  raising  the  stopper  occasionally,  in  order  to  hasten 
the  change  of  the  calcium  carbonate.  Now,  without  disturbing 
the  precipitate,  filter  the  clear  fluid  through  a  small  plaited  filter, 
which  will  take  a  very  short  time,  throw  the  filter  at  once  into 
the  flask  containing  the  precipitate  and  the  rest  of  the  fluid,  and 
proceed  according  to  II,  e.  This  process  has  been  in  use  for  10 
years  in  my  laboratory  for  all  mineral  water  analyses ;  it  is  ex- 
tremely simple  and  gives  excellent  results,*  If  the  water  con- 
tains alkali  carbonate,  put  a  quantity  of  calcium  chloride  suf- 
ficient to  decompose  the  alkali  carbonate  with  the  lime  in  the 
flask  before  adding  the  water. 

/?.  WITH    BARIUM    CHLORIDE    OR    CALCIUM    CHLORIDE    AND 
AMMONIA. 

Add  an  excess  of  ammonia  to  solution  of  barium  or  calcium 


*  Zeitschr.f.  analyt.  Chem.,  n,  49  and  341. 


182  DETERMINATION.  [ 

shloride,*  boil  for  a  short  time,  when  a  precipitate  of  barium  or 
calcium  carbonate  falls,  let  deposit,  and  filter  the  still  hot,  clear 
liquid  as  rapidly  as  possible,  with  as  little  exposure  to  air  as  pos- 
sible. Introduce  50  to  80  c.  c.  of  this  freshly  prepared  solution 
into  a  300-c.  c.  flask  provided  with  a  rubber  stopper.  For  filling 
with  carbonic-acid  water  one  of  the  methods  given  under  <x  may 
be  employed.  If  the  water  contains  only  free  carbonic  acid,  the 
mixture  remains  clear  at  first,  because  ammonium  carbonate 
(!N"H4[NHa]CO3)  forms ;  if  it,  however,  also  contains  carbonates, 
a  partial  precipitation  of  calcium  or  barium  carbonate  at  once 
forms.  Since  ammonium  carbonate  is  but  slowly  acted  upon  by 
cold  water,f  and  particularly  so  in  the  presence  of  free  ammonia, f 
the  liquid  must  be  heated  in  order  to  precipitate  all  the  carbonic 
acid  as  calcium  or  barium  carbonate.  The  best  method  of  heat- 
ing is,  according  to  my  experiments,  §  to  immerse  the  flask, 
weighted  down  by  a  leaden  ring,  in  water  contained  in  a  tall  jar, 
and  to  heat  the  water  to  boiling.  The  contents  of  the  flask  may 
thus  reach  a  temperature  of  98°,  and  the  precipitation  is  complete 
in  one  and  a  half  to  two  hours.  A  lower  temperature  will  require 
a  longer  time  to  effect  the  object,  but  active  ebullition  must  be 
avoided,  as  it  would  occasion  a  loss  of  ammonium  carbonate,  due 
to  the  action  of  ammonium  chloride  on  the  carbonate  of  the  alkali 
earth.  The  contents  of  the  flask  are  then  treated  according  to 
one  of  the  following  processes : 

aa.  Gravimetric  Determination :  Cool  the  supernatant  fluid 
obtained  as  above,  rapidly  filter,  avoiding  contact  with  air  so  far 
as  possible,  fill  the  flask  with  water  to  which  a  few  drops  carbo- 
nate-free ammonia  water  has  been  added,  stopper,  shake,  allow  to 
settle,  decant,  and  repeat  the  washing  by  decantation ;  then  bring 
the  precipitate  upon  a  filter,  wash  it  until  the  last  portions  of  the 
wash-water  gives  no  reaction  with  silver  nitrate,  dry,  gently 
ignite,  and  weigh  (§  101  .  2,  a).  The  weight  of  the  carbonic  acid 
may  be  calculated  from  that  of  the  barium  carbonate,  provided 
the  solution  contained  no  other  substances  precipitable  by  ammonia 

*  Barium  chloride  is  preferable  when  operating  according  to  aa  later  on; 
the  calcium  chloride  when  using  the  process  bb. 
\Zeitschr.f.  analyt.  Chem..,  v,  321. 

JEDW.  DIVERS,  Journ.  Chem.  Soc.,  London,  New  Ser.,  vni,  359. 
§Zeitschr.f.  analyt.  Chem  ,  u,  50. 


§  139.]  CARBONIC   ACID.  483 

and  barium  chloride.  Should  it,  however,  be  otherwise,  and  the 
precipitated  barium  carbonate  contains  calcium  carbonate,  barium 
phosphate,  ferric  hydroxide,  etc.,  the  precipitate  should  be  gently 
ignited,  but  not  weighed,  and  the  carbonic  acid  determined 
according  to  one  of  the  methods  given  under  II ;  e.g. ,  II,  c  (fusion 
with  borax  glass).  Free  the  filter  as  completely  as  possible  from 
the  precipitate,  incinerate,  moisten  the  ash  with  a  solution  of 
ammonium  carbonate,  gently  ignite,  and  add  to  the  precipitate. 
If  the  quantity  of  precipitate  is  considerable,  it  is  best  to  weigh  it 
all,  and  then  to  determine  the  carbonic  acid  in  a  weighed  portion 
of  the  uniformly  mixed  powder. 

Should  it  not  be  possible  to  remove  the  last  portions  of  the 
precipitate  from  the  flask  by  mechanical  means,  dissolve  them, 
after  thoroughly  washing  out  the  flask,  in  a  small  quantity  of 
diluted  hydrochloric  acid,  precipitate  with  sodium  carbonate,  and 
filter  off  the  trifling  precipitate  on  a  separate  filter,  which  then 
incinerate  with  the  larger  one. 

lb.  Volumetric  Determination.  Filter  as  in  aa,  but  in  this 
case  it  is  unnecessary  to  collect  the  entire  precipitate,  as  the  par- 
ticles adhering  to  the  flask  may  be  left  and  washed  by  decanta- 
tion.  The  washing  with  water,  to  which  a  few  drops  of  amnionia 
are  added,  is  continued  until  the  filtrate  gives  no  reaction  with 
silver  solution.  The  funnel  containing  the  filter  is  then  placed  on 
the  flask  in  which  the  precipitation  was  effected,  the  filter  point 
pierced,  and  the  precipitate  washed  into  the  flask  by  means  of  a 
wash-bottle.  The  filter  is  then  spread  open  on  a  glass  plate,  and 
any  adhering  particles  also  washed  into  the  flask ;  this  can  be  fully 
accomplished  and  with  ease.  The  precipitate,  even  though  fully 
washed  with  ammonia-free  water,  nevertheless  retains  a  slight 
quantity  of  ammonia,  hence,  in  order  to  drive  this  off,  the  con- 
tents of  the  flask  are  heated  to  gentle  boiling  for  half  an  hour. 
A  little  litmus  tincture  is  then  added,  sufficient  normal  (or  deci- 
normal,  as  the  case  may  be),  nitric  or  hydrochloric  acid  run  in 
from  a  burette  until  the  solution  is  colored  a  distinct  red,  the  car- 
bonic acid  expelled  by  heating,  and  the  liquid  titrated  with  soda 
solution.  After  a  memorandum  has  been  made  of  the  number  of 
c.  c.  of  acid  and  soda  solutions  used,  add  1  c.  c.  of  the  acid,  boil 
again,  and  once  more  titrate  with  soda  solution.  This  procedure 
should  be  repeated  a  number  of  times.  By  subtracting  from  the 


484  DETERMINATION.  [§  139. 

entire  volume  of  acid  used  the  volume  equivalent  to  the  soda 
taken,  the  difference  will  give  the  acid  which  was  required  to 
expel  the  carbonic  acid  (and  which  is  its  equivalent  therefore) 
from  the  calcium  or  barium  carbonate.  The  details  of  this  process 
are  given  under  §223.  The  coloring  matter  of  the  litmus  is 
occasionally  thrown  down  with  the  silicic  acid  separating  from  the 
precipitate,  hence  it  is  often  necessary  to  again  add  some  tincture 
of  litmus.  Should  this  not  yield  the  desired  result,  add  soda  solu- 
tion until  the  reaction  is  almost  complete,  read  off  the  height  of 
the  soda  solution  in  the  burette,  dilute  the  fluid  to  a  definite  vol- 
ume, filter,  and  to  half  of  the  volume  add  soda  solution  cautiously 
until  the  liquid  becomes  blue,  double  the  quantity  of  soda  taken, 
and  add  this  volume  to  that  first  used.  Instead  of  litmus,  cur- 
cuma paper,  etc.,  may  also  be  used,  just  as  in  y,  to  determine  the 
moment  of  alkalinity. 

The  methods  given  under  /?  give  good  results  only  when  all 
the  numerous  sources  of  error  which  may  vitiate  the  accuracy  of 
the  results  are  most  carefully  avoided.  Frequently  the  results 
obtained  are  too  high  because  the  clear  mixture  of  calcium  chlo- 
ride and  ammonia  contains  ammonium  carbonate,  either  from 
incomplete  decomposition  or  because  of  the  absorption  of  carbonic 
acid  from  the  atmosphere  during  filtration,  or,  particularly  when 
operating  according  to  Z>5,  when  there  has  been  any  delay  or 
neglect  in  completely  expelling  all  adherent  ammonia  from  the 
precipitate  by  prolonged  boiling.  These  circumstances,  tending 
to  give  results  too  high,  are  partly  compensated  for  by  the  fact 
that  the  carbonates  of  the  alkaline  earths  are  not  absolutely  in- 
soluble in  the  fluid  containing  ammonium  chloride  and  in  the 
wash-water.  If  the  mixture  of  carbonated  water  and  calcium  or 
barium  chloride  and  ammonia  is  not  heated  as  above  detailed,  the 
results  may  be  too  low,  either  from  imperfect  decomposition  of 
the  ammonium  carbonate  by  reason  of  insufficient  heating,  or  be- 
cause of  loss  of  ammonium  carbonate  by  too  active  an  ebullition. 
y.  AFTER  PETTENKOFER.* 

The  principle  of  this  simple  and  expeditious  process  consists  in 
mixing  the  carbonic-acid  water  with  a  measured  quantity  of  stand- 

*BUCHNEB'S  neuesRepert.,  x,  1;  Journ.  f.  prakt.  Chem.,  LXXXII,  32;  Annal. 
d.  Chem.  u.  Pharm.,  n,  Supplement,  i;  Zeitschr.  f.  analyt.  Chem.,  I,  92. 


§  139.  J  CARBONIC   ACID.  485 

ard  lime  water  (or,  under  certain  circumstances,  baryta  water)  in 
excess.  After  complete  separation  of  the  calcium  or  barium  carbo- 
nate, the  excess  of  calcium  or  barium  in  the  fluid  is  determined  in 
an  aliquot  part  by  means  of  standard  solution  of  oxalic  acid ;  the 
difference  gives  the  calcium  or  barium  precipitated  by  the  carbonic 
acid,  and  consequently  the  amount  of  the  latter  present. 

If  a  water  contains  only  free  carbonic  acid,  the  analyst  has  only 
to  bear  in  mind — if  lime  water  is  employed — that  the  calcium  car- 
bonate formed  is  at  first,  as  long  as  it  remains  amorphous,  very 
perceptibly  soluble  in  water,  to  which  it  communicates  an  alkaline 
reaction.  Hence  the  unprecipitated  lime  in  the  fluid  cannot  be 
estimated  till  the  calcium  carbonate  has  separated  in  the  crystalline 
form,  which  takes  8  or  10  hours,  unless  the  mixture  is  warmed  to 
70°  or  80°.  On  this  account  it  is  generally  best  to  use  baryta 
water  (see  "Analysis  of  Atmospheric  Air"). 

If,  on  the  contrary,  a  water  contains  an  alkali  carbonate  or  any 
other  alkali  salt  whose  acid  would  be  precipitated  by  lime  or  baryta, 
a  neutral  solution  of  calcium  or  barium  chloride  must  first  be  added 
to  decompose  the  same.  This  addition,  too,  prevents  any  incon- 
venience arising  from  the  presence  of  free  alkali  in  the  lime  or 
baryta  water,  or  of  magnesium  carbonate  in  the  carbonic  acid 
water;  this  inconvenience  consists  in  the  fact  that  oxalate  of  an 
alkali  or  of  magnesium  enters  into  double  decomposition  with  cal- 
cium carbonate  (which  is  seldom  entirely  absent  from  the  fluid  to 
be  analyzed),  forming  calcium  oxalate  and  carbonate  of  the  alkali 
or  of  magnesium,  which  latter  will  of  course  again  take  up  oxalic 
acid. 

In  the  presence  of  magnesium  salts  in  the  carbonic  acid  water, 
in  order  to  avoid  the  precipitation  of  the  magnesium,  a  little 
ammonium  chloride  must  also  be  added,  but  in  this  case  heat  must 
not  be  applied  to  induce  the  calcium  carbonate  to  become  more 
quickly  crystalline,  as  ammonia  would  be  thereby  expelled. 

In  making  the  determination  the  first  thing  to  be  done  is  to 
ascertain  the  relation  between  the  lime-  or  baryta  water  and  a 
standard  solution  of  oxalic  acid.  PETTENKOFEK  makes  the  latter 
solution  by  dissolving  2*8647  grin,  pure  uneflioresced  dry  crystal- 
lized oxalic  acid  to  1  litre ;  1  c.c.  of  this  is  equivalent  to  1  mgrm. 
carbonic  acid.  The  lime  water  is  standardized  as  follows  :  Measure 
45  c.c.  into  a  little  flask  which  can  be  closed  by  the  thumb,  and 
then  run  in  from  the  burette  the  solution  of  oxalic  acid  till  the 


486  DETERMINATION.  [§  139. 

alkaline  reaction  has  just  vanished.  During  the  operation  the 
flask  is  closed  with  the  thumb  and  gently  shaken.  The  end  is 
attained  as  soon  as  a  drop  taken  out  with  a  glass  rod  and  applied  to 
sensitive  turmeric  paper  *  produces  no  brown  ring.  The  first 
experiment  is  a  rough  one,  the  second  should  be  exact. 

The  analysis  of  a  carbonic  acid  water  (a  spring  water,  for 
instance)  is  performed  by  transferring  100  c.c.  to  a  dry  flask,  add- 
ing 3  c.c.  of  a  neutral  and  nearly  saturated  solution  of  calcium  or 
barium  chloride,  and  2  c.c.  of  a  saturated  solution  of  ammonium 
chloride,  then  45  c.c.  of  the  standard  lime  or  baryta  water ;  close 
the  flask  with  an  india-rubber  stopper,  shake  and  allow  to  stand  12 
hours.  The  fluid  contents  of  the  flask  measure  consequently  150 
c.c.  From  the  clear  fluidf  take  out  by  means  of  a  pipette  two  por- 
tions of  50  c.c.  each,  and  determine  the  free  lime  or  baryta  by 
means  of  oxalic  acid,  in -the  first  portion  approximately,  in  the 
second  exactly.  Multiply  the  c.c.  used  in  the  last  experiment  by 
3  and  deduct  the  product  from  the  c.c.  of  oxalic  acid  which  corre- 
spond to  45  c.c.  of  lime  or  baryta  water.  The  difference  shows  the 
lime  or  baryta  precipitated  by  carbonic  acid,  each  c.c.  corresponds 
to  1  mgrm.  carbonic  acid. 

The  method  is  convenient  and  good  ;  it  is  especially  to  be 
recommended  for  dilute  carbonic  acid  water.  When  calcium  sul- 
phate or  carbonate  is  present,  as  is  almost  always  the  case  in  spring 
water,  you  must  always  before  titrating  await  the  conversion  of  the 
amorphous  calcium  carbonate  to  the  crystalline  state,  even  if  baryta 
water  is  used  (K.  KNAPP  J).  Baryta  water  therefore  possesses  no 
advantages  over  lime  water  for  the  analysis  of  spring  waters. 

*For  the  preparation  of  this  bibulous  paper  should  be  used,  the  ash  of 
which  is  free  from  carbonate  of  lime.  Swedish  filtering-paper  answers  best. 
J.  GOTTLIEB  (Journ.f.  prakt.  Chem.,  cvn,  488;  Zeitschr.  f.  analyt.  Chem.,  ix, 
251)  prefers  aqueous  tincture  of  litmus,  prepared  from  litmus  first  exhausted 
with  alcohol,  and  used  in  a  very  dilute  state.  E.  SCHULZE  and  M.  MARCKER 
(Zeitschr.  /.  analyt.  Chem.,  ix,  334)  employ  corallin  or  rosolic  acid,  which  they 
say  is  specially  adapted  for  the  purpose.  The  alcoholic  solution  is  cautiously 
neutralized  with  potassa  and  a  drop  or  two  of  this  tincture  is  added.  F. 
SCHULZE  (Zeitschr.  f.  analyt.  Chem.,  ix,  292)  recommends  alcoholic  tincture  of 
turmeric. 

f  It  is  not  admissible  to  use  a  filter  (A.  MULLER,  Zeitschr.  f.  analyt.  Chem., 
I,  84). 

\Annal.  d.  Chem.  u.  Pharm.,  CLVIII,  112;  ZeitscUr.f.  analyt.  Chem.,  x,  361. 


§  139.]  CARBONIC    ACID.  487 

II.    Separation   of   Carbonic    Ac'nl  from    the   Basic 
Radicals,  and  its  Estimation  hi  Carbona1<  *. 

a.  Exit inatioii  in  Normal  Alkali  Carbonates  and  Alkali-earth 
Carbonates. 

.If  the  salts  are  unquestionably  normal  carbonates,  and  there  is 
no  other  salt  with  power  to  neutralize  an  acid  present,  we  may 
determine  the  quantity  of  the  basic  radical  by  the  alkalimetric 
method  (§§  219,  220,  223),  and  calculate  the  amount  of  CO, 
necessary  to  form  with  it  normal  carbonate. 

b.  Separation  from  Basic  Metals  in  Salts  which  upon  ignition 
readily  and  completely  yield  their  Carbonic  Acid. 

Such  are,  for  instance,  the  carbonates  of  zinc,  cadmium,  lead, 
copper,  magnesium,  &c. 

a.  Anhydrous  Carbonates. — Ignite  the  weighed  substance,  in  a 
platinum  crucible  (cadmium  and  lead  carbonates  in  a  porcelain 
crucible),  until  the  weight  of  the  residue  remains  constant.  The 
results  are,  of  course,  very  accurate.  Substances  liable  to  absorb 
oxygen  upon  ignition  in  the  air  are  ignited  in  a  bulb-tube,  through 
which  a  stream  of  dry  carbon  dioxide  gas  is  conducted.  The  car- 
bonic acid  is  inferred  from  the  loss. 

/?.  Hydrated  Carbonates. — The  substance  is  ignited  in  a  bulb- 
tube  through  which  dried  air  or,  in  presence  of  oxidizable  sub- 
stances, carbon  dioxide  is  transmitted,  and  which  is  connected  with 
a  calcium  ^chloride  tube,  by  means  of  a  dry,  close-fitting  cork. 
During  the  ignition,  the  posterior  end  of  the  bulb-tube  is,  by 
means  of  a  small  lamp,  kept  sufficiently  hot  to  prevent  the  con- 
densation of  water  in  it,  care  being  taken,  however,  to  guard  against 
burning  the  cork.  The  loss  of  weight  of  the  tube  gives  the  amount 
of  the  water -(-the  carbonic  acid;  the  increase  of  weight  gained  by 
the  calcium  chloride  tube  gives  the  amount  of  the  water,  and  the 
difference  accordingly  that  of  the  carbonic  acid.  A  somewhat 
wide  glass  tube  may  also  be  put  in  tiie  place  of  the  bulb-tube,  and 
the  substance  introduced  into  it  in  a  little  boat,  which  is  weighed 
before  and  after  the  operation. 

c.  Srjxi ration  front  all  fixed  Basic  Radicals,  without  exception, 
in  Anhydrous  Carbonat<  *. 

Fuse  vitrified  borax  in  a  weighed  platinum  crucible,  allow  to 
cool  in  the  desiccator,  weigh,  then  transfer  the  well-dried  substance 
to  the  crucible  and  weigh  again.  The  weights  of  both  carbonate 


488  DETERMINATION.  [§  139. 

and  borax  are  thus  ascertained.  They  should  be  in  about  the  pro- 
portion of  1  :  4.  Heat  is  then  applied,  which  is  gradually  increased 
to  redness,  and  maintained  at  this  temperature  until  the  contents 
of  the  crucible  are  in  a  state  of  calm  fusion.  The  crucible  is  now 
allowed  to  cool,  and  weighed.  The  loss  of  weight  is  carbonic  acid. 
The  results  are  very  accurate  (SCHAFFGOTSCH). 

I  must  add  that  borax-glass  may  be  kept  in  a  state  of  fusion  at 
a  red  heat  for  J  to  £  an  hour  without  the  occurrence  of  any  vola- 
tilization, but  that  at  a  white  heat  (by  igniting  over  the  gas-bel- 
lows), even  in  a  few  minutes,  it  suffers  a  decided  loss.*  A  few 
bubbles  of  carbonic  acid  remaining  in  the  fusing  mass  are  without 
any  influence  on  the  result. 

Instead  of  vitrified  borax  fused  potassium  dichromate  may  be 
used,  in  the  proportion  of  5  to  1  of  the  carbonate  (H.  RosEf).  The 
heat  applied  in  this  case  must  be  low,  and  great  caution  must  be 
used,  or  the  dichromate  will  lose  weight  of  itself.;};  The  carbonic 
acid  may  be  expelled  from  alkali  carbonates,  by  strong  ignition 
with  ignited  silica  (H.  KOSE§). 

d.  Separation  from  all  bases  without  exception.  (Estimation 
from  the  loss  of  weight.) 

aa.    Carbonates  of  Bases  yielding  Soluble  Salts  with 
Sulphuric  Acid. 

The  process  is  conducted  in  the  apparatus  illustrated  by  Fig.  93. 

The  size  of  the  flask  depends  upon  the  capacity  of  the  balance. 
B  may  be  smaller  than  A.  The  tube  a  is  closed  at  b  with  a  little 
wax  ball  or  a  small  piece  of  india-rubber  tube  stopped  with  half 
an  inch  of  rod ;  the  other  end  of  the  tube  a  is  open,  as  are  also  both 
ends  of  c  and  d.  The  flask  B  is  nearly  half  filled  with  concentrated 
sulphuric  acid,  free  from  oxides  of  nitrogen  and  sulphurous  acid. 
The  tubes  must  fit  air-tight  in  the  corks,  and  the  latter  equally  so 
in  the  flasks.  The  weighed  substance  is  put  into  A  ;  this  flask  is 
then  filled  about  one  third  with  waiter,  the  cork  properly  inserted, 
and  the  apparatus  tared  on  the  balance.  A  few  bubbles  of  air 
are  now  sucked  out  of  d,  by  means  of  an  india-rubber  tube.  This 
serves  to  rarefy  the  air  in  A  also,  and  causes  the  sulphuric  acid  in 
B  to  ascend  in  the  tube  c.  The  latter  is  watched  for  some  time, 

*Zeitschr.  f.  analyl.  Chem.,  i,  65.  f  Pogg.  An?tal.,  cxvr,  131. 

\Zdtschr.f.  analyt.  C/iem.,  I,  183.  §  Pogg.  Annal.,  cxvi,  686. 


§  139.]  CARBONIC   ACID.  489 

to  ascertain  whether  the  column  of  sulphuric  acid  in  it  remains 
stationary,  which  is  a  proof  that  the  apparatus  is  air-tight.  Air  is 
then  again  sucked  out  of  ^7,  which  causes  a  portion  of  the  sulphuric 
acid  to  flow  over  into  A.  The  carbonate  in  the  latter  flask  is 
decomposed  by  the  sulphuric  acid,  and  the  liberated  carbonic  acid, 
completely  dried  in  its  passage  through  the  sulphuric  acid  in  By 
escapes  through  d.  When  the  evolu- 
tion of  the  gas  slackens  a  fresh  portion 
of  sulphuric  acid  is  made  to  pass  over 
into  A,  by  renewed  suction  through 
d\  the  operation  being  repeated  until 
the  whole  of  the  carbonate  is  decom- 
posed. A  more  vigorous  suction  is 
now  applied,  to  make  a  large  amount 
of  sulphuric  acid  pass  over  into  A, 
whereby  the  contents  of  that  flask  are 
considerably  heated ;  when  the  evolu- 
tion of  gas  bubbles  has  completely 
ceased,  the  stopper  on  a  is  opened,  and 

suction  applied  to  d,  until  the  air  sucked  out  tastes  no  longer  of 
carbonic  acid.*  When  the  apparatus  is  quite  cold  it  is  replaced 
upon  the  balance,  and  the  equilibrium  restored  by  additional  weights. 
The  sum  of  the  weights  so  added  indicates  the  amount  of  carbonic 
acid  originally  present  in  the  substance. 

If  the  flasks  A  and  B  are  selected  of  small  size,  the  apparatus 
may  be  so  constructed  that,  together  with  the  contents,  it  need  not 
weigli  above  TO  grammes,  admitting  thus  of  being  weighed  on  a 
delicate  balance.  The  results  obtained  by  the  use  of  this  apparatus, 
first  suggested  by  WILL  and  myself,  are  very  accurate,  provided 
the  quantity  of  the  carbonic  acid  be  not  too  trifling.  Various 
modifications  of  the  apparatus  have  been  proposed,  principally  in 
order  to  make  it  lighter.  See  foot-note,  p.  492. 

If  sulphites  or  sulphides  are  present,  together  with  the  carbon- 
ates, their  injurious  influence  is  best  obviated  by  adding  to  the 
carbonate  solution  of  normal  potassium  chroinate  in  more  than 
sufficient  quantity  to  effect  their  oxidation.  If  chlorides  are  pres- 
ent, in  order  to  prevent  the  evolution  of  hydrochloric  acid,  add  to 

*  In  accurate  experiments  it  is  advisable  to  connect  the  end  b  of  the  tube  » 
with  a  calcium-chloride  tube  during  the  process  of  suction,  and  to  use  an  aspira- 
tor or  hydraulic  air-pump  instead  of  the  mouth. 


490  DETERMINATION.  [§  139. 

the  evolution  flask  a  sufficient  quantity  of  silver  sulphate  in  solu- 
tion, or  connect  the  exit  tube  d  with  a  small  prepared  TJ-tnbe, 
which  is,  of  course,  "first  tared  with  the  apparatus,  and  afterwards 
weighed  with  it.  This  U-tube  is  prepared — in  accordance  with  the 
happy  proposal  of  STOLBA — by  filling  with  fragments  of  pumice 
which  have  been  boiled  with  an  excess  of  concentrated  solution  of 
cupric  sulphate,  till  the  air  has  been  expelled,  and  then  dried  and 
heated  to  complete  dehydration  of  the  copper  salt.  If  the  U-tube 
is  only  8  cm.  high  and  has  a  bore  of  1  cm.,  it  answers  the  purpose 
very  well.  The  outer  end  is  provided  with  a  perforated  cork  and 
short  glass  tube.  We  apply  suction  to  this  by  means  of  a  flexible 
tube,  instead  of  to  d. 

"bb.    Carbonates  of  Bases  yielding  Insoluble  Salts  with  Sul- 
phuric Acid. 

The  method  aa  is  unsuitable  for  these  bases,  because  the 
insoluble  sulphate  formed,  e.g.,  calcium  sulphate,  partially  protects 
tne  still  undecompcsed  portion  of  carbonate  from  decomposition. 
The  apparatus  is  hence  modified  as  shown  in  Fig.  94. 

The  modification  consists  simply  in  the  tube  ab,  which,  as  the 
cut  shows,  is  provided  with  a  glass  bulb,  and  is  drawn  out  to  a 
fine  point  at  the  lower  end. 

The  process  is  carried  out  as  follows :  The  weighed  substance 
is  introduced  into  A,  with  some  water.  .  The  bulb-tube  ab  con- 
tains dilute  nitric  acid  (or,  if  substances  are  present  which  decom- 
pose nitric  acid,  e.g.,  ferrous  oxide,  10-per  cent,  hydrochloric 
acid)  in  quantity  more  than  sufficient  to  decompose  the  carbonate 
present.  The  end  b  is  closed  by  a  well-kneaded  piece  of  wax,  or 
with  a  short  section  of  rubber  tubing  closed  by  a  small  piece  of  a 
glass  rod,  in  order  to  prevent  the  acid  from  running  out.  The 
tip  of  the  tube  a  should  not,  at  first,  dip  into  the  water  in  A. 
Place  the  apparatus  on  a  balance  and  ascertain  its  tare,  then  care- 
fully push  the  tube  a  down  into  the  liquid  until  the  tip  nearly 
reaches  the  bottom,  then  loosen  the  wax  plug  or  open  the  rubber 
tube  for  a  moment,  and  allow  some  of  the  acid  to  run  out,  and 
repeat  this  now  and  again  until  all  the  carbonate  has  been  decom- 
posed. Now  heat  the  contents  of  A  to  incipient  boiling,  remove 
the  stopper  from  5,  and  draw  the  carbonic  acid  out  of  the 
apparatus  as  detailed  under  aa\  after  cooling,  determine  the  loss 
of  weight. 

It  will  be  seen  at  a  glance  that  the  apparatus  is  susceptible  of 


§  139.] 


CARBONIC   ACID. 


491 


a  different  construction ;   for  instance,  the  flask  E  may  be  omitted, 
and  the  tube  C  connected  instead  with  a  calcium-chloride  tube  or 


Fig.  94. 


Fig.  95. 


a  tube  containing  pumice-stone  or  asbestos  saturated  with  sulphuric 
acid ;  or  the  substance  to  be  decomposed  may  be  placed  in  a  small 
tube  arranged  to  stand  upright  at  first,  or  suspended  by  a  thread, 
and  which,  after  the  apparatus  is  tared,  may  be,  upset  or  lowered 
into  the  dilute  acid ;  the  closure  of  the  tube  J  may  also  be  effected 
by  a  pinch-cock,  etc.  Such  modifications,  if  made  judiciously, 
affect  the  results  but  little,  if  at  all.  An  apparatus  of  this  kind, 
modified  by  FR.  MOHR,  is  shown  in  Fig.  95. 

One  of  the  most  convenient  apparatus  is  that  proposed  by 
GEISSLER  *  and  shown  in  Fig.  96.  This  consists  of  two  parts,  AB 
and  C.  C  is  ground  into  the  neck  at  a  so  as  to  fit  airtight,  yet 
be  readily  removable,  whereby  A  may  be  filled.  C  carries  a 
tube,  £><?,  open  at  both  ends,  and  ground  to  fit  C  watertight  at  c ; 
by  means  of  a  movable  cork,  «",  it  is  kept  in  proper  position.  In 
other  respects,  the  apparatus  is  arranged  as  shown  in  the  cut. 
The  cork  e  must  fit  airtight,  and  so  too  must  the  tube  d  in  the 
cork  e.  The  weighed  substance  to  be  decomposed  is  introduced 
into  A,  water  added  to  the  extent  indicated,  and  the  substance 
shaken  to  one  side  of  the  flask.  C  is  now  nearly  filled  with 


*  Journ.f.  prakt.  CJiem..  LX,  35. 


492 


DETERMINATION-. 


[§  139. 


diluted  nitric  acid,  or  10-per  cent,  hydrochloric  acid,  by  means  of 

n  x^ette,  having  previously  slipped  the  cork  upwards,  without,  how- 
ever, unseating  Z»;  the  cork  i  is  then  slipped 
down  again,  ^inserted  into  A,  J5  filled  a 
little  over  half  full  with  pure  concentrated 
sulphuric  acid,  and  I  closed  with  a  wax 
plug  or  a  piece  of  rubber  tubing  and  a 
small  piece  of  glass  rod.  After  weigh- 
ing, the  decomposition  is  effected  by 
slightly  lifting  J  and  allowing  acid  to  flow 
into  A  from  O.  The  carbonic  acid  escapes 
B  through  Ji  into  the  sulphuric  acid  by 
which  it  is  dried,  and  passes  from  the 
apparatus  through  d.  "When  the  decom- 
position is  complete,  A  is  cautiously  heated 
to  incipient  boiling,  5  is  then  opened,  and 
the  carbonic  acid  exhausted  from  the  ap- 
paratus at  d  by  means  of  a  small  rubber 
tube.  After  cooling,  the  apparatus  is 
weighed.* 

If  it  is  necessary  to  use  hydrochloric 
acid  for  decomposing  the  carbonate,  the 
escaping  carbonic  acid  should  be  dried 
with  pieces  of  pumice- stone  impregnated 
with  anhydrous  cupric  sulphate,  which 

retains   both  the    water   and   hydrochloric-acid    gas    (STOLBAf). 

Its  preparation  is  described  under  aa.     It  is  well  to  fill  both  limbs 

*  Other  carbonic-acid  apparatus  effecting  similar  results  by  somewhat  dif- 
ferent modifications  have  been  proposed  by  H.  ROSE,  FKITZSCHE,  ROGERS  (see 
H.  ROSE'S  Handb.  der  analyt.  Chem.,  6.  Aufl.,  n,  784),  VOHL  (AnnaL  d.  Chem. 
u.  Pharm.,  LXVI,  247),  M.  SCHAFFNER (AnnaL  de  Chem.  u.  Pharm.,  LXXXII,  335), 
WERTHER  (Modification  of  GEISSLER'S  apparatus  described  above,  Journ.  f» 
prakt.  Chem.,  LXI,  99),  T.  D.  SMITH  (Chem.  Gaz.,  1855,  201),  A.  MAYER  (Journ.  f. 
prakt.  Chem.,  LXVII,  63),  TH.  SIMMLER  (Journ.  f.  prakt.  Chem.,  LXXI,  158),  AL. 
BAUER  (personal  communication),  P.  HART  (Chem.  Gaz.,  1859,  174),  C.  D. 
BRAUN  (Dingl.  polyt.  Journ.,  CLV,  301),  E.  J.  REYNOLDS  (Chem.  News,  1862,  143), 
STOLBA  (Zeitschr.  f.  analyt.  Chem.,  i,  368),  ULLGREN  (ib.,  VITT,  46),  JOHNSON 
(ib.,  ix,  90),  BUNSEN  (ib.,  x,  403),  and  others.  Johnson's  method  differs  from, 
the  usual  one  in  that  the  apparatus  is  filled  and  acid  saturated  with  carbonic 
acid  before  beginning,  and  therefore  does  not  remove  the  carbonic-acid  gas  on 
completing  the  analysis.  In  this  method  it  is  necessary,  of  course,  to  observe 
that  both  air-pressure  and  temperature  are  alike  during  both  weighings. 

f  Dingl.  polyt.  Journ.,  CLXIV,  128;  Zeitschr.  f.  analyt.  Chem.,  i,  368. 


Fig.  96. 


§  139.]  CARBONIC  ACID.  493 

of  a  light  U-tube  of  a  size  to  suit  the  apparatus.  The  tube  remains 
serviceable  so  long  as  one-third  of  its  contents  remains  uncolored. 
When  employing  a  measured  volume  of  standard  acid  accord- 
ing to  one  of  the  methods  described  under  d,  Ib,  the  estimation 
of  the  carbonic  acid  and  the  base  may  be  effected  in  one  operation 
according  to  §  139,  II.,  a\  this  is  often  advantageous  with  pasty 
precipitates.  The  standard  acid  is  run  in  from  a  burette  having  a 
fine  point,  into  the  bulb-tube  of  the  apparatus,  Fig.  94,  first,  how- 
ever, closing  the  lip  of  the  latter  with  a  little  tallow.  After  the 
apparatus  is  tared  it  is  warmed  to  melt  the  tallow,  and  the  opera- 
tion is  then  carried  out 


e.   From  all  Bases  without  Exception   (Determination  of  Acid 
from  the  Increase  in  Weight  of  an  Absorption  Apparatus). 

This  method,  which  was  formerly  seldom  employed,  has  been 
highly  recommended  by  KoLBEf.  I  have  striven,  by  utilizing 
all  the  results  recorded  by  G.  J.  MULDER,  STOLBA,  and  KOLBE, 
to  make  it  as  practical  as  possible,  and  for  the  last  ten  years  I 
have  used  it  almost  exclusively.  The  accuracy  of  the  results 
afforded  by  it  is  approached  by  but  few  others. 

The  apparatus  I  use  has  the  arrangement  shown  in  Fig.  97. 
A  is  a  150-  to  300-c.  c.  flask  provided  with  a  doubly  perfor- 
ated rubber  stopper.  is  a  twice-bent  tube  expanded  at  c  to  a 

bulb  ;  it  may  be  connected  by  means  of  a  piece  of  rubber  tubing, 
bearing  a  pinch-cock,  d,  either  with  the  funnel  e,  or  with  the 
soda-lime  tube,  f,  which,  in  turn,  is  connected  with  the  flask  g 
containing  potassa  solution.  The  tube  A,  also  having  a  bulb,  has 
its  lower  end  ground  obliquely.  The  U-tube  i  has  a  height  of 
17  cm.  and  a  bore  of  16  mm.  ;  only  the  lower  bend  contains  some 
calcium  chloride.  J  The  equally  large  U-tubes  Jc  and  I  contain 
respectively  calcium  chloride  and  pumice  copper  sulphate  (see 
page  490).  The  smaller  tubes,  w,  w,  o,  and  p,  are  1  1  cm.  high 
and  are  12  mm.  bore;  m  contains  calcium  chloride,  whereas  n  and 

*  Journ.  /.  prakt.  Chem.,  xcvn,  312  ;  Zeitschr.f,  analyt.  Chem.,  v,  208,  and 
vi,  444. 

\  Annal.  de  Chem.  u.  Phnrm.t  cxix,  130. 

i  It  need  scarcely  be  mentioned  that  all  the  calcium  chloride  must  be  free  from 
alkalinity,  and  should  be  tested  for  this.  Any  alkalinity  is  readily  obviated  by 
adding  a  little  ammonia  to  the  calcium  chloride  solution  before  evaporating. 


494 


DETERMINATION. 


[§  139. 


o,  are  |-  filled  with  coarsely  granulated  soda-lime  (about  20  grm.) 
and  the  remaining  -g-  with  coarsely  granulated   calcium  chloride ; 


Fig.  97. 

the  outer  limb  of  p  contains  soda-lime,  the  inner  calcium  chloride. 
^',  #,  £,  and  m  serve  to  deprive  the  carbonic-acid  gas  of  water 
and  hydrochloric  acid;  the  soda-lime  in  n  and  o  completely 
absorb  the  carbonic  acid,  and  the  calcium  chloride  they  contain 
prevents  any  evaporation  of  water  from  the  soda-lime  (as  this 
becomes  warmed  during  the  absorption  of  carbonic  acid) ;  p  serves 
to  prevent  ingress  of  moisture  into  n  and  o  from  without.  The 
corks  of  n  and  o  should  be  coated  with  sealing-wax.  The  other 
tubes  may  be  provided  with  perforated  rubber  stoppers  or  corks 
coated  with  sealing-wax.  The  apparatus,  once  in  order,  may 
serve  for  use  for  a  long  time,  but  before  each  new  experiment, 
it  is  necessary  to  renew  the  calcium  chloride  in  ?',  and  to  refill  n^ 
and  occasionally  o  also. 

After  weighing  the  substance  introduce  it  into  JL,  together 


§  139.]  CAKBONIC    ACID.  495 

with  a  little  water ;  then  weigh  n  and  0,  connect  the  various  parts 
of  the  apparatus,  join  ~b  and  <?,  close  d,  and  apply  suction  by 
means  of  a  hydraulic  air-pump  or  aspirator  to  the  end  of  the  tube 
s,  which  is  connected  both  with  the  U-tube  r,  containing  a  little- 
water,  and  with  j?.  The  pinch-cock  ^is  in  the  meantime  opened. 
The  apparatus  may  be  known  to  be  air-tight  when  the  passage  of 
air-bubbles  through  the  water  in  r  soon  ceases.  As  soon  as  this 
occurs,  fill  e  with  dilute  hydrochloric  acid  (or  with  nitric  acid, 
according  to  circumstances),  and  allow  a  little  to  flow  into  A  by 
cautiously  opening  d.  Evolution  of  carbonic-acid  gas  begins  at 
once,  and  its  rise  noted  by  the  passage  of  air-bubbles  through 
the  water  in  r.  When  the  evolution  slackens,  allow  a  further 
quantity  of  acid  to  flow  into  A,  and  if  the  quantity  of  acid  has- 
been  properly  adjusted  the  decomposition  of  the  carbonate  will  be 
complete  on  adding  the  last  portion  of  acid,  e  is  then  rinsed  with 
a  little  water,  which  is  allowed  to  flow  into  A,  after  which  remove 
0,  connect  d  withy,  and,  cautiously  opening  d,  draw  a  gentle  cur- 
rent of  air  continuously  through  the  apparatus,  while  the  con- 
tents of  A  are  heated  to  incipient  boiling. 

As  soon  as  the  carbonic  acid  reaches  the  soda-lime  tubes  these 
become  warm,  and  this  progressive  heating  affords  a  good  indi- 
cation as  to  the  extent  to  which  the  soda-lime  has  been  saturated 
by  the  carbonic  acid.  As  soon  as  the  soda-lime  tubes  have  be- 
come perfectly  cold  the  larger  part  of  the  carbonic  acid  has  been 
absorbed,  and  if  the  current  of  air  is  slowly  drawn  through  the 
tubes  5  or  10  minutes  longer  all  the  carbonic  acid  will  surely  have 
been  removed  from  A,  «',  &,  I,  and  m.  If  the  flask  has  been 
properly  and  judiciously  heated,  but  very  little  water  will  have 
reached  ?',  so  that  the  calcium  chloride  in  its  lower  part  will  have- 
become  moist,  yet  will  not  have  quite  deliquesced. 

After  the  experiment  is  finished,  stop  the  suction  at  s,  and 
remove  and  weigh  n  and  o.  The  increase  in  weight  will  be  the 
exact  expression  of  the  carbonic  acid  present  in  the  carbonate  ex- 
amined. The  concordance  and  accuracy  of  the  results  leave 
nothing  to  be  desired.*  The  bases  are  retained  perfectly  pure 
and  completely  dissolved  in  the  hydrochloric  (or  nitric)  acid. 

In  making  a  second  test  empty  t,  and  recharge  both  it  and  n. 

*  ZeitscJir.  f.  analyt.  C7iem.,  ir,  49,  and  n,  341. 


496  DETERMINATION.  [§  139. 

o  need  not  be  refilled  as  a  rule,  but  it  is  advisable  to  transpose  these 
two,  i.e.,  put  in  the  place  of  o  the  freshly  charged  tube  n. 

If  it  is  preferred  to  decompose  the  carbonate  in  the  dry  way, 
this  may  be  accomplished.by  fusing  the  finely  powdered  carbonate 
(alkali  carbonates  need  not  be  powdered)  with  six-  to  ten -fold  its 
quantity  of  fused  potassium  dichromate.  The  operation  should  be 
conducted  in  a  piece  of  combustion  tubing,  slightly  bent  in  the 
middle,  and  one  end  of  which  is  connected  with  an  apparatus  for 
purifying  the  air,  the  other  being  connected  with  a  calcium -chloride 
tube  for  drying  the  carbonic  acid,  soda-lime  tubes  for  absorption,  a 
guard  tube,  and  an  aspirator  or  hydraulic  air-pump.  After  a  gentle 
current  of  air  through  the  apparatus  has  been  established,  heat  the 
tube  to  decompose  the  carbonate,  and  produce  a  slow  evolution  of 
carbonic-acid  gas.  As  soon  as  the  mass  fuses  calmly  the  operation 
is  completed.  The  current  of  air  is  continued  a  little  longer,  and 
then  the  increase  in  weight  of  the  absorption  tubes  determined  by 
weighing.  The  method  requires  no  modification  even  when  sul- 
phites or  thiosulphates  are  present  (PERSOZ  *). 

The  process  as  used  by  H.  ROSE  is  as  follows ;  and  the  apparatus 
employed  is  shown  in  Fig.  98. 

The  flask  for  decomposing  the  carbonate  should  be  small  (150 
c.c.),  in  order  to  facilitate  subsequent  removal  of  carbonic  acid  by 
aspiration,  unless  the  substance  froths  strongly  during  its  decom- 
position, in  which  case  a  larger  flask  must  be  used.  The  end  of 
the  funnel  tube,  after  it  is  inserted  in  the  rubber  stopper  which  is 
fitted  to  the  flask,  is  drawn  to  a  less  diameter  and  bent  upwards  in 
the  form  of  a  hook,  to  prevent  the  entrance  of  gas-bubbles.  Above 
the  stop-cock  its  internal  diameter  should  not  be  so  small  as  to  pre- 
vent water  when  poured  in  from  filling  it,  and  this  portion  should 
be  so  long  that  the  pressure  of  the  liquid  filling  it  wrill  suffice  to 
force  gas  through  the  apparatus.  A  piece  of  glass  tube  bent  at  a 
right  angle  is  fitted  to  the  funnel  by  means  of  a  piece  of  rubber 
tube  slipped  over  it. 

The  nearly  horizontal  glass  tube  (about  0'7  metre  long)  is  of 
thin  glass,  and  of  a  diameter  not  less  than  12  millimetres.  It  is 
inclined  to  such  extent  that  water  condensing  in  it  may  flow  back. 
The  upper  half  is  filled  with  granulated  dried  calcium  chloride, 
secured  in  place  by  a  little  cotton  or  asbestos  at  each  end.  In  the 

*  Compt.  rend.,  LIU,  239.—Zeit8chr.f.  analyt.  Chem.,  I,  83. 


§  139.]  CARBONIC   ACID.  497 

end  of  the  large  tube  a  small  tube  is  fitted  by  means  of  a  rubber 
stopper,  and  to  this  is  joined  by  a  rubber  tube  the  potash  appara- 
tus and  soda-lime  tube  (weighable  either  jointly  or  separately) 
charged  with  absorbents,  as  described  §  174.  The  flask  is 
removed  to  receive  the  weighed  substance,  and  replaced  without 
disturbing  the  position  of  the  rest  of  the  apparatus.  It  can  now 
be  ascertained  whether  the  apparatus  will  leak  gas  by  forcing  a 
little  air  (free  from  carbonic  acid)  through  the  funnel  tube,  closing 
the  stop-cock,  and  observing  whether  the  unequal  height  of  liquid 
in  the  two  limbs  of  the  potash  apparatus  remains  for  a  few  minutes. 
Introduce  a  little  water  through  the  funnel  tube,  and  next  acid 
slowly  by  turning  the  stop-cock  until  evolution  of  CO2  ceases. 
The  small  right-angled  tube,  to  which  is  attached  a  large  tube 
filled  with  fragments  of  caustic  potassa,  is  now  inserted  in  the  glass 
funnel,  and  a  slow  current  of  air  (1  bubble  per  second)  is  drawn 
through  the  apparatus  by  means  of  an  aspirator  (Fig.  100)  con- 
nected with  the  soda-lime  tube.  The  aspirator  should  not  be  con- 
nected directly  to  the  soda-lime  tube,  but  to  a  calcium -chloride 
tube,  which  ought  to  be  connected  with  the  latter  during  the 
whole  operation.  As  soon  as  the  current  of  air  is  established, 


Fig.  98. 

apply  the  smallest  possible  flame  of  a  Bunsen  lamp,  best  main- 
tained constant  by  capping  the  burner  with  wire  gauze  until  the 
fluid  just  boils.  Keep  up  the  gentle  boiling  a  few  minutes  until 
water  condenses  in  the  tube,  but  not  until  condensed  drops  appear 


498 


DETERMINATION. 


[§  139. 


quite  up  to  the  calcium  chloride.  Remove  then  the  lamp,  and 
aspirate  a  while  longer  somewhat  faster.  The  volume  of  air  neces- 
sary to  remove  the  carbonic  acid  depends  upon  the  size  of  the 
decomposing  flask.  When  the  operation  is  completed,  disconnect 
the  absorbing  apparatus,  close  the  ends  with  caps  of  rubber  tubing, 
and  weigh  after  lapse  of  half  an  hour. 

For  liberating  the  carbonic  acid,  sulphuric  acid  (the  concen- 
trated diluted  with  4  or  5  times  its  volume  of  water)  is  best 
adapted,  provided  it  readily  decomposes  the  substance  without 
formation  of  insoluble  sulphates. 

When  there  are  objections  to  using  sulphuric  acid,  dilute  hydro- 
chloric acid  (containing  about  10  per  cent.)  may  be  used,  or  more 
rarely  nitric  acid.  Nitric  acid  cannot  be  used  when  substances  are 
present  which  cause  its  decomposition;  e.g.,  ferrous -salts  and  sul- 
phides. 

When  sulphuric  acid  is  used,  the  evolution  of  HaS  from  sul- 


Fig.  100. 

phides,  if  present,  may  be  prevented  by  adding  first  a  solution  of 
chromic  acid  or  mercuric  chloride.     If  sulphites  are  present,  use 


§  139.]  CARBONIC   ACID.  499 

chromic  acid  or  potassium  cliromate.  When  hydrochloric  acid  is 
employed,  the  disturbing  influence  of  compounds  which  cause  evo- 
lution of  chlorine  may  be  prevented  by  allowing  some  concentrated 
solution  of  stannous  chloride  to  run  into  the  flask  before  addition 
of  the  acid.  "When  hydrochloric  acid  is  used,  or  even  sulphuric  in 
the  presence  of  chlorides,  it  is  best  to  guard  against  the  possibility 
of  carrying  HC1  gas  into  the  potash  apparatus  by  substituting 
STOLBA'S  preparation  of  anhydrous  copper  sulphate  and  pumice- 
stone  (see  page  490)  for  that  portion  of  the  calcium  chloride 
which  fills  10  to  15  cm.  of  the  end  of  the  tube. 

A  modification  *  of  the  above-described  apparatus,  possessing 
some  obvious  advantages,  is  shown  by  Fig.  99.  In  place  of  the 
empty  part  of  the  long  glass  tube  shown  in  Fig.  98,  there  is  sub- 
stituted a  smaller  strong  tube,  provided  with  a  cooling  apparatus 
through  which  water  circulates.  This  is  connected  by  a  piece  of 
close-fitting  rubber  tube  with  the  remaining  part  d.  Some  suitable 
form  of  apparatus  for  absorbing  CO,  must,  of  course,  be  attached 
to  d  in  the  manner  shown  by  Fig.  98.  The  calcium- chloride  tube, 
used  to  prevent  moist  air  from  entering  the  absorbing  apparatus, 
is  conveniently  supported  by  attaching  it  to  the  aspirator  (Fig. 
100).  The  aspirator  may  be  connected  with  the  apparatus  from 
the  beginning  to  the  end  of  the  operation,  with  its  stop-cock  so 
adjusted  that  water  flows  from  it  drop  by  drop.  In  conducting 
the  operation,  a  little  variation  from  the  before  described  manipu- 
lation is  admissible  on  account  of  the  presence  of  the  condensing 
apparatus.  After  enough  acid  has  been  admitted  to  effect  decom- 
position, the  stop-cock  of  a  is  closed,  a  little  liquid  still  being 
allowed  to  remain  above  it.  Heat  is  then  applied  as  before 
directed,  but  continued  longer  until  the  COa  is  almost  or  quite 
expelled  from  the  flask  by  steam.  This  point  is  indicated  by 
almost,  or  nearly,  entire  cessation  of  dropping  of  water  from  the 
aspirator.  Diminish  now  the  heat,  and  immediately  after  open 
the  stop-cock  of  a  and  let  air  (free  from  COa)  enter  and  replace 
the  condensing  steam.  Boil  again  to  expel  the  air  which  has 
entered,  after  which  a  small  volume  of  air  drawn  through  the 
apparatus  by  the  aspirator  will  ensure  the  bringing  of  all  the  CO, 
into  the  absorbing  apparatus. 

*  Devised  by  H.  L.  WELLS,  of  the  Sheffield  Laboratory. 


500  DETERMINATION.  [§  139. 

f.  Separation  from  all  Bases  without  Exception.  (Determi- 
nation of  Acid  by  Expulsion,  Absorption,  and  Volumetric 
Estimation.) 

If  the  carbonic  acid  is  disengaged  in  the  evolution  apparatus 
described  under  e  (which  I  consider  the  most  satisfactory),  or  in  a 
similar  one,  the  quantity  of  the  carbonic  acid  expelled  may  be 
also  ascertained  by  one  of  the  methods  given  above  for  estimating 
free  carbonic-acid  gas,  i.e.,  it  may  be  passed  into  a  carbonate-free 
mixture  of  barium-  or  calcium  chloride  and  ammonia,  as  described 
in  §  139,  I,  b,  /?,  and  the  analysis  then  made  as.  in  §  139, 1,  J,  /?,  bb. 
This  method  is,  however,  far  more  inconvenient,  and  much 
slower,  than  that  described  in  §  139,  II,  e,  and  affords  good 
results  only  when  all  the  sources  of  error  already  pointed  out  are 
avoided. 

On  the  other  hand,  it  is  sometimes  advantageous,  particularly 
when  determining  very  small  quantities  of  carbonic  acid,  to  ab- 
sorb this  in  a  definite  volume  of  standard  baryta  water,  and  to 
proceed  with  the  analysis  according  to  PETTENKOFER'S  process 
(§  139,  i,  I,  y).  As  this  method  is  employed  in  the  determina- 
tion of  carbonic  acid  with  atmospheric  air,  I  refer  to  this  section, 
merely  remarking  that  AL.  MULLER,*  E.  ScnuLZE,f  and  P. 
WAGNER  ^  have  devised  special  apparatus  and  given  rules  for 
carrying  out  the  process  in  the  most  satisfactory  manner. 

g.   Estimation  by  Measuring  the   Gas. 

a.  According  to  C.  SCHEIBLER  §  this  process  is  applicable 
in  the  case  of  all  salts  which  are  decomposed  by  hydrochloric 
acid  in  the  cold.  It  is  distinguished  for  rapid  and  convenient 
execution  and  very  satisfactory  results,  but  requires  a  special 
apparatus.  It  is  much  employed  in  determining  calcium  carbon- 
ate in  bone  black. 

Fig.  101  shows  the  ingenious  apparatus  devised  for  this 
method.  The  carbonate  to  be  decomposed  is  introduced  into 
A.  The  decomposition  is  effected  by  simply  lifting  the  bottle, 
as  shown,  thus  allowing  the  hydrochloric  acid  contained  in  the 

*  Zeitschr.  /.  analyt.  Chem.,  i,  47.  f  lb  ,  ix,  290. 

$  lb.,  ix,  445. 

§Anleitung  zum  OebraucJi  des  Apparates  zur  Pestimmung  dei  KoJUensaurm 
Kalkerde  in  der  Knochenkohle  etc.  Dr.  C.  SCHEIBLEK.  Printed  ID  manuscript. 
Berlin,  1862. 


§  139.] 


CARBONIC   ACID. 


501 


gutta-percha  cylinder  S  to  flow  out.  The  glass  stopper  closing 
A.  must  be  ground  accurately  to  fit  tightly,  and  must  be  greased, 
so  as  to  be  perfectly  air-tight;  it  should,  further,  be  perfor- 


Fig.  101. 

ated,  and  have  a  short  glass  tube  cemented  into  the  perforation. 
The  carbonic- acid  gas  evolved  passes  through  this  tube  and  the 
rubber  tubing  ?',  which  is  connected  with  it,  then  through  a 
glass  tube  cemented  into  one  of  the  perforations  of  the  flask  B, 
and  finally  into  a  bladder  of  very  thin  caoutchouc,  J5T,  connected 


502  DETERMINATION.  [§139. 

air- tight  with  A.  One  of  the  two  other-  perforations  of  the 
stopper  of  B  carries  a  short  rubber  tube  closed  by  a  pinch- cock ;  in 
the  other  a  glass  tube,  u,  is  fixed,  and  is  connected  with  the 
measuring  apparatus.  This  last  consists  of  a  150-c.  c.  tube,  (7, 
graduated  in  0*5  c.  c.,  and  connected  with  a  similar,  but  n'ot 
graduated  tube,  D,  as  shown  in  the  cut.  In  the  rubber  stopper 
closing  the  lower  end  of  this  tube  there  is  also  fixed  a  short  glass 
tube  closed  by  a  piece  of  rubber  tubing  carrying  a  pinch-cock, 
jP,  in  turn  connected  with  a  glass  tube  cemented  into,  and  nearly 
reaching  to,  the  bottom  of.  the  flask  E.  In  the  second  tubulure 
of  E\§  cemented  a  short  glass  tube,  to  which  the  rubber  tube,  -y, 
is  connected.  The  flask  E  serves  as  a  reservoir;  on  opening  _P 
the  water  contained  in  the  tubes  G  and  D  flow  into  E\  on  blow- 
ing into  v  the  water  may  be  driven  up  through  the  open  pinch- 
cock  back  into  the  tube  again.  At  the  beginning  E  is  nearly 
filled  with  water  through  D. 

All  parts  of  the  apparatus  excepting  the  decomposing  bottle 
being  permanently  connected,  it  is  advisable  to  fasten  them  to  a 
wooden  stand  by  means  of  suitable  brass  fastenings.  The  stand 
should  also  carry  a  thermometer. 

Every  experiment  is  begun  by  first  filling  C  and  D  with  water 
to  the  zero  point,  this  being  accomplished  by  blowing  into  v,  the 
stopper  of  A  being  removed.  As  soon  as  the  water  level  is 
slightly  above  the  zero  point,  P  is  closed,  and  then  slightly  opened 
to  allow  water  to  drop  out  until  the  level  is  reached.  Of  course 
a  certain  amount  of  caution  is  necessary  in  blowing  into  vy  as  well 
as  in  managing  the  pinch-cock,  for  were  water  blown  up  into  u  and 
J2,  the  entire  apparatus  would  have  to  be  taken  apart  and  the  water 
removed.  While  water  is  filling  0,  the  air  it  displaces  enters  B 
and  compresses  the  caoutchouc  bladder.  Should  this  compression 
not  be  sufficiently  accomplished,  blow  carefully  into  q  until  the 
bladder  has  completely  collapsed.  When  a  number  of  consecu- 
tive experiments  are  made,  the  bladder  will  always  empty  itself. 
Should  it  happen  that  the  bladder  becomes  empty  before  the 
water  in  the  tubes  has  reached  the  zero  mark,  then  the  water 
in  the  tubes  will  not  be  in  equilibrium ;  in  such  a  case  open  q  for 
a  moment  only.  The  apparatus  should  be  used  in  a  place  where 
the  temperature  is  as  constant  as  possible,  and  care  should  be 
taken  that  neither  the  direct  ravs  of  the  sun  nor  the  radiations 


§  139.]  CARBONIC   ACID.  603 

from  a  stove  strike  the  apparatus,  because  sudden  changes  of 
temperature  during  the  experiment  will  naturally  interfere  with 
the  accuracy  of  the  results. 

To  perform  the  analysis,  introduce  the  finely  triturated  sub- 
stance into  the  decomposition  flask  A,  fill  the  gutta-percha  cylinder 
with  10  c.  c.  hydrochloric  acid  (sp.  gr.  1-12)  by  means  of  a  pipette, 
carefully  place  the  cylinder  within  the  flask  A,  and  tightly  insert 
the  stopper  previously  greased  with  tallow.  As  this  will  cause 
the  water-level  to  somewhat  fall  in  C  and  rise  in  D,  open  q  for  a 
moment,  when  the  equilibrium  will  be  restored.  Now  note  the 
thermometer  and  barometer,  grasp  the  flask  with  the  right  hand 
around  the  neck  in  order  to  avoid  warming  the  contents,  tilt 
slightly  and  allow  the  hydrochloric  acid  to  flow  out  while  the 
pinch-cock  is  opened  with  the  left  hand,  and  in  such  a  manner 
that  the  water-level  in  both  tubes  is  exactly  at  the  same  height ; 
these  operations  are  continued  without  intermission  so  long  as  any 
disengagement  of  carbonic  acid  takes  place  and  a  lowering  of  the 
water-level  in  C  is  observed.  When  the  level  remains  constant 
for  a  few  seconds  the  experiment  is  finished.  Care  must  be  taken 
that  the  levels  in  C  and  D  are  at  the  same  height,  then  read  off 
the  height  and  _  observe  whether  the  temperature  has  remained 
constant.  If  it  has,  the  c.  c.  read  off  denote  the  volume  of  car- 
bonic acid  disengaged  j  since,  however,  a  small  quantity  has  re- 
mained behind  dissolved  in  the  hydrochloric  acid,  a  small  correc- 
tion has  to  be  made.  SCHEIBLER  has  determined  the  quantity  of 
carbonic  acid  remaining  behind  dissolved  in  10  c.  c.  of  hydro- 
chloric acid  at  a  medium  temperature,  and  he  directs  adding 
3*2  c.  c.  to  the  volume  read  off  and  then  reducing  the  whole  to 
0°,  760  mm.,  in  the  dry  condition  (see  §  198).*  Every  1000  c.  c. 
of  carbonic  acid  thus  reduced  to  normal  conditions  weighs 
1-96507  grin. 

If  it  is  desired  to  dispense  with  all  corrections,  begin  each 
series  of  experiments  by  establishing  the  relation  between  the  car- 


*  This  method  of  correction  involves  some  uncertainty,  since  the  quantity  of 
carbonic  acid  remaining  absorbed  depends  upon  the  concentration  of  the  saline 
solution  resulting,  as  well  as  upon  the  quantity  of  air  with  which  the  carbonic 
acid  is  mixed;  hence  rises  and  falls  according  to  the  total  quantity  of  carbonic 
acid  evolved.  Compare  SCHEIBLER'S  later  directions,  and  DIETRICH  (ZeiUchr.  /. 
analyt.  Chem.,  in,  165). 


504 


DETERMINATION. 


[§  139. 


bonic  acid  obtained  (and  with  the  correction  3*2  c.  c.,  repre- 
senting the  carbonic  acid  remaining  behind  in  solution)  and  a 
weighed  portion  of  finely  triturated  and  dried  pure  calcium  car- 
bonate (or  Iceland  spar).  The  relation  will,  of  course,  depend 
upon  the  conditions  (temperature  and  pressure)  prevailing  on  that 
particular  day.  Suppose,  for  instance,  from  0-2737  grm.  of  cal- 
cium carbonate,  containing  0*120307  grm.  of  carbon  dioxide,  we 
had  obtained,  after  adding  the  correction  3*2  c.  c.,  63 '76  c.  c.  ;  sup- 
pose, too,  that  an  analysis  of  0-2371  grm.  of  dolomite  made  under 
similar  conditions  had  yielded  (including 
the  3'2  c.  c.)  57*3  c.  c. ;  the  dolomite  would 
have  contained  63-70  :  0*120307  :  :  57*3  :  x 
»=  0-10812  grm.  of  carbonic  acid,  or  45 -62 
per  cent.  Even  this  method,  however,  can 
give  accurate  results  only  when  the  saline 
solutions  resulting  and  the  quantities  of 
carbonic  acid  evolved  are  fairly  alike  in 
the  first  experiment  and  in  the  analysis. 

/?.  B.  E.  DIETKICH  *  has  devised  a  very 
convenient  apparatus  in  which  the  evolved 
carbonic  acid  is  measured  over  mercury. 
He  has  also  calculated  tables  (given  on 
pages  506-508)  showing  the  weight  of  1  c.  c. 
of  carbonic  acid  at  pressures  of  720  to  700 
mm.  and  temperatures  of  10°  to  25°,  and 
the  quantity  of  CO,  absorbed  by  5  c.  c.  of 
hydrochloric  acid  (sp.  gr.  1*125)  for  evolved 
quantities  between  1  and  100  c.  c.  of  gas. 
Using  DIETRICH'S  apparatus  and  tables, 
estimations  of  CO,  may  be  very  rapidly 
and  accurately  made,  and  the  method  is  particularly  to  be  recom- 
mended when  a  long  series  of  experiments  are  to  be  made. 

[The  azotometer,  Fig.  102,  is  employed,  and  the  details  of 
the  process  are  for  the  most  part  similar  to  those  followed  in  the 
estimation  of  ammonia  as  described  on  page  257.  The  weighed 
carbonate  is  put  in  the  bottle  #,  and  the  tube  f  is  charged  with  5 
C.  c.  of  HC1.,  sp.  gr.  1  -125.  "When  the  burette  is  adjusted  to  zero. 


Fig.  102. 


*Zeitschr.f.  analyt.  Chern.,  m,  162,  iv,  141,  and  v,  49. 


§  140.]  SILICIC    ACID.  505 

the  acid  is  poured  at  once  upon  the  carbonate.  The  precautions 
to  be  observed  in  the  measurement  of  the  gas  are  as  detailed  on 
page  2 57.  It  is  not  needful  to  wait  so  long  for  the  gas  to  cool. 
The  necessary  corrections  are  applied  by  aid  of  the  tables  given 
by  DIETKICH,  pages  506-508.  Their  use  is  perfectly  similar  to 
that  of  the  tables  given  on  pages  259-261.] 

y.  G.  RUMPF  *  describes  »  very  simple  apparatus  which  may 
be  readily  constructed  in  the  laboratory.  Since,  however,  there 
is  no  advantage  in  using  it  unless  RUMPF'S  tables  are  at  hand,  and 
as  lack  of  space  forbids  their  insertion  here,  I  must  simply  refer 
to  the  original  paper. 

d.  Small  quantities  of  carbonic  acid  in  minerals  can  be- 
estimated  as  follows :  By  means  of  an  air-pump  fill  with  mer- 
cury a  graduated  tube  provided  at  its  upper  end  with  a  well 
greased  glass  cock.  Fold  up  the  mineral  in  blotting-paper  and 
send  it  up  the  tube;  follow  this  by  a  measured  quantity  of 
hydrochloric  acid,  sent  up  by  means  of  a  pipette  having  a  bent-up 
tip,  and  then  measure  the  evolved  gas.  To  this  volume  there 
must,  of  course,  be  added  also  the  quantity  of  gas  absorbed  by 
the  hydrochloric  acid.  For  the  method  of  calculating  the  weight 
from  the  volume,  see  §  198. 

§  140. 

2.   SILICIC  Acm. 
I.     DETERMINATION. 

The  direct  estimation  of  silicic  acid  is  almost  invariably  effected 
by  converting  the  soluble  modification  of  the  acid  into  the  insol- 
uble modification,  by  evaporating,  and  completely  drying;  the 
insoluble  modification  is  then,  after  removal  of  all  foreign  matter, 
strongly  ignited  (over  the  bellows  blowpipe)  and  weighed.  For 
the  properties  of  silicic  acid,  see  §  93,  9. 

For  the  guidance  of  the  student  I  would  observe  here  that,  to 
guard  against  mistakes,  he  should  always  test  the  purity  of  the 
iveighed  silicic  acid.  The  methods  of  testing  will  be  found 
below. 

If  you  have  free  silicic  acid  in  the  state  of  hydrate,  in  an 

*  Zeitsrhr.f  analyt.  Chem.,  vi,  398. 


506 


DETEKMINATION. 


[§  139. 


TABLE  OF  THE  WEIGHT  OF  A  CUBIC 

In  Milligrammes  from  720  to  770  mm.  of  press- 

MILLIMETRES. 


720 

722 

724 

726 

728 

730 

732 

734 

736 

738 

740 

742 

744 

10° 

1.77446 

1.77945 

1.78445 

1.78944 

1.79443 

1.79942 

1.80441 

1.80941 

:i.  81440 

1.81940 

1.82438 

1.82937 

1.83437 

11° 

1.76668 

1.77165 

1.77662 

1.78160 

1.78657 

1.79155 

1.79652 

1.80149 

1.80647 

1.81144 

1.81642 

1.82139 

1.82636 

12° 

1.75881 

1.76377 

1.76873 

1.77368 

1.77864 

1.78359 

1.78855 

1.79351 

1.79846 

1.80342 

1.80838 

1.81333 

1.81829 

13° 

1.75092 

1.75587 

1.76081 

1.76576 

1.77070 

1.77565 

1.78059 

1.78554 

1.79048 

1.79543 

1.80037 

1.80532 

1.81026 

14° 

1.74301 

1.74795 

1.75288 

1.75781 

1.76275 

1.76768 

1.77261 

1.77754 

1.78248 

1.78741 

.79234 

1.79728 

1.80221 

15° 

1.73502 

1.73993 

1.74484 

1.74974 

1.75465 

1.75955 

1.76446 

1.76937 

1.77427 

1.77918 

1.78408 

1.78899 

1.79390 

16° 

1.72699 

1.73188 

1.73677 

1.74166 

1.74655 

1.75144 

1.75633 

1.76122 

1.76611 

1.77100 

1.77590 

1.78078 

1.78567 

17° 

1.71888 

1.72376 

1.72862 

1.73349 

1.73836 

1.74322 

1.74809 

1.75296 

1.75783 

1.76269 

1.767'56 

1.77243 

1.77729 

18° 

1.71069 

1.71554 

1.72040 

1.72525 

1.73011 

1.73497 

1.73982 

1.74468 

1.74953 

1.75439 

1.75925 

1.76410 

1.76896 

19° 

1.70239 

1.70723 

1.71207 

1.71691 

1.72175 

1.72659 

1.73143 

1.73627 

1.74111 

1.74595 

1.75078 

1.75562 

1.76046 

20° 

1.69412 

1.69894 

1.70377 

1.70859 

1.71341 

1.71823 

1.72305 

1.72788 

1.73270 

1.73725 

1.74234 

1.74716 

1.75199 

21° 

1.68571 

1.69051 

1.69532 

1.70012 

1.70493 

1.70974 

1.71454 

1.71935 

1.72415 

1.72896 

1.73377 

1.73857 

1.74338 

22o 

1.67722 

1.68201 

1.68680 

1.69151 

1.69638 

1.70117 

1.70596 

1.71075 

1.71554 

1.72033 

1.72512 

1.72991 

1.73470 

23° 

1.66862 

1.67340 

1.67817 

1.68294 

1.68772 

1.69249 

1.69727 

1.70204 

1.70681 

1.71159 

1.71636 

1.72114 

1.72591 

24° 

1.65994 

1.66470 

1.66945 

1.67421 

1.67897 

1.68372 

1.68848 

1.69324 

1.69799 

1.70275 

1.70751 

1.71227  1.71702 

25° 

1.65113 

1.65587 

1.66061 

1.66535 

1.67009 

1.67484 

1.67958 

1.68432 

1.68906 

1.69380 

1.69854 

1.70329 

1.70803 

720 

722 

724 

726 

728 

730 

732 

734 

736 

738 

740 

742 

744 

MILLIMETRES. 


§  139.] 


CARBONIC   ACID. 


507 


CENTIMETRE   OF   CARBONIC   ACID. 

ure  of  mercury,  and  from  10°  to  25°  Cent. 

MILLIMETRES. 


746 

748 

750     752 

754 

756 

758 

760  ;  762 

764 

766 

768 

770 

1.83936 

1.84435 

1.849341.85433 

1.85933 

1.86432 

1.86931 

1.87430 

1.87930 

1.88429 

1.88928 

1.89427 

1.89926 

10° 

1.83134 

1.83631 

1.84129  1.84626 

1.85123 

1.85621 

1.86118 

1.86616 

1.87113 

1.87610 

1.88108 

1.88605 

1.89103 

11° 

1.82324 

1.82820 

1.833151.83811 

1.84307 

1.84802 

1.85298 

1.85793 

1.86289 

1.86785 

1.87280 

1.87776 

1.88271 

12° 

1.81521 

1.H2015 

1.825101.83004 

1.88499 

1.83993 

1.84488 

1.84982 

1.85477 

1.85971 

1.86466 

1.86960 

1.87455 

13° 

1.80714 

1.81208 

1.81701 

1.82194 

1.82687 

1.83181 

1.83674 

1.84167 

1.84661 

1.85154 

1.85647 

1.86141 

1.86634 

14° 

1.79880 

1.80371 

4.80861 

1.81352 

1.81843 

1.82333 

1.82824 

1.83314 

1.83805 

1.84296 

1.84786 

1.85277 

1.85767 

15° 

1.79056 
1.78216 
1.77381 
1  76530 

1.79545 

1.78703 

1.80034 
1.79189 

1.80523 
1.79676 

1.81012 
1.80163 

1.81501 
1.80650 

1.81990 
1.81136 

1.82479 
1.81623 

1.82968 
1.82110 

"""" 

1.82596 

1.83946 
1.83083 

1.84435 
1  83570 
1.82723 
1.81853 
1.80985 

1.84924 
1.84056 
1.83209 
1.82337 
1.81467 

16° 
17° 
18° 
19°| 
20° 

1.77867 
1  77014 

1.78353 
1.77498 

1.78838 
1  77982 

1.79324 
1.78466 

1.79809 
1.78950 

1.80295 
1.79434 

1.80781 
1.79917 

1.81266 
1  80401 

1.81752 
1.80885 

1.82337 
1  81369 
1.80503 

1.75681 

1.76113 

1.76645 

1.77127 

1.77610 

i.  78092 

1.78574 

1.79056 

1.79538 

1.80021 

1.74818 

1.75299 

1.75780 

1.76260 

1.76741 

1.77221 

1.77702 

1.78183 

1.78663 

1.79144 

1.79624 

1.80105 

1.80586 

81« 

1.73949 

1.74428 

1.74907 

1.75386 

1.75865 

1.76344 

1.76823 

1.77302 

1.77781 

1.78260 

1.78739 

1.79218 

1.79697 

22«. 

1.73068 

1.73546 

1.74023 

1.74501 

1.74978 

1.75455 

1.75933 

1.76410 

1.76888 

1.77365 

1.77842 

1.78320 

1.78797 

23° 

1.72178 

1.72654 

1.73129 

1.73605 

1.74081 

1.74556 

1.75032 

1.75508 

1.75984 

1.76459 

1.76935 

1.77411 

1.77886 

24" 

1.71277 

1.71751 

1.72225 

1.72699 

1.73173 

1.73648 

1.74122 

1.74596 

1.75070 

1.75544 

1.76018 

1.76492 

1.76967 

25° 

746 

748 

750 

752 

754 

756 

758 

760 

762 

764 

766 

768 

770 

MILLIMETRES. 


508 


DETERMINATION. 


[§  139. 


§  5 

^  S 

§  « 

10 

s  5 

s  s 

TH           0 

s  s 

^H 

*  g 

s  S 

s  5 

CO 

s  S 

s  s 

MH 

*  s 

a  2 

»J 

CO        o 

0:1      co' 

CO 

5    w. 

^* 

»  5 

s  ! 

e  e 

1O 

si 

s  S 

«  s 

S    9 

10 

s  -. 

IO 

a  S 

S    8 

•^ 

8   3 

^  3 

e  ! 

-  S 

3    1 

*;-3 

-  1 

S    8 

to 

s  s. 

10 

a  g 

a  S 

s  | 

S    § 

<*    2 

a  S 

2    « 

CO 

a  5 

s  S 

-    1 

§    8 

10 

*  ! 

0 

s  S 

?0 

S         °= 

»0 

">* 

c  S 

C3 

s  S 

s  S 

»    S 

§  1 

e  s 

10 

J  1 

c=    8 

CO 

«  1 

5* 

^         CO 

id 

T—  t 

O5         ^Q 

0      10' 

§  s 

id 

CO 
GO        ^ 

cci 

«  1 

S8    S 

10 

§        ^ 

id 

88    » 

10 

!>       8. 
09 

«  S 

J  i 

fe  S 

-  S 

C5 

co      « 

05 

„. 

CO         § 

05  S 

«  1 

-  s 

a  1 

J> 

1C        "^ 
05 

a  1 

*  2 

g  » 

10 

s  s 

IO 

1—  1 
-<#        *^ 
«N 

-  3 

3     S 

10 

^      ^   . 
co         . 

1O 

*S 

CO 
CO        -1-1 
(M 

s  3- 

CD 
r3        ^ 
10 

CO        g 

id 

aj 

O 

«      o 

w 

s  3 

s    ^ 

10' 

§      « 

lO 

a  e 

10 

-  s 

T-I 

1C 

s  3 

5!     S 
to 

i—  i      S 

10   S 

r-          g 

00  5 

Evolved  
Absorbed  

Evolved  
Absorbed  

Evolved.  
Absorbed  

Evolved  
Absorbed  

Evolved  
Absorbed  

§  140.]  SILICIC   ACID.  509 

aqueous  or  acid  solution  free  from  other  fixed  bodies,  simply 
evaporate  the  solution  in  a  platinum  dish,  ignite  and  weigh  the 
residue. 

Respecting  a  volumetric  estimation  of  silicic  acid  (conversion 
into  and  acidimetric  determination  of  potassium  silicofluoride,  see 
§97,  5),  I  must  refer  to  STOLBA.* 

II.  SEPARATION  OF  SILICIC  ACID  FROM  THE  BASIC  RAD- 
ICALS. 

a.  In  all  compounds  which  are  decomposed  by  Hydrochloric 
or  Nitric  Acid,  on  digestion  in  open  vessels. 

To  this  class  belong  the  silicates  soluble  in  water,  as  well  as 
many  of  the  insoluble  silicates,  as,  for  instance,  nearly  all  zeolites. 
Several  minerals  not  decomposable  of  themselves  by  acids  become 
so  by  persistent  ignition  in  a  state  of  fine  powder  (F.  MOHR  f).  If 
the  ignition  is  too  strong,  particles  of  alkali  may  be  lost. 

The  substance  is  very  finely  powdered,  J  dried  at  100°,  and  put 
into  a  platinum  or  porcelain  dish  (in  the  case  of  silicates  whose  solu- 
tion might  be  attended  with  disengagement  of  chlorine,  platinum 
cannot  be  used) ;  a  little  water  is  then  added,  and  the  powder 
mixed  to  a  uniform  paste.  Moderately  concentrated  hydrochloric 
acid,  or — if  the  substance  contains  lead  or  silver — nitric  acid,  is  now 
added,  and  the  mixture  digested  at  a  very  gentle  heat,  with  con- 
stant stirring,  until  the  substance  is  completely  decomposed,  in 
other  terms,  until  the  glass  rod,  which  is  rounded  at  the  end, 
encounters  no  more  gritty  powder,  and  the  stirring  proceeds 
smoothly  without  the  least  grating. 

The  silicates  of  this  class  do  not  all  comport  themselves  in  the 
same  manner  in  this  process,  but  show  some  differences ;  thus  most 
-of  them  form  a  bulky  gelatinous  mass,  whilst  in  the  case  of  others 
the  silicic  acid  separates  as  a  light  pulverulent  precipitate ;  again, 
many  of  them  are  decomposed  readily  and  rapidly,  whilst  others 
require  protracted  digestion. 

When  the  decomposition  is  effected,  the  mixture  is  evaporated 
to  dry  ness  on  the  water-bath,  and  the  residue  heated,  with  frequent 
stirring,  until  all  the  small  lumps  have  crumbled  to  pieces,  and  the 

*  Zeitschr.  f.  annlyt.  Chcm  ,  iv,  163.  f  lb.t  vn,  293. 

$  Very  hard  silicates  cannot  be  powdered  in  an  agate  mortar  without  taking 
up  silica  ;  these  must,  therefore,  be  powdered  in  a  steel  mortar,  sifted,  and  freed 
from  particles  of  steel  with  the  magnet. 


510  DETERMINATION.  [§  140. 

whole  mass  is  thoroughly  dry,  and  until  no  more  acid  fumes  escape. 
It  is  always  the  safest  way  to  conduct  the  drying  on  the  water-bath. 
Occasionally  it  is  well  to  moisten  the  dry  mass  with  water  and  evap- 
orate again.  In  cases  where  it  appears  desirable  to  accelerate  the 
desiccation  by  the  application  of  a  stronger  heat,  an  air-bath  may 
be  had  recourse  to ;  which  may  be  constructed  in  a  simple  way,  by 
suspending  the  dish  containing  the  substance,  with  the  aid  of  wire, 
in  a  somewhat  larger  dish  of  silver  or  iron,  in  a  manner  to  leave 
everywhere  between  the  two  dishes  a  small  space  of  uniform  width. 
Direct  heating  over  the  lamp  is  not  advisable,  as  in  the  most 
strongly  heated  parts  the  silicic  acid  is  liable  to  unite  again  with 
the  separated  bases  to  compounds  which  are  not  decomposed,  or 
only  imperfectly,  by  hydrochloric  acid. 

When  the  mass  is  cold,  it  is  brought  to  a  state  of  semi-fluidity 
by  thoroughly  moistening  it  with  hydrochloric  acid  ;  after  wrhich 
it  is  allowed  to  stand  for  half  an  hour,  then  warmed  on  a  water- 
bath,  diluted  with  hot  water,  stirred,  allowed  to  deposit,  and  the 
fluid  decanted  on  to  a  filter ;  the  residiiary  silicic  acid  is  again 
stirred  with  hydrochloric  acid,  warmed,  diluted,  and  the  fluid  once 
more  decanted  ;  after  a  third  repetition  of  the  same  operation,  the 
precipitate  also  is  transferred  to  the  filter,  thoroughly  washed  with 
hot  water,  well  dried,  and  ignited  at  last  as  strongly  as  possible,  as 
directed  in  §  52  or  53.  For  the  properties  of  the  residue,  see  §  93,  9. 
The  results  are  accurate.  The  basic  metals,  which  are  in  the  filtrate 
as  chlorides,  are  determined  by  the  methods  given  above.  Devia- 
tions from  the  instructions  here  given  are  likely  to  entail  loss  of 
substance  ;  thus,  for  instance,  if  the  mass  is  not  thoroughly  dried, 
a  not  inconsiderable  portion  of  the  silicic  acid  passes  into  the  solu- 
tion, whereas,  if  the  instructions  are  strictly  complied  with,  only 
traces  of  the  acid  are  dissolved  ;  in  accurate  analyses,  however,  even 
such  minute  traces  must  not  be  neglected,  but  should  be  separated 
from  the  metals  precipitated  from  the  solution.  The  separation 
may,  as  a  rule,  be  readily  effected  by  dissolving  them,  after  ignition 
and  weighing,  in  hydrochloric  or  sulphuric  acid,  by  long  digestion 
in  the  heat,  the  traces  of  silicic  acid  being  left  undissolved.  Some- 
times it  is  better  to  fuse  the  metallic  oxides  with  potassium  disul- 
phate,  or  to  reduce  them  to  the  metallic  state  by  ignition  in  hydro- 
gen, and  then  to  treat  with  hydrochloric  acid.  Again,  if  the  silicic 
acid  is  not  thoroughly  dried  previous  to  ignition,  the  aqueous  vapor 
disengaged  upon  the  rapid  application  of  a  strong  heat  may  carry 


§  140.]  SILICIC   ACID.  511 

away  particles  of  the  light  and  loose  silica.  If  a  suction  appara- 
tus has  been  used,  however,  and  the  precipitate  has  been  quite 
thoroughly  freed  from  water,  the  precipitate  may  be  ignited  at 
once,  as  described  in  §  52,  p.  116.  In  this  case,  however,  the 
incineration  of  the  filter  is  frequently  imperfect. 

The  silicic  acid  may  be  tested  as  follows  :  This  testing  must  on 
no  account  be  omitted  if  the  silica  has  been  separated  in  a  pulveru- 
lent and  not  in  a  gelatinous  form.  Heat  a  portion  on  a  water- bath 
with  moderately  concentrated  solution  of  sodium  carbonate  for  an 
hour  in  a  platinum  or  silver  dish ;  with  less  advantage  in  a  porce- 
lain dish.  EGGERTZ*  recommends,  for  0*1  grm.  silicic  acid,  6  c.c. 
of  a  saturated  solution  of  sodium  carbonate  and  12  c.c.  of  water. 
Pure  silica  would  dissolve.  If  a  residue  remains,  pour  off  the  clear 
fluid  and  heat  again  with  a  small  quantity  of  sodium  carbonate.  If 
a  residue  still  remains,  weigh  the  rest  of  the  impure  silica  and 
treat  it  according  to  &,  to  estimate  the  amount  of  impurity. 

If  you  have  pure  hydrofluoric  acid,  you  may  also  test  the  silicic 
acid  in  a  very  easy  manner,  by  treating  it  with  this  acid  and  a  few 
drops  of  sulphuric  acid  in  a  platinum  dish  ;  upon  the  evaporation 
of  the  solution,  the  silicic  acid,  if  pure,  will  volatilize  completely 
(as  silicon  fluoride).  If  a  residue  remains,  moisten  this  once 
more  with  hydrofluoric  acid,  add  a  few  drops  of  sulphuric  acid, 
evaporate,  and  ignite  ;  the  residue  consists  of  the  sulphates  of  the 
metals  retained  by  the  silicic  acid,  as  well  as  any  titanic  acid  that 
was  present  (BERZELIUS).  Ammonium  fluoride  may  be  used 
instead  of  hydrofluoric  acid. 

1).  Compounds  which  are  not  decomposed  "by  Hydrochloric  or 
Nitric  Acid  on  digestion  in  open  vessels. 

a.  Decomposition  by  fusion  with  Alkali  Carbonate. 

Reduce  the  substance  to  an  impalpable  powder,  by  trituration 
and,  if  necessary,  sifting  (§  25) ;  transfer  to  a  platinum  crucible, 
and  mix  with  about  4  times  the  weight  of  pure  anhydrous  sodium 
carbonate  or  sodium  and  potassium  carbonate,  with  the  aid  of  a 
rounded  glass  rod ;  wipe  the  rod  against  a  small  portion  of  sodium 
carbonate  on  a  card,  and  transfer  this  also  from  the  card  to  the 
crucible.  Cover  the  latter  well,  and  heat,  according  to  size,  over  a 
gas  or  spirit-lamp  with  double  draught,  or  a  blast  gas-lamp  ;  or 

*  Zeitschr.  f.  analyt.  Chem.,  vir,  502. 


512  DETERMINATION.  [§  140. 

insert  in  a  Hessian  crucible,  compactly  filled  up  with  calcined 
magnesia,  and  heat  in  a  charcoal  fire. 

Apply  at  first  a  moderate  heat  for  some  time  to  make  the  mass 
simply  agglutinate  ;  the  carbonic  acid  will,  in  that  case,  escape  from 
the  porous  mass  with  ease  and  unattended  with  spirting.  Increase 
the  heat  afterwards,  finally  to  a  very  high  degree,  and  terminate 
the  operation  only  when  the  mass  appears  in  a  state  of  calm  fusion, 
and  gives  no  more  bubbles. 

The  platinum  crucible  in  which  the  fusion  is  conducted  must 
not  be  too  small ;  in  fact,  the  mixture  should  only  half  fill  it.  The 
larger  the  crucible,  the  less  risk  of  loss  of  substance.  As  it  is  of 

O  " 

importance  to  watch  the  progress  of  the  operation,  the  lid  must  be 
easily  removable  ;  a  concave  cover,  simply  lying  on  the  top,  is  there- 
fore preferable  to  an  overlapping  lid.  If  the  process  is  conducted 
over  the  spirit  or  simple  gas-lamp,  the  mixed  sodium  and  potas- 
sium carbonates  are  preferable  to  sodium  carbonate,  as  they  fuse 
much  more  readily  than  the  latter.  In  heating  o\rer  a  lamp,  the 
crucible  should  always  be  supported  on  a  triangle  of  platinum  wire, 
with  the  opening  just  sufficiently  wide  to  allow  the  crucible  to 
drop  into  it  fully  one  third,  yet  to  retain  it  firmly,  even  with  the 
wire  at  an  intense  red  heat.  When  conducting  the  process  over  a 
spirit-lamp  with  double  draught,  or  over  a  simple  gas-lamp,  it  is 
also  advisable,  towards  the  end  of  the  operation,  when  the  heat  is 
to  be  raised  to  the  highest  degree,  to  put  a  chimney  over  the  cruci- 
ble, with  the  lower  border  resting  on  the  ends  of  the  iron  triangle 
which  supports  the  platinum  triangle  ;  this  chimney  should  be 
about  12  or  14  cm.  high,  and  the  upper  opening  measure  about  4 
cm.  in  diameter.  The  little  clay  chimneys  recommended  by  O.  L. 
ERDMANN  are  still  more  serviceable  (Fig.  20,]).  22,  fclQnal.  Anal."). 
When  the  fusion  is  ended,  the  red-hot  crucible  is  removed  with 
tongs,  and  placed  on  a  cold,  thick,  clean  iron  plate,  on  which  it 
will  rapidly  cool ;  it  is  then  generally  easy  to  detach  the  fused  cake 
in  one  piece. 

The  cake  (or  the  crucible  with  its  contents)  is  put  into  a  beaker, 
from  10  to  15  times  the  quantity  of  water  poured  over  it,  and  heat 
Jipplied  for  half  an  hour,  then  hydrochloric  acid  is  gradually  added, 
or,  under  certain  circumstances,  nitric  acid ;  the  beaker  is  kept 
covered  with  a  glass  plate,  or,  which  is  macli  better,  with  a  large 
watch-glass  or  porcelain  dish,  perfectly  clean  outside,  to  prevent 
the  loss  of  the  drops  of  fluid  which  the  escaping  carbonic  acid  car- 


§  140.]  SILICIC   ACID.  613 

ries  along  with  it ;  the  drops  thus  intercepted  by  the  cover  are 
afterwards  rinsed  into  the  beaker.  The  crucible  is  also  rinsed  with 
water  mixed  with  dilute  acid,  and  the  solution  obtained  added  to 
the  fluid  in  the  beaker. 

"The  solution  is  promoted  by  the  application  of  a  gentle  heat, 
which  is  continued  for  some  time  after  this  is  effected  to  insure  the 
complete  expulsion  of  the  carbonic  acid  ;  since  otherwise  some  loss 
of  substance  might  be  incurred,  in  the  subsequent  process  of  evapo- 
ration, by  spirting  caused  by  the  escape  of  that  gas.  If  in  the  pro- 
cess of  treating  the  fused  mass  with  hydrochloric  acid,  a  saline 
powder  subsides  (sodium  or  potassium  chloride),  this  is  a  sign  that 
more  water  is  required. 

If  the  decomposition  of  the  mineral  has  succeeded  to  the  full 
extent,  the  hydrochloric  acid  solution  is  either  perfectly  clear,  or 
light  flakes  of  silicic  acid  only  float  in  it.  But  if  a  heavy  powder 
subsides,  which  feels  gritty  under  the  glass  rod,  this  consists  of 
undecomposed  mineral.  The  cause  of  such  imperfect  decomposi- 
tion is  generally  to  be  ascribed  to  imperfect  pulverization.  In 
such  cases  the  undecomposed  portion  may  be  fused  once  more  with 
alkali  carbonate  ;  the  better  way,  however,  is  to  repeat  the  process 
with  a  fresh  portion  of  mineral  more  finely  pulverized. 

The  hydrochloric  or  nitric  acid  solution  obtained  is  poured, 
together  with  the  precipitate  of  silicic  acid,  which  is  usually  floating 
in  it,  into  a  porcelain  or,  better,  into  a  platinum  dish,  and  treated 
as  directed  in  II.,  a.  That  the  fluid  may  not  be  too  much  diluted, 
the  beaker  should  be  rinsed  only  once,  or  not  at  all,  and  the  few 
remaining  drops  of  solution  dried  in  it ;  the  trifling  residue  thus 
obtained  is  treated  in  the  same  way  as  the  residue  left  in  the  evapo- 
rating basin.  This  is  the  method  most  commonly  em  ployed 'to 
effect  the  decomposition  of  silicates  that  are  undecomposable  by 
acids  ;  that  it  cannot  be  used  to  determine  alkalies  in  silicates  is 
self-evident. 

ft.  Decomposition  ~by  means  of  Hydrofluoric  A  cid. 

aa.  By  Aqueous  Hydrofluoric  Acid. 

The  silicate  should  be  finely  pulverized,  dried  at  100°  (in  some 
cases  ignition  is  advisable"-).  It  is  mixed,  in  a  platinum  dish,  with 

*  Many  minerals  are  much  more  readily  decomposed  by  hydrofluoric  acid 
also  if  they  are  previously  ignited  in  a  state  of  fine  division  (HERMANN,  RAM- 
MELSBERG,  FR.  MOHR,  Zeitschr.  /.  analyt.  Chem.,  vn,  291). 


514  DETERMINATION.  [§  140, 

rather  concentrated,  slightly  fuming  hydrofluoric  acid,  the  acid 
being  added  gradually,  and  the  mixture  stirred  with  a  thick  plati- 
num wire.  The  mixture,  which  has  the  consistence  of  a  thin  paste, 
is  digested  some  time  on  a  water-bath  at  a  gentle  heat,  and  pure 
concentrated  sulphuric  acid,  diluted  with  an  equal  quantity  of 
water,  is  then  added,  drop  by  drop,  in  more  than  sufficent  quantity 
to  convert  all  the  basic  metals  present  into  sulphates.  The  mixture 
is  now  evaporated  on  the  water-bath,  during  which  operation  sili- 
con fluoride  gas  and  hydrofluoric  acid  gas  are  continually  volatiliz- 
ing ;  then  it  is  finally  exposed  to  a  stronger  heat  at  some  height  above 
the  lamp,  until  the  excess  of  sulphuric  acid  is  almost  completely 
expelled.  The  mass,  when  cold,  is  thoroughly  moistened  with  con- 
centrated hydrochloric  acid,  and  allowed  to  stand  at  rest  for  one 
hour ;  water  is  then  added,  and  a  gentle  heat  applied.  If  the 
decomposition  has  fully  succeeded,  the  whole  must  dissolve  to  a 
clear  fluid.  If  an  undissolved  residue  is  left,  the  mixture  is  heated 
for  some  time  on  the  water-bath,  then  allowed  to  deposit,  the  clear 
supernatant  fluid  decanted  as  far  as  practicable,  the  residue  dried, 
and  then  treated  again  with  hydrofluoric  acid  and  sulphuric  acidr 
and,  lastly,  with  hydrochloric  acid,  which  will  now  effect  complete 
solution,  provided  the  analyzed  substance  was  very  finely  pulver- 
ized, and  free  from  barium,  strontium  (and  lead).  The  solution  i& 
added  to  the  first.  The  basic  metals  in  the  solution  (which  con- 
tains them  as  sulphates,  and  contains  also  free  hydrochloric  acid) 
are  determined  by  the  methods  which  will  be  found  in  Section  V. 

This  method,  which  is  certainly  one  of  the  best  to  effect  the 
decomposition  of  silicates,  was  proposed  by  BERZELIUS.  It  has- 
been  but  little  used  hitherto,  because  we  did  not  know  how  to  pre- 
pare hydrofluoric  acid,  except  with  the  aid  of  a  distilling  appa- 
ratus of  platinum,  or,  at  least,  with  a  platinum  head  ;  nor  to  keep 
it,  except  in  platinum  vessels.  These  difficulties  can  now  be  con- 
sidered as  overcome,  comp.  §  58,  2.  Xever  omit  testing  the  acid 
before  using  it. 

The  hydrofluoric  acid  may  also  be  employed  in  combination 
with  hydrochloric  acid ;  thus  1  grm.  of  finely  elutriated  felspar,, 
mixed  with  40  c.c.  water,  7  c.c.  hydrochloric  acid  of  25$  and  SJc.c. 
hydrofluoric  acid,  and  heated  to  near  the  boiling  point,  dissolves- 
completely  in  three  minutes.  4  c.c.  sulphuric,  acid  are  then  added, 
the  barium  sulphate  which  may  separate  is  filtered  oft',  and  the 


§  140.]  SILICIC    ACID.  515 

filtrate    evaporated    till    no   more  hydrofluoric  acid  escapes  (AL. 

MlTSCHEKLICH  *). 

The  execution  of  the  method  requires  the  greatest  possible 
care,  both  the  liquid  and  the  gaseous  hydrofluoric  acid  being  most 
injurious  substances.  The  treatment  of  the  silicate  with  the  acid 
and  the  evaporation  must  be  conducted  in  the  open  air,  otherwise 
the  windows  and  all x  glass  apparatus  will  be  attacked.  As  the 
silicic  acid  is  in  this  method  simply  inferred  from  the  loss,f  a 
combination  with  method  a  is  often  resorted  to. 

lb.   By  Gaseous  Hydrofluoric  Acid. 

Instead  of  the  aqueous  solution  of  hydrofluoric  acid,  the 
gaseous  acid  may  also  be  used  for  decomposing  silicates.  This 
method,  which  was  formerly  much  used,  was  proposed  by  BRTJN- 
NER,;£  and  is  as  follows:  Place  1  or  2  grammes  of  the  very  finely 
powdered  silicate  in  as  thin  a  layer  as  possible  in  a  shallow  platinum 
dish,  moisten  the  powder  with  diluted  sulphuric  acid,  and  place 
the  dish  on  a  leaden  tripod  or  other  support  within  a  leaden  box 
about  6  inches  in  diameter  and  about  6  inches  high,  and  on  the 
bottom  of  which  there  has  been  just  placed  a  half -inch  layer  of 
powdered  fluor-spar  mixed  into  a  paste  with  concentrated  sul- 
phuric acid.  (Take  care  to  avoid  the  vapors  evolved ;  the  mixing 
of  the  fluor-spar  and  sulphuric  acid  should  be  done  with  a  long 
glass  rod,  or,  better  still,  with  a  leaden  rod).  As  soon  as  the  plat- 
inum dish  has  been  placed  in  the  box,  by  the  aid  of  a  pair  of  pin- 
cers or  tongs,  put  on  a  tightly-fitting  cover,  lute  the  joints  airtight 
with  plaster-of-paris,  and  set  the  whole  for  6  to  8  days  in  a  warm 
place.  If  it  is  desired  to  facilitate  the  process  do  not  lute  air- 
tight, but  heat  the  apparatus  in  the  open  air  §  over  a  gas-  or 
alcohol-lamp ;  by  this  method  1  to  2  grammes  of  the  silicate  may 
be  decomposed  in  a  few  hours,  provided  the  silicate  has  been 
spread  out  in  a  very  thin  layer,  or  else  stirred  from  time  to  time, 
an  operation  which  must  be  cautiously  effected. 

*  Journ.  f.  prakt.  Chem.,  LXXXI,  108. 

f  The  silicon  escaping  in  the  form  of  fluoride  may  sometimes  be  determined 
directly  by  the  method  of  STORY  M. \SKEL YNE  (  Zeitschr.  f.  analyt.  Chem.,  ix, 
380),  which,  however,  requires  a  platinum  retort  of  peculiar  construction. 

J  Pogg.  Annal. ,  XLIV,  134. 

§  An  apparatus  that  may  be  used  in  the  laboratory  has  been  described  by 
A.  MULLER  (Journ.  f.  prakt.  CTiem.,  xcv,  51). 


516  DETERMINATION.  [§  140. 

If  the  decomposition  lias  succeeded,  tile  residue  in  the  plati- 
num dish  will  consist  of  silicofluorides  of  the  metals,  and  sulphates. 
Place  the  dish  now  in  a  larger  platinum  dish,  add  sulphuric  acid 
drop  by  drop,  using  a  little  more  than  is  sufficient  to  convert  the 
bases  into  sulphates,  evaporate  in  an  air-bath,  nearly  but  not  quite 
expel  finally  the  excess  of  sulphuric  acid  over  the  naked  flame, 
and  treat  the  residue  with  hydrochloric  acid  and  water  as  detailed 
under  aa.  The  decomposition  may  be  considered  as  complete 
only  when  a  perfect  solution  results  (apart  from  the  presence  of 
a  little  barium  sulphate). 

If  a  platinum  tube  adapted  for  the  purpose  is  at  hand  the  de- 
composition may  also  be  effected  by  heating  the  finely  powdered 
mineral  placed  in  a  platinum  boat  inserted  into  the  tube  while, 
passing  through  the  latter  a  current  of  dry  hydrofluric-acid  gas. 
The  platinum  tube  is  bent  downwards  in  front,  and  the  end  should 
dip  into  water ;  the  water  takes  up  the  volatile  fluorides,  while 
the  iion- volatile  remain  ^in  the  platinum  boat.  (SAINT-CLAIRE 
DEVILLE,  KUHLMANN.*) 

cc.  By  Ammonium  Fluoride. 

Mix  the  very  finely  powdered  substance  in  a  platinum  dish 
witli  four  times  its  weight  of  ammonium  fluoride,  moisten  well 
with  concentrated  sulphuric  acid,  heat  on  the  water-bath  till  the 
evolution  of  silicon  fluoride  and  hydrofluoric  acid  slackens,  add 
more  sulphuric  acid,  heat  again,  finally  somewhat  more  strongly 
till  the  greater  part  of  the  sulphuric  acid  has  escaped,  and  treat 
the  residue  according  to  aa  (L.  v.  BABO,  J.  POTYKA,  R.  HOFF- 
MANN f).  H.  ROSE  J  first  warms  the  silicate  gently  with  seven 
times  its  amount  of  the  fluoride  and  some  water,  then  heats  gradu- 
ally to  redness  till  no  more  fumes  escape,  and  finally  treats  with 
sulphuric  acid. 

dd.  By  Hydrogen  Potassium  Fluoride,  etc. 

In  silicates,  which  more  or  less  resist  the  action  of  hydrofluoric 
acid,  such  as  zircon  and  beryl,  the  basic  metals  with  the  exception 
of  the  alkalies  may  be  determined  by  fusing  with  hydrogen 
potassium  fluoride  (MARIGNAC,  GIBBS  §),  or  by  mixing  with 
3  parts  of  sodium  fluoride,  adding  12  parts  of  potassium  disulphate 

*  Compt.  Rend.,  LVITT,  545.  f  Zeitschr.f.  analyt.  Chem.,  vi,  366. 

\Pogg.  Annal.,  cvin,  20.  %  Zeitschr.f.  analyt.  Chem.,  mr  399. 


§  140.]  SILICIC   ACID.  517 

to  the  crucible,  and  then  heating  at  first  very  gently,  afterwards 
more  strongly  till  the  mass  fuses  calmly.  The  residue  is  dissolved 
in  water  or  hydrochloric  acid  (CLARKE*). 

y.  Decomposition  by  Fusion  with  Barium  Hydroxide  or 
Bariwn  Carbonate. 

The  fusion  of  silicates  with  barium  carbonate  requires  a  very 
high  heat,  obtainable  only  with  a  good  blast-lamp,  a  SEFSTROM  fur- 
nace, a  DEVILLE  turpentine  lamp,  etc. ;  even  the  highest  temper- 
ature afforded  by  a  wind  furnace  is  insufficient  to  effect  the  melt- 
ing together  of  the  barium  carbonate  and  silicate,  and  only  when 
this  occurs  is  decomposition  complete.  When  this  does  occur, 
however,  it  is  so  energetic  that  even  the  most  refractory  fossils 
are  easily  and  completely  decomposed.  From  4  to  6  parts  of 
barium  carbonate  are  taken  for  1  part  of  the  very  finely  powdered 
mineral.  The  fusion  is  effected  in  a  platinum  crucible,  which, 
if  a  SEFSTROM  furnace  is  used,  is  placed  within  another  crucible 
of  refractory  fire-clay  filled  with  magnesia.  The  crucible  is  left 
in  the  furnace  for  at  least  half  an  hour.  The  greater  the  quan- 
tity of  barium  carbonate  taken  the  greater  is  the  danger  of  alka- 
lies volatilizing.  DEVILLE,  in  fact,  recommends  taking  only  0*8 
part  barium  carbonate  for  1  part  of  felspathic  mineral. 

More  readily  decomposable  minerals  may  be  more  easily  de- 
composed by  means  of  barium  hydroxide  freed  from  its  water  of 
crystallization.  To  1  part  of  the  mineral,  from  4  to  5  parts  of 
barium  hydroxide  are  taken,  the  whole  intimately  mixed  and 
covered  with  a  layer  of  barium  carbonate.  The  fusion  may  be 
effected  over  an  ordinary  gas-  or  BERZELIUS  alcohol -lamp ;  and  it 
is  best  to  use  silver  crucibles,  as  platinum  is  attacked.  The  mass 
fuses  either  completely  or  at  least  melts  together  into  a  mass. 
In  order  to  render  platinum  crucibles  also  applicable,  v.  FELLEN- 
BERtt-RrviER  f  recommends  melting  4  or  5  parts  of  calcium  chlo- 
ride in  the  platinum  crucible,  shaking  the  crucible  around  while 
cooling,  then  adding  1  part  of  barium  hydroxide  and  fusing  this 
in  turn.  After  cooling,  1  part  of  the  finely  powdered  silicate  is 
introduced  and  heat  is  applied,  gently  at  first,  but  strongly  later 
when  no  gas  appears  to  be  evolved.  SMITH  $  recommends  fusing 

*Zeil8chr.f.  analyt.  Chem  ,  vn,  463.  \  2b.,  ix,  459. 

\Journ.f.  prakt.  Chem.,  LX,  246. 


518  DETERMINATION.  [§  140. 

1  part  silicate  with  3  to  4  parts  barium  carbonate  and  2  parts 
barium  chloride. 

When  the  operation  is  at  an  end — no  matter  whether  barium 
carbonate  or  barium  hydroxide  has  been  used — allow  the  crucible 
to  cool,  clean  its  outside,  cover  it  with  10  to  15  parts  of  water  in 
a  beaker,  allow  to  macerate  for  some  time,  then  add  hydrochloric 
or  nitric  acid,  and  proceed  as  in  &,  a.  Care  must  be  taken  to 
avoid  adding  too  much  hydrochloric  acid  at  a  time,  because  the 
barium  chloride  is  difficultly  soluble  in  it,  and  hence  may  retard 
or  check  the  solution  of  the  still  un dissolved  portion  by  forming 
over  this  an  insoluble  protective  coating.  In  the  filtrate  from 
silicic  acid  the  bases  are  estimated  according  to  the  methods  given 
under  Section  Y.  The  silicic  acid  is  to  be  tested  as  to  its  purity 
according  to  the  method  described  in  #,  before  the  operation  may 
be  regarded  as  having  been  successful.  These  methods,  which 
were  formerly  much  employed  in  determining  the  alkalies  in  sili- 
cates, have  been  more  or  less  superseded  by  decomposition  with 
aqueous  hydrofluoric  acid  and  with  ammonium  fluoride,  as  both 
of  these  are  now  readily  obtainable  commercially. 

d.  Decomposition  ~by  fusion  with  Calcium  Carbonate  and 
Ammonium  Chloride. 

DEVILLE  *  recommends  fusing  1  part  of  the  powdered  silicate 
with  0'3  to  0'8  part  calcium  carbonate,  but  I  have  not  found  the 
process  to  answer  with  many  silicates.  L.  SMITH  f  recommends 
fusing  0*5  to  1  grm.  of  powered  silicate  with  1  grm.  finely  granu- 
lated ammonium  chloride  (prepared  by  interrupted  crystallization) 
and  8  grm.  pure  calcium  carbonate  (obtained  by  precipitation 
with  ammonium  carbonate  with  heat).  Should  the  temperature 
during  fusion  rise  too  high,  however,  a  portion  of  the  alkali 
chloride  may  be  lost  by  volatilization.  SMITH  employs  crucibles 
9  mm.  high,  22  mm.  wide  at  the  mouth,  and  16  mm.  wide 
at  the  base.  The  crucibles  are  fixed  in  a  metal  clamp  or  in  the 
iron  plate  of  a  special  gas  furnace,^:  and  in  such  a  manner  that 
about  15  mm.  remains  outside.  Gentle  heat  is  applied  first  to 

*  Journ.  f.  prakt.  Chem.,  LX,  246. 

•\  Ib.,  LX,  246;  also  Chem.  News,  xxnt,  222  and  234;   Zeitschr.  /.  analyt. 
Chem.,  xi,  85. 

%  Zeitschr.  f.  analyt.  Chem.,  xi,  87. 


§  140.]  SILICIC    ACID.  519 

the  part  of  the  crucible  above  the  mixture,  and  is  then  gradually- 
moved  downwards,  so  that  in  about  5  minutes  all  the  ammonium 
chloride  is  decomposed.  The  heat  is  then  increased  and  the 
crucible  kept  at  a  bright-red  heat  for  40  to  60  minutes.  By  this 
method  of  heating  all  fear  of  volatilization  of  alkali  chloride  is 
avoided.  After  cooling,  proceed  with  the  semi-fused  mass  accord- 
ing to  y.  SMITH  states,  however,  that  a  solution  of  the  total 
alkalies  may  also  be  obtained  by  heating  the  ignited  mass  with 
water  for  several  hours,  filtering,  and  washing  the  residue.  From 
this  solution  of  alkalies,  calcium  chloride,  and  calcium  hydroxide, 
the  calcium  is  precipitated  by  ammonium  carbonate  and  a  little 
oxalate. 

[Prof.  J.  L.  SMITH'S  METHOD  in  detail  for  separating  alkalies : 
Mix  1  part  of  the  pulverized  silicate  with  1  part  of  dry  ?  mi  no- 
mum  chloride,  by  gentle  trituration  in  a  smooth  inortar,  then  add 
8  parts  of  calcium  carbonate  ("  Qual.  Anal."  p.  87)  and  mix  inti- 
mately. Bring  the  mixture  into  a  platinum  crucible,  rinsing  the 
mortar  with  a  little  calcium  carbonate.  Warm  the  crucible  gradu- 
ally over  a  small  Bunsen  burner  until  fumes  of  ammonium  salts  no 
longer  appear,  then  heat  with  the  flame  of  a  Bunsen  burner  until 
the  lower  three-fourths  only  of  the  crucible  are  brought  to  a  red 
heat.  Keep  this  temperature  constant  from  40  to  60  minutes. 
The  temperature  desired  is  that  which  suffices  to  keep  in  state  of 
fusion  the  calcium  chloride  formed  by  the  reaction  of  ammonium 
chloride  with  calcium  carbonate.  The  mass,  however,  does  not 
become  liquid  since  the  fused  calcium  chloride  is  absorbed  by  the 
large  quantity  of  calcium  carbonate  present.  If  the  silicate  is 
fused  by  application  of  too  strong  heat,  disintegration  of  the  mass 
at  the  end  of  the  operation  with  water  cannot  be  effected.  More- 
over, too  high  a  temperature  causes  volatilization  of  alkali  chlo- 
rides. Certain  silicates — e.g.,  those  which  contain  much  ferrous 
iron — may  fuse  when  heated  with  the  above  mixture,  even  if  no 
higher  temperature  is  employed  than  is  necessary  to  effect  decom- 
position. If  this  occurs,  it  is  better  to  repeat  the  ignition  with  a 
new  portion  jof  the  silicate,  using  8  to  10  parts  of  calcium  carbo- 
nate. The  mass  contracts  in  volume  during  the  ignition,  and  is 
usually  easily  detached  from  the  crucible.  Boil  it  in  a  covered 
porcelain  dish,  with  50-75  c.c.  water,  half  an  hour,  replacing  water 
lost  by  evaporation.  Decant  the  solution  from  the  residue  upon  a 
filter,  boil  the  residue  a  few  minutes  with  water,  and  decant  again. 


620  DETERMINATION.  [§  140. 

If  the  residue  is  now  all  in  a  finely  disintegrated  state,  it  may  be 
brought  upon  the  filter  and  washed.  But  if,  as  is  often  the  case,  a 
portion  remains  coherent  or  in  a  coarsely  granular  state,  it  must  be 
reduced  to  a  fine  state  of  division  by  trituration  with  a  porcelain 
or  agate  pestle  in  the  dish,  and  boiling  with  water  again.  By  a 
few  repetitions  of  the  trituration,  boiling  and  decanting,  allowing 
the  fine  suspended  portion  to  pass  upon  the  filter  each  time,  the 
whole  can  usually  be  transferred  to  the  filter  in  properly  disinte- 
grated condition  in  course  of  an  hour.  Next  wash  until  a  few  drops 
of  the  washings  acidified  with  nitric  acid  give  but  a  slight  turbid- 
ity with  silver  nitrate.  The  filtrate  now  contains  the  alkalies  of 
the  silicate  as  chlorides  together  with  calcium  chloride  and  hydrox- 
ide. It  is  not  advisable  to  concentrate  this  filtrate  in  a  glass  vessel, 
since  it  might  take  an  appreciable  quantity  of  sodium  from  the 
glass.  Precipitate,  therefore,  the  calcium  at  once  with  ammonium 
carbonate  ;  allow  the  precipitate,  to  settle,  and  concentrate  the 
supernatant  solution  in  a  porcelain  (or  platinum)  dish,  decanting  it 
into  the  latter,  portionwise  if  necessary,  rinsing  finally  the  precipi- 
tate into  the  porcelain  dish.  When  the  whole  is  thus  reduced  to 
about  30  c.c.,  add  a  little  more  ammonium  carbonate  and  ammonia, 
heat  and  filter  into  a  platinum  (or  porcelain)  dish,  evaporate  to 
dry  ness  on  a  water-bath,  expel  ammonium  chloride  by  ignition, 
dissolve  the  residual  alkali  chlorides  in  3  to  5  c.c.  of  water.  A 
little  black  or  dark-brown  flocculent  matter  usually  remains  undis- 
solved,  while  the  solution  may  still  contain  traces  of  calcium.  Add 
two  or  three  drops  of  ammonium  carbonate  and  ammonia,  warm 
gently,  and  filter  through  a  very  small  filter  into  a  weighable  plati- 
num vessel.  Evaporate  to  dryness  on  a  water-bath,  heat  to  in- 
cipient fusion  of  the  alkali  chlorides,  and  after  cooling  weigh. 

Prof.  SMITH'S  method  is  the  most  convenient  of  all  methods 
for  extracting  alkalies  from  silicates,  and  is  universally  applicable, 
except  perhaps  in  presence  of  boric  acid.  When  carried  out  as 
here  described,  the  results  are  sufficiently  accurate  in  most  cases. 
If,  however,  the  silicate  is  rich  in  alkalies,  a  loss  amounting  to 
0-1  or  0-2  per  cent  of  the  mineral  is  possible.  If  great  accuracy 
is  desired  in  such  cases,  a  repetition  of  the  whole  process  may  be 
applied  to  the  residue  left  by  treatment  of  the  ignited  mass  with 
water.  It  need  hardly  be  mentioned  that  unless  care  be  taken  to  use 
reagents  perfectly  free  from  soda,  and  to  avoid  the  action  of  solu- 


§  141.]  CHLORINE.  521 

tions  on  glass,  an  amount  of  soda  may  be  introduced  from  these 
sources  equal  to  O'l  or  0*2  per  cent  of  the  silicate.] 

e.   Decomposition  with  Hydrochloric  or  Sulphuric  Acid 
in    Sealed    Tubes    (under    Pressure),    according    to    AL. 

MlTSCHERLICH.* 

Many  silicates,  as  well  as  altiminates,  which  are  scarcely  or  not 
at  all  attacked  on  being  digested  with  hydrochloric  or  sulphuric 
acid  in  open  vessels,  are  completely  decomposed  on  being  heated 
for  two  hours  in  sealed  tubes  to  200°  to  210°  with  25-per  cent 
hydrochloric  acid  or  with  a  mixture  of  1  part  by  weight  of 
water  and  3  parts  by  weight  of  concentrated  sulphuric  acid.  To 
carry  out  the  process  introduce  about  1  grm.  of  the  finely  elu- 
triated or  sifted  substance  into  a  tube  of  difficultly  fusible  Bohe- 
mian glass  sealed  at  one  end  and  drawn  out  somewhat  at  the 
other ;  then  add  the  acid,  carefully  seal  the  tube,  enclose  it  in  the 
wrought-iron  tube  of  a  metallic  bath,f  and  heat  in  the  manner 
prescribed.  After  cooling,  carefully  open  the  tube,  rinse  out  its 
contents  into  a  platinum  or  porcelain  dish,  and  proceed  as  in 
§  140,  II,  a.  This  method  possesses  the  advantage  over  others 
that  any  ferrous  salt  present  is  obtained  in  solution  as  such  and 
can  be  readily  determined. 

Second  Group. 

CHLORINE — BROMINE — IODINE — CYANOGEN — SULPHUB. 
§141. 

1.  CHLORINE. 

I.  Determination. 

Chlorine  may  be  determined  very  accurately  in  the  gravimetric 
as  well  as  in  the  volumetric  way.;f 

a.  Gravimetric  Method. — Determination  as  Silver  Chloride. 

Solution  of  silver  nitrate,  mixed  with  some  nitric  acid,  is  added 
in  excess  to  the  solution  of  the  chloride,  the  precipitated  chloride 
is  made  to  unite  by  heating  and  agitating,  washed  by  decantation 

*  Jour ii,  f.  prakt.  Chem.,  LXXXI,  108,  and  LXXXIII,  455. 

fSuch  a   bath   is   described   and  illustrated  in  the  Joitrn.  f.  prakt.   Chem., 
LXXXIII,  489,  and  in  the  ZeitscJir.  f.  analyt.  C/tem.,  i,  55. 

\  For  the  acidimetric  estimation  of  free  hydrochloric  acid,  see  §  215. 


DETERMINATION.  [§  141. 

and  filtration,  dried,  and  ignited.  The  details  of  the  process  have 
been  given  in  §  115,  1,  a.  Care  must  be  taken  not  to  heat  the 
solution  mixed  with  nitric  acid,  before  the  nitrate  of  silver  has 
been  added  in  excess.  As  soon  as  the  latter  is  present  in  excess, 
the  silver  chloride  separates  immediately  and  completely  upon 
shaking  or  stirring,  and  the  supernatant  fluid  becomes  perfectly 
clear  after  standing  a  short  time  in  a  warm  place.  The  determina- 
tion of  chlorine  by  means  of  silver  is  therefore  more  readily  effected 
than  that  of  silver  by  means  of  hydrochloric  acid. 

5.  Volumetric  Methods. 

a.  By  Solution  of  Silver  Nitrate. 

In  §  115,  5,  we  have  seen  how  the  silver  in  a  fluid  may  be  esti- 
mated by  adding  a  standard  solution  of  sodium  chloride  until  no 
further  precipitation  ensues ;  in  the  same  way  we  may  determine 
also,  by  means  of  a  standard  solution  of  silver,  the  amount  of  hydro- 
chloric acid  in  a  fluid,  or  of  chlorine  in  combination  with  a  metal. 
PELOUZE  has  used  this  method  for  the  determination  of  several 
atomic  weights.  LEVOL*  proposed  a  modification  which  serves  to 
indicate  more  readily  the  exact  point  of  complete  precipitation. 
To  the  fluid,  which  must  be  neutral,  he  added  one  tenth  volume 
of  a  saturated  solution  of  sodium  phosphate.  When  the  whole  of 
the  chlorine  has  been  precipitated  by  the  silver,  the  further  addi- 
tion of  the  solution  of  silver  produces  a  yellow  precipitate  which 
does  not  disappear  upon  shaking  the  vessel.  FE.  MOHE  has  since 
replaced,  with  the  most  complete  success,  the  sodium  phosphate  by 
potassium  chromate. 

This  convenient  and  accurate  method  requires  a  perfectly  neu- 
tral solution  of  silver  nitrate  of  known  value.  The  strength  most 
convenient  is,  1  litre  =  0*1  at.  Cl.  I  recommend  the  following 
method  of  preparation :  Dissolve  18-80  to  18-85  grin,  pure  fused 
.silver  nitrate  in  1100  c.c.  water,  and  filter  the  solution  if  required ; 
the  solution  is  purposely  made  too  strong  at  first.  Now  weigh  off 
exactly  four  portions  of  pure  sodium  chloride,  each  of  0' 10  to  0-18 
grm.,  one  after  another.  The  salt  should  be  moderately  ignited, 
not  fused,  powdered  roughly  while  still  warm,  and  introduced  into 
a  small  dry  tube,  that  can  be  well  closed.  The  weighing  off  is  per- 
formed by  first  weighing  the  filled  tube,  then  shaking  out  into  a 
dry"  beaker  the  quantity  required,  weighing  again,  dropping  a 

*  Jo-urn./,  prakt.  Chem.,  LX,  384. 


§  141.]  CHLORINE. 

second  portion  into  beaker  No.  2,  weighing  again,  and  so  on. 
Each  portion  is  dissolved  in  20  to  30  c.c.  water,  and  about  3  drops 
of  a  cold  saturated  solution  of  pure  normal  potassium  chromate 
added. 

Fill  a  MOHE'S  burette  (in  very  accurate  analysis  an  ERDM ANN'S 
float  should  be  used)  with  the  silver  solution,  and  run  it  slowly, 
with  constant  stirring,  into  the  light  yellow  solution  contained  in 
one  of  the  beakers.  Each  drop  produces,  where  it  falls,  a  red  spot, 
which  on  stirring  disappears,  owing  to  the  instant  decomposition 
of  the  silver  chromate  writh  the  sodium  chloride.  At  last,  how- 
ever, the  slight  red  coloration  remains.  Now  all  chlorine  has  com- 
bined with  silver,  and  a  little  silver  chromate  has  been  permanently 
formed.  Read  off  the  burette  and  reckon  how  much  silver  solu- 
tion would  have  been  required  for  O'l  mol.  sodium  chloride,  i.e., 
5-85  grm.  Suppose  we  have  used  to  0*11  sodium  chloride  18*7 
c.  c.  silver  solution : 

0-11  :  5-85  ::  18'T  :  x\     x=  994-5. 

Now,  without  throwing  away  the  contents  of  the  first  beake* , 
make  a  second  and  third  experiment  in  the  same  manner,  of  course 
always  taking  notice  to  regard  the  same  shade  of  red  as  the  sign  of 
the  end.  The  results  of  these  are  reckoned  out  in  the  same  way 
as  the  first.  Suppose  they  gave  for  5;85  NaCl  995*0  and  993-0 
respectively,  we  take  the  mean  of  the  three  numbers,  which  is 
994-2,  and  we  now  know  that  we  have  only  to  take  this  number  of 
c.c.  of  silver  solution,  and  make  it  up  to  1000  c.c.  with  5-8  water, 
in  order  to  obtain  a  solution  of  the  required  strength,  i.e.,  1000  c.c. 
=  0-1  mol.  NaCl.  But  if  994-2  requires  5'8  water,  1000  requires 
5-83.  Hence  we  fill  a  litre-flask  (previously  dried  or  rinsed  with 
a  small  portion  of  the  solution)  up  to  the  "  holding "  mark  with 
the  solution,  add  5*83  c.c.  water,  insert  a  caoutchouc  stopper,  and 
shake. 

The  solution  must  now  be  correct ;  however,  to  make  quite 
sure,  we  perform  another  experiment  with  it.  To  this  end  rinse 
the  empty  burette  with  the  new  solution,  fill  it  with  the  same  and 
test  with  the  portion  of  salt  in  beaker  No.  4.  The  c.c.  used  of 
silver  solution  must  now, if  multiplied  by  0-00585,  give  exactly  the 
weight  of  the  salt. 

Being  now  in  possession  of  a  standard  silver  solution,  and  bei'iiir 
practised  in  exactly  hitting  the  transition  from  yellow  to  the  sh:idr 


524  DETERMINATION.  [§  141. 

of  red,  we  are  in  the  position  to  determine  with  precision  chlorine 
in  the  form  of  hydrochloric  acid  or  of  a  metallic  chloride  soluble  in 
water.  The  fluid  to  be  tested  must  be  neutral — free  acids  dissolve 
the  silver  chromate.  The  solution  of  the  substance  is  therefore, 
if  necessary,  rendered  neutral  by  addition  of  nitric  acid  or  sodium 
carbonate  (it  should  be  rather  alkaline  than  acid),  about  3  drops 
of  the  solution  of  chromate  added,  and  then  silver  from  the  burette, 
till  the  reddish  coloration  is  just  perceptible.  The  number  of  c.c. 
used  has  only  to  be  multiplied  by  the  atomic  weight  of  chlorine 
or  the  mol.  weight  of  the  metallic  chloride  and  divided  by  10,000 
to  give  the  amount  of  these  respectively  present. 

If  the  operator  fears  he  has  added  too  much  silver  solution,  i.e., 
if  the  red  color  is  too  strongly  marked,  he  may  add  1  c.c.  of  a  solu- 
tion of  sodium  chloride  containing  5*85  in  a  litre  (and  therefore 
corresponding  to  the  silver  solution),  and  then  add  the  silver  drop 
by  drop  again.  Of  course  in  this  case  1  c.c.  must  be  deducted  from 
the  amount  of  silver  solution  used. 

The  results  are  very  satisfactory.  The  fluid  to  be  analyzed 
should  be  about  the  same  volume  as  the  solutions  employed  in 
standardizing  the  silver  solution,  and  also  about  the  same  strength, 
otherwise  the  small  quantity  of  silver  which  produces  the  colora- 
tion wrill  not  stand  in  the  same  proportion  to  the  chlorine  present. 
This  small  quantity  of  silver  solution  is  extremely  small,  varying 
between  0'05  and  0  '1  c.c.  :  the  inaccuracy  hereby  arising  even  in  the 
case  of  quantities  of  chlorine  differing  widely  from  that  originally 
used  in  standardizing  the  silver  solution  is  therefore  almost  incon- 
siderable. If  the  amount  of  silver  solution  necessary  to  impart  the 
coloration  always  remained  the  same,  we  should  have  simply  to 
deduct  the  amount  in  question  in  all  experiments,  in  order  to 
avoid  this  small  inaccuracy  entirely ;  since,  however,  the  greater 
the  quantity  of  silver  chloride  the  more  silver  chromate  is  required 
for  visible  coloration,  this  method  of  proceeding  would  not  increase 
the  exactness  of  the  results. 

ft.  By  Solution  of  Silver  Nitrate  and  Iodide  of  Starch 
(PISANI'S  method*). 

Add  to  the  solution  of  the  chloride,  acidified  with  nitric  acid,  a 
slight  excess  of  standard  solution  of  silver  nitrate,  warm,  and  filter. 
Determine  the  excess  of  silver  in  the  filtrate  by  means  of  solution 

*  Annal.  d.  Mines,  \,  83;  LTEBIG  and  KOPP'S  Jahrefbericht,  1856,  751. 


§  141.]  CHLORINE.  525 

of  iodide  of  starch  (see  p.  349),  and  deduct  tliis  from  the  amount 
of  silver  solution  used.  The  difference  shows  the  quantity  of 
silver  which  has  combined  with  the  chlorine;  calculate  from  this 
the  amount  of  the  latter.  Results  satisfactory. 

y.  By  Mercuric- Nitrate  Solution  (LiEBio's  method,*  par- 
ticularly recommended  for  estimating  chlorine  in  the  chlorides  in 
urine). 

aa.  Principle  of  the  method.  Mercuric-nitrate  solution  causes 
an  immediate,  dense,  white  precipitate  in  a  solution  of  urea; 
mercuric  chloride,  however,  does  not.  On  mixing  a  mercuric- 
nitrate  solution  with  an  alkali  chloride,  mercuric  chloride  and 
alkali  nitrate  are  formed.  Hence  on  adding  sodium  chloride  to  a 
urea  solution  and  then  dropping  in  a  dilute  mercuric-nitrate  solu- 
tion a  white  cloudiness  forms  at  the  point  where  the  drops  fall, 
but  on  shaking  it  disappears  immediately  so  long  as  the  mercu- 
ric nitrate  continues  to  react  with  the  sodium  chloride.  The 
moment  the  double  decomposition  is  complete,  however,  an 
additional  drop  of  mercuric-nitrate  solution  causes  a  permanent 
turbidity.  Hence,  if  the  volume  and  strength  of  the  mercuric- 
nitrate  solution  added  be  known,  the  chlorine  strength  of  the  salt 
solution  is  also  known,  since  1  eq.  of  mercury  in  the  mercuric 
solution  corresponds  to  2  eq.  of  chlorine. 

bb.  Preparation  of  the  mercuric-nitrate  solution.  As  this 
solution  must  be  perfectly  free  from  other  metals,  it  is  advisable 
to  prepare  it  from  mercury  oxide  obtained  by  precipitating  mer- 
curic chloride  with  soda  solution  and  thoroughly  washing  the 
precipitate.  10 '8  grin,  of  the  dried  oxide  so  obtained  are  dissolved 
in  nitric  acid,  the  solution  evaporated  to  a  syrupy  consistency, 
and  then  diluted  to  550  c.  c.  with  water.  The  solution  may  also 
be  made  by  dissolving  repeatedly  recrystallized  mercurous  nitrate 
in  water,  with  the  addition  of  some  nitric  acid,  boiling,  adding 
strong  nitric  acid  until  red  fumes  no  longer  are  evolved,  evapo- 
rating to  syrupy  consistency,  and  diluting  with  enough  water  to 
yield  a  solution  of  approximately  correct  strength. 

cc.  Determining  the  strength  of  the  solution.  This  is  effected 
by  means  of  a  sodium-chloride  solution  of  known  strength,  pre- 
pared according  to  LIEBIG  by  mixing  20  c.  c.  of  a  saturated  (at 


*  Annal.  d.  Chem.  u.  Pharm.,  LXXXV,  297. 


526  DETERMINATION.  [§  141* 

ordinary  temperatures)  solution  of  pure  rock  salt  or  chemically 
pure  sodium  chloride,  with  298 '4  c.  c.  water.  Every  c.  c.  of  the 
solution  will  contain  20  mg.  of  sodium  chloride. 

Of  this  solution  measure  10  c.  c.  into  a  beaker  and  add  3  c.  c. 
of  a  urea  solution  containing  4  grm.  in  every  100  c.  c. 

Drop  the  mercury  solution  to  be  standardized  from  a  burette 
into  the  mixture,  with  shaking,  until  a  just  perceptible  precipitate, 
which  fails  to  dissolve  on  shaking,  forms.* 

dd.  Having  thus  ascertained  how  many  c.  c.  of  mercuric- 
nitrate  solution  are  equivalent  to  10  c.  c.  of  sodium-chloride  solu- 
tion (=0*2  grm.  NaCl),  the  mercuric  solution  is  applicable 
for  immediate  use,  if  a  little  calculation  is  not  objected  to.  If 
this  is  rather  avoided,  dilute  the  mercuric  solution  so  that  every 
c.  c.  may  correspond  to  a  given  number  of  milligrammes  of 
sodium  chloride  or  chlorine.  LIEBIG  dilutes  the  solution  so  that 
1  c.  c.  corresponds  to  0*01  grm.  of  sodium  chloride. 

ee.  If  the  test-fluid  is  to  be  used  for  testing  solutions  which 
contain  much  foreign  salts  or  an  excess  of  urea,  add  to  10  c.  c.  of 
the  sodium- chloride  solution  3  c.  c.  of  the  urea  solution  and  also 
5  c.  c.  of  a  cold  saturated  sodium-sulphate  solution  before  drop- 
ping in  the  mercuric  solution  f .  Results  accurate. 

d.  Alkalimetrically  (according  to  BOHLJG  £).  Add  to  the 
solution,  if  necessary,  potassium  carbonate  in  not  too  great  excess, 
to  precipitate  the  alkali  earths,  earths,  or  metallic  oxides,  dilute  to 
250  c.  c.,  mix,  filter,  and  determine  the  alkalinity  of  50  c.  c.  of  the 
filtrate  according  to  §  220.  To  125  c.  c.  of  the  filtrate  in  a  250- 
c.  c.  flask  add  an  excess  of  pure  silver  oxide,  fill  to  the  mark  with 
water  and  shake  repeatedly,  with  exclusion  of  light.  After  a  few 
minutes  filter  through  a  dry  folded  filter,  pipette  oft*  100  c.  c. 
of  the  filtrate  (corresponding  to  50  c.  c.  of  the  original  liquid),, 
and  determine  its  alkalinity  also.  The  difference  in  the  c.  c.  of 

*  A  mere  opalescence  of  the  fluid  is  to  be  disregarded,  as  this  depends  upon 
a  trace  of  foreign  metals,  and  has  no  bearing  on  the  reaction,  as  may  be  readily 
seen  from  the  fact  that  the  cloudiness  is  not  increased  by  a  further  addition  of 
the  mercuric  solution. 

t  The  reason  for  this  addition  is  that  the  mercuric  nitrate  and  urea  are  more 
readily  soluble  in  pure  water  than  in  saline  solution,  hence  the  solvent  powers  oi 
the  solutions  should  be  as  nearly  alike  as  possible  when  standardizing  and  wh^n> 
performing  the  analysis,  if  accurate  results  are  desired. 

\Zeitschr.f.  analyt.  Chem.,  ix,  314 


§  141.]  CHLOKINE.  527 

standard  acid  used  in  the  two  determinations  of  alkalinity  cor- 
responds to  the  chlorine  content  of  the  solution.  The  result  is 
naturally  correct  only  when  another  portion  of  the  filtrate  has 
been  tested  and  found  free  from  chlorine.  BOHLIG'S  method  is 
particularly  well  adapted  for  technical  purposes. 

Of  these  volumetric  methods  of  estimating  chlorine,  the  first 
deserves  the  preference  in  all  ordinary  cases.  It  is  not,  however, 
applicable  in  urinalysis,  because  compounds  of  the  silver  oxide, 
with  coloring  matters,  etc. ,  are  precipitated  with  the  silver  chloride 
(C.  NEUBAUEK*).  PISANI'S  method  (b,  ft)  is  especially  suited  for 
the  estimation  of  very  minute  quantities  of  chlorine,  but  is  not 
applicable  when,  as  in  nitre  analyses,  large  quantities  of  alkaline 
nitrate  are  present  (p.  344). 

II.  Separation  of  Chlorine  from  the  Metals, 
a.  In  Soluble  Chlorides. 

The  same  method  as  in  I,  a.  The  metals  in  the  filtrate  are 
separated  from  the  excess  of  the  salt  of  silver  by  the  methods 
which  will  be  found  in  Section  Y.  Chlorides  soluble  in  water  may 
also  be  completely  decomposed  by  cold  digestion  with  oxide  or 
carbonate  of  silver.  Silver  chloride  is  obtained,  while  the  metal 
combined  with  the  chlorine  is  converted  into  oxide  or  carbonate 
and  either  remains  in  solution  or  falls  down  with  the  silver  chlo- 
ride. Take  care  that  no  traces  of  oxide  or  carbonate  of  silver  pa~*$ 
into  the  filtrate. 

Stannous  chloride,  mercuric  chloride,  platinic  chloride,  the 
chlorides  of  antimony,  and  the  green  chloride  of  chromium,  form 
exceptions  from  the  rule. 

a.  From  stannic  chloride,  silver  nitrate  would  precipitate, 
besides  silver  chloride,  a  compound  of  stannic  oxide  and  silver 
oxide.  To  precipitate  the  tin,  therefore,  the  solution  is  mixed  with 
concentrated  solution  of  ammonium  nitrate,  boiled,  allowed  to 
deposit,  decanted,  and  filtered  (compare  §  126,  1,  J),  and  the  chlo- 
rine in  the  filtrate  is  precipitated  with  solution  of  silver.  LOWEN- 
THAL,  the  inventor  of  this  method,  has  proved  its  accuracy. f 

*  To  apply  this  method  to  urine  also,  R.  PRIBRAM  (Zeitschr.  f.  analyt.  Chem., 
ix,  428)  heats  10  c.  c.  of  urine  with  50  c.  c.  of  a  solution  of  pure  potassium 
permanganate  (1  or  2  : 1,000)  to- gentle  boiling,  filters  off  the  brown  flocks,  washes 
these,  and  determines  the  chlorine  in  the  filtrate  according  to  b,  a. 

\Journ.  /.  prakt.  Chem.,  LXVJ,  371. 


528  DETERMINATION.  [§  141. 

ft.  When  'mercuric  chloride  is  precipitated  with  solution  of 
silver  nitrate,  the  silver  chloride  thrown  down  contains  an  admix- 
ture of  mercury.  The  mercury  is,  therefore,  first  precipitated  by 
hydrogen  sulphide,  and  the  chlorine  in  the  filtrate  determined  as 
directed  in  §  169. 

•  y.  The  chlorides  of  antimony  are  also  decomposed  in  the  man- 
ner described  in  ft.  The  separation  of  basic  salt  upon  the  addi- 
tion of  water  may  be  avoided  by  addition  of  tartaric  acid.  The 
antimonous  sulphide  should  be  tested  for  chlorine. 

$.  Solution  of  silver  fails  to  precipitate  the  whole  of  the  chlo- 
rine from  solution  of  the  green  chloride  of  chromium  (PELIGOT). 
The  chromium  is,  therefore,  first  precipitated  with  ammonia,  the 
fluid  filtered,  and  the  chlorine  in  the  filtrate  precipitated  as  in  I.,  a. 

e.  From  platinic  chloride  silver  nitrate  throws  down  a  com-, 
pound  of  platinons  chloride  and  silver  chloride  (COMAILLE  *).  We 
may  either  ignite  the  platinic  chloride  in  a  current  of  hydrogen 
and  pass  the  hydrochloric  acid  produced  into  solution  of  silver 
(BONSDORFF),  or  we  may  evaporate  the  solution  with  sodium  car- 
bonate, fuse  the  residue  in  a  platinum  crucible,  and  determine  the 
chloride  in  the  aqueous  solution  of  the  fusion.  Or,  thirdly,  we 
may  (after  TOPSOE  f)  digest  the  moderately  dilute  solution  in  the 
cold  with  zinc  clippings  till  hydrogen  ceases  to  escape,  add  ammo- 
nia in  excess,  heat  on  a  water-bath  till  the  fluid  is  fully  decolorized, 
all  the  platinum  being  precipitated,  and  finally  determine  the  chlo- 
rine in  the  filtrate. 

&.  In  Insoluble  Chlorides. 

a.  Chlorides  soluble  in  Nitric  A.cid. 

Dissolve  the  chloride  in  nitric  acid,  without  applying  heat,  and 
proceed  as  in  I.,  a. 

ft.    Chlorides  insoluble  in  Nitric  Acid  (lead  chloride, 
silver  chloride,  mercurous  chloride). 

aa.  Lead  chloride  is  decomposed  by  digestion  with  alkali 
hydrogen  carbonate  and  water.  The  process  is  exactly  the  same 
as  for  the  decomposition  of  lead  sulphate  (§  132,  II.,  &,  ft). 

~bb.  Silver  chloride  is  ignited  in  a  porcelain  crucible,  with  3 
parts  of  sodium  and  potassium  carbonate,  until  the  mass  com- 
mences to  agglutinate.  Upon  treating  with  water,  the  metallic 

*  Zeitschr.f.  analyt.  Chem.,  vi,,121.         *  f7&.,  ix,  30. 


§  142.]  CHLORINE.  529 

silver  is  left  undissolved ;  the  solution  contains  the  alkali  chloride, 
which  is  then  treated  as  in  I.,  a. 

Silver  chloride  may  also  be  readily  decomposed  by  long  diges- 
tion with  pure  iron  (reduced  by  hydrogen)  and  dilute  sulphuric 
acid.  Zinc  may  be  used  instead  of  iron,  but  it  does  not  answer  so 
well.  The  separated  metallic  silver  may  be  washed,  heated  with 
dilute  sulphuric  acid,  washed  again  and  weighed ;  it  must  after- 
wards be  ascertained,  however,  whether  it  dissolves  in  nitric  acid. 
The  chlorine  is  determined  in  the  chloride  of  iron  or  zinc  as  in 
L,a. 

cc.  Mercurous  chloride  is  decomposed  by  digestion  with  solu- 
tion of  soda  or  potassa.  The  hydrochloric  acid  in  the  filtrate  is 
determined  as  in  I.,  a.  The  mercurous  oxide  is  dissolved  in  nitric 
or  nitrohydrochloric  acid,  and  the  mercury  determined  as  directed 
in  §  117  or  §  118. 

c.  The  soluble  chlorides  of  the  metals  of  the  fourth,  fifth,  and 
mxth  groups  may  generally  be  decomposed  also  by  hydrogen  sul- 
phide or  ammonium  sulphide.     The   chlorine   in  the  filtrate  is 
determined  as  in  §  169.     It  must  not  be  omitted  to  test  the  pre- 
cipitated sulphides  for  chlorine.     Several  chlorides,  cadmium  chlo- 
ride for  instance,  give  sulphides  free  from  chlorine  with  ammonium 
sulphide,  but  not  with  hydrogen  sulphide. 

d.  In  many  metallic  chlorides,  for  instance  in  those  of  the  first 
and  second  groups,  the  chlorine  may  be  determined  also  by  evapo- 
rating with  sulphuric  acid,  converting  the  metal  thus  into  a  sul- 
phate, which  is  then  ignited  and  weighed  as  such ;  the  chlorine 
being  calculated  from  the  loss.     This  method  is  not  applicable  in 
the  case  of  silver  chloride  and  lead  chloride,  which  are  only  imper- 
fectly and  with  difficulty  decomposed  by  sulphuric  acid ;  nor  in 
the  case  of  mercuric  chloride  and  stannic  chloride,  which  sulphuric 
acid  fails  almost  or  altogether  to  decompose. 

Supplement. 

§142. 
DETERMINATION  OF  CHLORINE  IN  THE  FEEE  STATE. 

Chlorine  in  the  free  state  may  be  determined  both  in  the  volu- 
metric and  in  the  gravimetric  way.  The  volumetric  methods, 
however,  deserve  the  preference  in  most  cases.  They  are  very 
numerous. 


530 


DETERMINATION. 


[§ 


I  shall  only  here  adduce  that  one  which  is  undoubtedly  the 
most  accurate  and  at  the  same  time  the  most  convenient.* 

1.  Volumetric  Method. 

With  Potassium  Iodide  (after  BUNSEN). 

Bring  the  chlorine,  in  the  gaseous  form  or  in  aqueous  solution^ 
into  contact  with  an  excess  of  solution  of  potassium  iodide  in  water. 
Each  at.  chlorine  liberates  1  at.  iodine,  which  remains  dissolved  in 
the  excess  of  potassium  iodide.  By  determining  the  liberated  iodine 
by  means  of  sodium  thiosulphate  as  in  §  146,  you  will  accordingly 
learn  the  quantity  of  chlorine,  and,  in  fact,  with  the  greatest  accuracy. 
If  you  have  to  determine  the  chlorine  of  chlorine  water,  measure 
a  portion  off  with  a  pipette.  So  as  to  prevent  any  of  the  gas 
entering  the  mouth,  connect  the  upper  end  of  the  pipette  with  a 
tube  containing  moist  potassium  hydroxide  laid  between  cotton. 
When  the  pipette  has  been  correctly  filled  allow  its  contents  to  flow, 
with  stirring,  into  an  excess  of  solution  of  potassium  iodide  (1  in  10). 
There  is  no  difficulty  about  knowing  whether  the  latter  is  sufficiently 
in  excess,  for  if  not,  a  black  precipitate  is  formed.  If  the  chlorine 
is  evolved  in  the  gaseous  condition,  you  may  employ  either  the 
apparatus  given  in  §  130,  I,  e^  /?,  or  the  following,  Fig.  103, 


Pig.  103. 

which  is  especially  suitable  where  the  chlorine  is  not  pure,  but 
is  mixed  with  other  gases. 

*  Compare  "  Chlorimctry  "  in  the  [Special  Part. 


§  142.]  CHLORINE.  531 

a  is  a  little  flask,  from  which  the  chlorine  is  evolved  by  boiling 
the  substance  with  hydrochloric  acid,  a  small  lump  of  magnesite 
being  added  ;  it  is  connected  with  the  tube,  b,  by  means  of  a  flexible 
tube.  The  latter  must  be  free  from  sulphur — should  it  contain 
sulphur  it  is  well  boiled  with  dilute  potassa  and  then  thoroughly 
washed.  The  thinner  tube,  c,  which  has  been  fused  to  the  bulb  of  b, 
leads  through  the  caoutchouc  stopper  (which  has  been  deprived  of  sul- 
phur) to  the  bulbed  U-tube,  d,  which  contains  solution  of  potassium 
iodide,  and  which  for  safety  is  connected  with  the  plain  U-tube,  e,  also 
containing  potassium  iodide  solution.  Both  tubes  stand  in  a  beaker 
filled  with  water.  The  apparatus  offers  the  advantages  that  the 
fluid  cannot  return,  that  the  potassium  iodide  remains  cold,  and 
that  the  absorption  is  complete.  After  all  the  chlorine  has  been 
expelled  by  boiling  long  enough,  rinse  d  and  e  out  into  a  beaker  and 
titrate  with  standard  sodium  thiosulphate  (§  146). 

2.   Gravimetric  Method. 

The  fluid  under  examination,  which  must  be  free  from  sulphu- 
ric acid,  say,  for  instance,  30  grin,  chlorine  water,  is  mixed  in  a  stop- 
pered bottle,  with  a  slight  excess  of  sodium  thiosulphate,  say  0*5 
grin.,  the  stopper  inserted,  and  the  bottle  kept  for  a  short  time  in  a 
warm  place ;  after  which  the  odor  of  chlorine  is  found  to  have 
gone  off.  The  mixture  is  then  heated  to  boiling  with  some  hydro- 
chloric acid  in  excess,  to  destroy  the  excess  of  sodium  thiosulphate, 
filtered,  and  the  sulphuric  acid  in  the  filtrate  determined  by  barium 
chloride  (§  132).  1  mol.  sulphuric  acid  corresponds  to  4  at.  chlorine 

(WlCKE*). 

In  fluids  containing,  'besides  free  chlorine,  also  hydrochloric  acid, 
or  a  metallic  chloride,  the  chlorine  existing  in  a  state  of  combination 
may  be  determined,  in  presence  of  the  free  chlorine,  in  the  follow- 
ing way : 

A  weighed  portion  of  the  fluid  is  mixed  with  solution  of  sulphur- 
ous acid  in  excess,  after  some  time  nitric  acid  is  added,  and  then  potas- 
sium chromate  to  destroy  the  excess  of  sulphurous  acid,  and  the 
whole  of  the  chlorine  is  precipitated  as  silver  chloride.  The  quantity 
of  the  free  chlorine  is  then  determined  in  another  weighed  portion, 
by  means  of  potassium  iodide ;  the  difference  gives  the  amount  of 
combined  chlorine. f 

*  Annal.  d.  Chem.  u.  Pharm.,  xcix,  99. 

f  If  chlorine  water  is  mixed  at  once  with  silver  nitrate,  |  only  of  the  chlorine 


532  DETERMINATION.  [§  143. 

Having  thus  seen  in  how  simple  and  accurate  a  manner  the 
quantity  of  free  chlorine  may  be  determined  by  BTJNSEN'S  method, 
it  will  be  readily  understood  that  all  oxides  and  peroxides  which 
yield  chlorine  when  heated  with  hydrochloric  acid,  may  be  analyzed 
by  heating  them  with  concentrated  hydrochloric  acid,  with  addition 
of  a  small  lump  of  magnesite,  and  determining  the  amount  of 
chlorine  evolved.  As  regards  the  procedure,  compare  §  142,  1. 

§  143. 
2.  BROMINE. 

I.  Determination. 

a.  Gravimetric  Methods. 

Estimation  as  silver  bromide.  Free  hydrobromic  acid — in  & 
solution  free  from  hydrochloric  acid  or  chlorides — is  precipitated 
by  silver  solution,  and  the  further  process  is  conducted  as  in  the 
case  of  hydrochloric  acid  (§  141).  For  the  properties  of  silver  bro- 
mide, see  §  94,  2.  The  results  are  perfectly  accurate. 

b.  Volumetric  Methods. 

Like  chlorine  in  hydrochloric  acid  and  alkali  chlorides,  bromine 
may  be  estimated  in  the  analogous  compounds  by  standard  silver 
solution  (§  141, 1.,  b,  <*),  by  solution  of  silver  and  iodide  of  starch 
(§  141,  I.,  5,  /?),  and  also  alkalimetrically  (§  141,  I.,  6,  6).  But 
these  methods  are  seldom  applicable,  as  they  cannot  be  used  in  the 
presence  of  hydrochloric  acid  and  metallic  chlorides. 

The  following  methods  must  therefore  be  detailed ;  they  are 
especially  useful  for  the  estimation  of  small  quantities  of  bromine 
in  solutions  containing  chlorides,  but  in  point  of  accuracy  they 
leave  much  to  be  desired.* 

of.  With  chlorine  water  and  chloroform  (after  A.  REiMANxf). 
This  method  depends  on  the  facts  that  chlorine,  when  added  to 
bromides  first  liberates  the  bromine  and  then  combines  with  it,  and 
that  bromine  colors  chloroform  yellow  or  orange,  while  bromine 

is  obtained  as  silver  chloride  :  6C1  -f  3Ag2O  —  5AgCl  +  AgClO3  (H.  ROSE, 
WELTZIEN,  Annal.  d.  Chem.  u.  Pharm.,  xci,  45).  If  chlorine  water  is  mixed 
with  ammonia  in  excess,  there  are  formed  at  first  ammonium  chloride  and  am- 
monium hypochlorite;  the  latter  then  gradually  decomposes  into  nitrogen  and 
ammonium  chloride.  However,  a  little  ammonium  chlorate  is  also  formed  be- 
sides (SciioNBEiN,  Journ.  /.  prakt.  Cliem.,  LXXXIV,  386  ;  Zeitschr.  /.  analyt. 
Chem.,  u,  59). 

*  Compare  §  169.  f  Annal.  a.  Onem.  u.  Pharm.,  cxv,  140. 


§  143.]  BROMINE.  533 

chloride  merely  communicates  a  yellowish  tinge  to  that  fluid.  The 
process  is  as  follows :  Mix  the  liquid  containing  a  bromide  of  an 
alkali  metal  in  neutral  solution,  in  a  stoppered  bottle  with  a  drop 
of  pure  chloroform  about  the  size  of  a  hazel-nut,  then  add  standard 
chlorine  water  from  a  burette,  protected  from  the  light  by  being 
surrounded  with  black  paper.  On  shaking,  the  chloroform  becomes 
yellow,  on  further  addition  of  chlorine  water,  orange,  then  yellow 
again,  and  lastly — at  the  moment  when  2  at.  chlorine  ha've  been 
used  for  1  at.  bromine — yellowish  white  (KBr  -f-  2C1  =  KC1  + 
BrCl).  Considerable  practice  and  skill  are  required  before  the 
operator  can  tell  the  end-reaction.  He  will  be  assisted  by  placing 
the  bottle  on  white  paper  and  comparing  the  color  of  the  chloro- 
form with  that  of  a  dilute  solution  of  yellow  potassium  chromate 
of  the  required  color.  The  strength  of  the  chlorine  water  should 
depend  on  the  amount  of  the  bromine  to  be  determined.  It  should 
be  so  adjusted  that  about  100  c.c.  may  be  used.  The  chlorine  water 
is  standardized  with  potassium  iodide  and  sodium  thiosulphate 
(§  142,  1).  The  method  is  especially  suited  for  the  determination 
of  small  quantities  of  bromine  in  mother  liquors,  kelp,  &c.  The 
results  are  approximate:  e.g.,  0*018  instead  of  0*0185 — 0*055 
instead  of  0*059—0*0112  instead  of  0*01,  &c.  If  the  fluid  con- 
tains organic  substances,  it  is — after  being  rendered  alkaline  with 
caustic  soda — evaporated  to  dryness,  the  residue  ignited  in  a  silver 
dish,  extracted  with  water,  the  solution  neutralized  exactly  with 
hydrochloric  acid,  and  then  tested. 

/3.  With  cMorine  water  and  heat  (after  FIGUIER*). 

The  principles  underlying  this  method  are  that  in  a  solution 
of  a  metallic  bromide,  1  eq.  of  bromine  is  liberated  by  1  eq.  of 
chlorine,  and  that  bromine  gives  an  aqueous  yellow  solution  from 
which  it  readily  escapes  on  boiling  and  leaves  behind  a  colorless 
liquid . 

The  chlorine  is  used  in  the  form  of  a  dilute  solution.  It  must 
be  standardized  immediately  before  use,  its  strength  being  deter- 
mined by  its  action  on  a  sodium-bromide  solution  of  known 
strength  acidulated  with  a  few  drops  of  hydrochloric  acid  (or  more 
simply  by  testing  with  potassium  iodide  and  sodium  thiosulphate 
according  to  §  142,  1).  The  mother-liquor  is  heated  almost  to, 

*  Annal.  d.  Chem.  et  de  Pkys.,  xxxm,  303;  Journ.  f.  prakt.  Chem.,  LIV,  293. 
Proposed  for  the  determination  of  bromine  in  mother-liquors. 


534  DETERMINATION.  [§  143. 

boiling  in  a  flask,  then  the  chlorine  water  is  allowed  to  flow  in 
from  the  burette  covered  with  black  paper,  and  the  mixture  heated 
for  about  three  minutes,  whereby  the  liquid  becomes  again  color- 
less. After  allowing  to  cool  for  two  minutes,  drop  some  more 
chlorine  water  into  the  mixture  and  continue  in  this  manner  until 
the  liquid  is  no  longer  colored  on  adding  chlorine  water.  If  the 
experiments  take  several  hours  to  carry  out,  titrate  the  chlorine 
w^ater  again  at  the  close  of  the  operations  and  base  the  calcula- 
tions on  the  mean  of  the  two  chlorine  determinations.  Alkaline 
fluids  should  be  acidulated  with  a  little  hydrochloric  acid.  Fer- 
rous and  manganous  salts,  iodine,  and  organic  substances  must  be 
absent.  Mother-liquors  colored  yellow  by  organic  matters  are 
best  evaporated  to  dry  ness,  gently  ignited,  arid  the  residue  treated 
with  water  and  filtered.  On  evaporating  the  solution  to  dryness, 
sodium  carbonate  must  be  added,  because  in  this  process  magnesium 
chloride  or  bromide  evolves  hydrochloric  or  hydrobromic  acid. 

According  to  my  investigations,  the  process  is  best  carried  out 
by  heating  the  mother-liquor  in  a  flask  the  stopper  of  which  has 
three  perforations.  Through  one  of  these  carbonic  acid  is  con- 
ducted nearly  to  the  bottom  of  the  flask  and  escapes,  together 
with  the  liberated  bromine,  through  another,  while  the  chlorine 
water  is  introduced  through  the  middle  perforation,  in  which  is 
inserted  the  somewhat  elongated  tip  of  the  burette.  The  process 
is  carried  out  while  the  liquid  is  kept  gently  boiling.  The  deter- 
minations may  in  this  manner  be  quite  rapidly  carried  out,  and 
then  afford  satisfactory  results. 

ft.  HEINE'S  colorimetric  method.*  The  bromine  is  liberated 
by  means  of  chlorine,  and  taken  up  with  ether;  the  solution  is 
compared,  with  respect  to  color,  with  an  ethereal  solution  of  bro- 
mine of  known  strength,  and  the  quantity  of  bromine  in  it  thus 
ascertained.  FEHLING  f  obtained  satisfactory  results  by  this  method. 
It  will  at  once  be  seen  that  the  amount  of  bromine  contained  in 
the  fluid  to  be  analyzed  must  be  known  in  some  measure  before 
this  method  can  be  resorted  to.  As  the  brine  examined  by  FEHL- 
ING could  contain  at  the  most  0*02  grm.  bromine  in  60  grm.,  he 
prepared  ten  different  test  fluids  by  adding  to  ten  several  portions 
of  60  grm.  each  of  a  saturated  solution  of  common  salt,  increasing 

*Journ.f.  prakt.  Chem.,  xxxvi,  184.     Proposed  to  effect  the  determination 
of  bromine  in  mother- liquors.  \Journ.f.  prakt.  Chem.,  XLV,  269. 


§  143.J  BROMINE.  535 

quantities  of  potassium  bromide,  containing  respectively  from  0*002 
grm.  to  0-02  grm.  bromine.  He  added  an  equal  volume  of  ether 
to  the  test  fluids,  and  then  chlorine  water,  until  there  was  no  fur- 
ther darkening  observed  in  the  color  of  the  ether.  It  being  of  the 
highest  importance  to  hit  this  point  exactly,  since  too  little  as  well 
as  too  much  chlorine  makes  the  color  appear  lighter,  FEHLING  pre- 
pared three  samples  of  each  test  fluid,  and  then  chose  the  darkest 
of  them  for  the  comparison.  60  grm.  are  now  taken  *  of  the 
mother  liquor  to  be  examined,  the  same  volume-  of  ether  added  as 
was  added  to  the  test  fluids,  and  then  chlorine  water.  Every 
experiment  is  repeated  several  times.  Direct  sunlight  must  be 
avoided,  and  the  operation  conducted  with  proper  expedition.  In 
my  opinion  it  is  well  to  replace  the  ether  by  chloroform  or  car- 
bon disulphide.  CAIGNET  f  substituted  sodium  hypochlorite  for  the 
chlorine  water,  and  removed  the  colored  carbon  disulphide  from 
time  to  time. 

II.  Separation  of  Bromine  from,  the  Metals. 
The  metallic  bromides  are  analyzed  exactly  like  the  correspond- 
ing chlorides  (§  141*  II.,  a  to  d)9  the  whole  of  these  methods  being 
applicable  to  bromides  as  well  as  chlorides.  In  the  decomposition 
of  bromides  by  sulphuric  acid  (§  141,  II.,  $),  porcelain  crucibles 
must  be  used  instead  of  platinum  ones,  as  the  latter  would  be 
attacked  by  the  liberated  bromine.  Some  bromides,  it  must  be 
remembered,  are  not  completely  decomposed  by  sulphuric  acid ; 
for  instance,  mercuric  bromide  is  not.  The  soluble  bromides  may 
be  converted  into  chlorides  by  evaporation  with  hydrochloric  acid 
and  excess  of  chlorine  water ;  but  this  process  cannot  be  applied 
where  the  chloride  is  liable  to  be  carried  away  with  the  steam ;  for 
instance,  in  the  case  of  mercuric  bromide. 


*  The  best  way  is  to  take  them  by  measure, 
f  ZeiUclir.  /.  analyt.  Chem.,  ix,  427. 


536  DETERMINATION.  [§§  144,  145. 

Supplement. 


DETERMINATION  OF  FREE  BROMINE. 

Free  bromine  in  aqueous  solution,  or  evolved  in  the  gaseous 
form,  is  caused  to  act  on  excess  of  solution  of  potassium  iodide. 
Each  at.  bromine  liberates  1  at.  iodine,  which  is  most  conveniently 
determined  by  means  of  sodium  thiosulphate  (§  146).  As  regards 
the  best  mode  of  bringing  about  the  action  of  the  bromine  on  the 
potassium  iodide,  compare  §  142,  1. 

The  determination  of  free  bromine  in  presence  of  hydrobromie 
acid  or  metallic  bromides  is  effected  in  the  same  manner  as  that  of 
free  chlorine  in  presence  of  hydrochloric  acid  (see  §  142). 

§145. 
3.  IODINE. 

I.  Determination* 

a.  Gravimetric  Methods. 

a.  Estimation  as  •  silver  iodide.  If  you  have  hydriodic  acid  in 
solution,  free  from  hydrochloric  and  hydrobromie  acids,  precipitate 
with  silver  nitrate,  and  proceed  exactly  as  with  hydrochloric  acid 
(§  141).  If  the  solution  is  colored  with  free  iodine,  first  add 
sulphurous  acid  cautiously  till  the  color  is  removed.  The  particles 
of  silver  iodide  adhering  to  the  filter  are  not  reduced  on  incinera- 
tion, but  a  little  of  the  iodide  is  liable  to  volatilize  if  the  heat  is 
too  high.  Hence  the  filter  should  be  got  as  clean  as  possible,  and 
the  heat  during  incineration  should  not  be  unduly  raised.  For  the 
properties  of  silver  iodide,  see  §  94,  3.  The  results  are  perfectly 
accurate. 

ft.  Estimation  as  palladious  iodide.  The  following  method, 
recommended  first  by  LASSAIGNE,  is  resorted  to  exclusively  to  effect 
the  separation  of  iodine  from  chlorine  and  bromine,  for  which  pur- 
pose it  is  extremely  well  adapted.  The  solution  may  not  contain 
any  alcohol.  Acidify  it  slightly  with  hydrochloric  acid,  and  add  a 
solution  of  palladious  chloride,  as  long  as  a  precipitate  forms  ;  let 

*  For  the  methods  to  be  adopted  in  the  presence  of  bromine  and  chlorine,, 
see  §  169. 


§  145.]  IODINE.  537 

the  mixture  stand  from  2-t  to  48  hours  in  a  warm  place,  filter  the 
brownish-black  precipitate  off  on  a  weighed  filter,  wash  with  warm 
water,  and  dry  at  100°,  until  the  weight  remains  constant.  For 
the  properties  of  the  precipitate,  see  §  94,  3.  This  method  gives 
very  accurate  results.  Instead  of  simply  drying  the  palladious 
iodide,  and  weighing  it  in  that  form,  you  may  ignite  it  in  a  current 
of  hydrogen  in  a  crucible  of  porcelain  or  platinum,*  and  calculate 
the  iodine  from  the  residuary  palladium  (H.  ROSE).  Compare 
§  122,  1. 

b.  Volumetric  Methods. 

a.  The  methods  given  for  hydrochloric  acid  by  precipitating 
with  silver  solution  (§  141,  I.,  b,  a);  by  silver  solution  and 
iodide  of  starch  (§141,  I.,  6,  /?),  and  also  alkalimetrically  (§  141,, 
I.,  6,  tf),  may  be  used  for  hydriodic  acid  and  alkali  iodides;  the 
absence  of  chlorine  and  bromine  being  of  course  presupposed. 

j3.  With  nitrous  acid  and  carbon  disidphide.  This  excellent 
method  has  been  in  frequent  use  in  my  laboratory  for  a  length  of 
time ;  it  may  be  used  for  small  or  large  quantities  of  iodine.  We 
require : 

aa.  Solution  of  potassium  iodide  of  known  strength.  Made  by 
drying  the  pure  salt  at  180°  (see  p.  149)  and  dissolving  an  exactly 
weighed  quantity  (about  5  grm.)  to  1  litre. 

bb.  Solution  of  sodium  thiosulphate  containing  about  13  or  13'5 
grm.  of  the  pure  crystallized  salt  in  1  litre. 

cc.  Solution  of  nitrous  acid  in  sulphuric  acid.  Prepared  by 
passing  nitrous  acid  gas  into  sulphuric  acid  to  saturation. 

dd.  Pure  carbon  disulphide. 

ee.  Solution  of  sodium  hydrogen  carbonate.  Made  by  dissolv- 
ing 5  grm.  in  1000  c.c.  cold  water  and  adding  1  c.c.  of  hydrochloric 
acid  to  the  solution. 

Begin  by  standardizing  the  thiosulphate  as  follows:  Take  a 
well-stoppered  bottle  of  about  400  c.c.  capacity,  transfer  to  it  50 
c.c.  of  the  potassium  iodide  solution,  add  about  150  c.c.  water,  20 
c.c.  carbon  disulphide,  some  dilute  sulphuric  acid,  and  10  drops  of 
the  solution  of  nitrous  acid  in  sulphuric  acid.  Insert  the  stopper 
and  shake  the  bottle  violently  for  some  time,  allow  to  settle,  and 
ascertain  by  adding  a  few  more  drops  of  the  nitrous  acid  that  the 
whole  of  the  iodine  has  been  liberated.  Shake  again,  allow  to- 

*  This  substance  is  not  injured  by  the  operation. 


538  DETERMINATION.  [§  145. 

settle,  and  pour  the  supernatant  fluid  as  completely  as  possible 
into  a  flask,  leaving  the  carbon  disulphide  in  the  bottle,  add  200 
c.c.  water  to  the  latter,  shake  well,  pour  off  the  water  into  the  flask 
and  repeat  the  washing  till  the  last  water  has  no  acid  reaction.  To 
the  contents  of  the  flask  add  10  c.c.  carbon  disulphide,  shake  well, 
pour  off  into  a  second  flask,  wrash  the  disulphide  a  little,  and  finally 
shake  the  contents  of  the  second  flask  again  with  some  fresh  disul- 
phide, which  should  now  be  barely  tinged.  Collect  the  disulphide 
from  both  flasks  on  a  filter  moistened  with  water,  wash  it  till  the 
washings  are  no  longer  acid,  place  the  funnel  in  the  bottle  and 
pierce  the  point  of  the  filter  so  that  the  disulphide  from  all  the 
operations  may  be  mixed.  Add  30  c.c.  of  the  sodium  hydrogen 
carbonate  and  then  the  thiosulphate  from  a  burette,  with  continual 
shaking,  till  the  disulphide  has  lost  its  color.  The  number  of  c.c. 
of  thiosulphate  used  will  correspond  to  the  iodine  in  50  c.c.  of 
potassium  iodide  solution. 

The  analysis  is  performed  exactly  as  above.  The  thiosulphate 
requires  to  be  standardized  before  every  fresh  series  of  experi- 
ments, as  it  is  liable  to  slight  alteration.  The  presence  of  chlorides 
has  no  influence  whatever  on  the  results.  In  determining  minute 
quantities  of  iodine  let  the  solutions  be  ten  times  weaker,  and  use 
smaller  quantities  and  smaller  vessels. 

The  results  are  entirely  concordant  and  exact. 

y.    With  potassium  permanganate,  according  to  REINIGE.* 

This  method,  which  is  accurate  and  gives  good  results,  depends 
upon  the  fact  that  all  alkali  iodides  decompose  potassium-perman- 
ganate according  to  the  following  equation : 

KI  +  2KMnO4  =  KIO3  +  K2O  +  2MnOa. 

This  reaction  was  first  recommended  by  PEA-N  DE  SAINT-GILLES  t 
as  a  basis  for  the  volumetric  determination  of  iodine.  Boiling 
facilitates  the  decomposition ;  in  very  dilute  solutions  a  little 
alkali  carbonate  is  added  in  order  to  start  the  reaction.  Metallic 
chlorides  and  bromides  have  no  disturbing  influence  on  the  reaction, 
since  they  are  unaffected  by  the  permanganate. 

In  this  process  there  are  required  a  solution  of  potassium 
permanganate  (standardized  according  to  §  112,  2,  a,  or  by  a  po- 

* Zeitschr.  f.  analyt.  Chem.,  ix,  39.  f  Compt.  rend.,  XLVI,  624. 


§  145.]  IODINE.  539 

tassium  -iodide  solution  of  known  strength,  as  described  below) 
and  a  dilute  solution  of  sodium  thiosulphate,  each  solution  con- 
taining about  5  grm.  per  litre.  The  thiosulphate  solution  being 
used  to  determine  the  excess  of  permanganate,  it  must  therefore 
be  standardized  against  the  latter,  according  to  the  following 
reaction  : 


2KMn04  +  6Xa2S,03  =  2MnO,  +  3^aaS4O6  +  KSO  +  3]STaaO. 

To  effect  standardization,  measure  off  1  c.  c.  of  the  permanganate 
solution,  add  to  it  a  large  volume  of  water  and  a  few  drops  of  a 
sodium-carbonate  solution,  and  then  run  in  sodium-thiosulphate 
solution  until  the  red  color  just  disappears,  a  point  readily 
observed  in  dilute  solutions  notwithstanding  the  presence  of  the 
precipitated  hydrated  manganese  peroxide. 

After  having  added  a  little  potassium-  or  sodium  carbonate  to 
the  solution  to  be  analyzed,  and  containing  all  the  iodine  as  an 
alkali  iodide,  heat  to  gentle  boiling  and  gradually  add  perman- 
ganate solution  until  the  liquid  containing  the  suspended  precipi- 
tate of  hydrated  manganese  peroxide  acquires  a  decided  red  tint, 
which  it  retains  even  after  repeated  boiling.  To  better  observe  the 
color,  remove  the  heat  a  few  seconds  after  each  'ebullition  in  order 
to  allow  the  precipitate  to  settle.  Now  pour  the  whole  into  a  500- 
c.  c.  flask,  allow  to  cool,  fill  up  to  the  mark,  pipette  off  100  c.  c.  of 
the  liquid,  and  add  to  this  thiosulphate  solution  until  decoloriza- 
tion  ensues.  Multiply  the  number  of  c.  c.  required  for  this 
purpose  by  5,  calculate  from  this  the  equivalent  of  permanganate 
solution,  and  deduct  this  from  the  permanganate  solution  used  ;  the 
remainder  corresponds  to  the  metallic  iodide  decomposed,  accord- 
ing to  the  equation  given  above.  The  direct  titration  of  the  excess 
of  the  permanganate  in  the  liquid  containing  the  suspended 
hydrated  manganese  peroxide,  as  recommended  by  HEINIGE,  is 
less  easily  accomplished. 

It  need  scarcely  be  mentioned  that  organic  and  other  reducing 
substances  must  be  carefully  excluded. 

tf.    With  silver  solution  and  starch  iodide  (PISANI  *). 

For  this  process  there  are  required  a  titrated  decinormal  silver 
solution  (p.  522)  and  standardized  starch-iodide  solution  (p.  34-9). 

To  the  solution,  containing  the  iodine  as  an  alkali  iodide,  and 

*  Compt.  rend.,  XLIV,  352  ;  Journ.  f  prakt.  CTiem.,  LXXII,  266. 


540  DETERMINATION.  [§  145. 

which  must  be  neutral  or  faintly  acid,  add  first  a  little  pure, 
precipitated  calcium  carbonate,  then  0*5  to  1  c.  c.  of  the  starch- 
iodide  solution,  and  then  run  in  from  a  burette  the  silver  solution, 
with  constant  stirring,  until  the  starch  iodide  is  just  decolorized. 
The  volume  of  silver  solution  used,  after  deducting  the  small 
quantity  required  to  decolorize  the  0*5  or  1  c.  c.  starch-iodide 
solution  taken,  corresponds  to  the  iodine  content.  The  method 
depends,  as  seen,  upon  the  fact  that  silver  solution  decom- 
poses metallic  iodides  first,  then  starch  iodide,  and  lastly  any 
chloride  that  may  be  present.  The  process  is  rapid  and  gives 
good  results  in  the  absence  of  metallic  chlorides  and  bromides.  If 
but  little  chloride  is  present,  the  results  are  still  approximate,  but 
if  considerable  is  present,  the  results  are  altogether  unreliable,  as 
the  silver  chloride  precipitated  is  not  decomposed  with  sufficient 
rapidity  by  the  metallic  iodide  and  starch  iodide  present.  Metallic 
bromides  interfere  to  a  still  greater  extent  than  the  chlorides. 

e.  By  distillation  with  ferric  chloride  (DUFLOS).  When 
hydriodic  acid  or  a  metallic  iodide  is  heated  in  a  retort  with  solu- 
tion of  pure  ferric  chloride,  the  whole  of  the  iodine  escapes  with 
the  aqueous  vapor,  and  ferrous  chloride  is  formed  (FeaCl8  -f-  2HI  = 
2FeCl9-f-  2HC1  -|-  21).  The  iodine  passing  over  is  received  in  so- 
lution of  potassium  iodide  and  determined  by  sodium  thiosulphate, 
as  directed  in  §  146.  In  employing  this  method  it  must  be  borne 
in  mind  that  the  ferric  chloride  must  be  free  from  chlorine  and 
nitric  acid.  It  is  best  to  prepare  it  from  ferric  oxide  and  hydro- 
chloric acid.  We  must  not  forget  too  that  the  separated  iodine 
is  liable  to  act  on  cork  and  caoutchouc ;  the  apparatus  should 
therefore  be  constructed  according  to  Fig.  81. 

C.  KERSTING'S  method,*  depending  upon  precipitation  with 
palladious-chloride  solution  until  no  further  precipitate  forms, 
gives  good  results,  but  is  rather  inconvenient,  and  is  hence  but 
little  used.  The  same  may  be  said  of  the  method  devised  by  A. 
and  F.  DUPKE  f,  which  depends  upon  the  action  of  chlorine 
water  on  an  alkali  iodide.  This  method  gives  good  results  only 
when  metallic  chlorides  are  absent  \. 

*  Annal.  d.  CJiem.  u.  Pharm.,  LXXXVII,  25. 
f  76. ,  xciv,  365. 

j  H.  ROSE'S  Handbuch  der  analyt.  Chem.,  6.  Aufl.  von  FINKENEK,  n,  628  j 
also  my  own  experiments. 


§  145.]  IODINE.  541 

V.  II.  STRUVE'S*  colorimetric  method  may  be  used  in  many 
cases.  In  this  method  the  amount  of  iodine  is  estimated  by  the 
depth  of  color  which  the  separated  iodine  gives  to  a  measured 
quantity  of  carbon  disulphide. 

II.  Separation  of  Iodine  from  the  Metals. 
The  metallic  iodides  are  in  general  analyzed  like  the  corre- 
sponding chlorides.  From  iodides  of  the  alkali  metals  containing 
free  alkali  the  iodine  may  be  precipitated  as  silver  iodide,  by  first 
saturating  the  free  alkali  almost  completely  with  nitric  acid,  then 
adding  solution  of  silver  nitrate  in  excess,  and'  finally  nitric  acid  to 
strongly  acid  reaction.  If  an  excess  of  acid  were  added  at  the 
beginning,  free  iodine  might  separate,  which  is  not  converted  com- 
pletely into  silver  iodide  by  solution  of  silver  nitrate.  In  com- 
pounds soluble  in  water  the  iodine  may  generally  be  precipitated 
as  palladious  iodide ;  you  may  also  determine  the  base  in  one  por- 
tion (decomposing  the  compound  with  concentrated  sulphuric 
acid)  and  the  iodine  in  another  portion  according  to  §  145,  I., 

J,    6. 

Iodine  cannot  be  separated  from  platinum  directly  with  silver 
nitrate,  as  insoluble  platinum  salts  would  be  thrown  down  with  the 
silver  iodide.  For  this  purpose  H.  TOPSOE  f  recommends  the  fol- 
lowing process :  Dissolve  the  substance  in  a  good  amount  of  water, 
add  solution  of  sodium  hydrogen  sulphite  and  sulphurous  acid, 
heat  on  a  water-bath  till  the  cplor  has  entirely  disappeared,  and  the 
platinum  is  consequently  converted  into  platinous  sulphite.  In 
this  operation  a  white  flocculent  precipitate  of  sodium  platinous 
sulphite  which  is  difficultly  soluble  separates ;  it  redissolves  on 
addition  of  sulphurous  acid.  After  heating  on  the  water-bath  for 
some  time,  allow  to  cool  completely,  precipitate  with  silver  solu- 
tion, which  should  not  be  added  in  large  excess,  add  nitric  acid, 
heat  for  about  an  hour  to  redissolve  the  silver  sulphite  first  thrown 
down  with  the  iodide,  and  then  filter  off  the  latter.  Occasionally 
it  is  to  be  preferred  to  add  sulphurous  acid  instead  of  the  sulphite, 
and  then,  when  the  fluid  has  been  heated  and  the  color  has  gone, 
to  add  an  excess  of  ammonia.  In  this  way  the  platinum  compound 

*  Zeitschr.  /.  analyt.  Chem.,  vm,  230. 
f  lb.,  ix,  30. 


542  DETERMINATION.  [§  146. 

is  not  thrown  down,  and  the  silver  sulphite  does  not  separate  after 
the  addition  of  silver  solution  till  nitric  acid  is  added,  and  is  imme- 
diately redissolved  by  the  excess  of  the  same. 

For  the  analysis  of  insoluble  iodides,  especially  silver  and  lead 
iodides,  mercurous  and  cuprous  iodides,  E.  MEUSEL*  strongly 
recommends  sodium  thiosulphate,  in  which  these  salts  dissolve. 
Very  little  water  should  be  used,  and  as  small  a  quantity  of  the 
thiosulphate  as  possible.  The  metal  is  precipitated  from  the  solu- 
tion by  ammonium  sulphide  in  the  form  of  sulphide.  Evaporate 
the  filtrate  with  soda,  and  heat  the  residue  in  a  platinum  dish  to 
incipient  redness  to  destroy  sodium  thiosulphate  and  tetrathionate. 
Dissolve  the  melt  in  water  by  the  aid  of  heat,  and  determine  the 
iodine  in  it  by  §  145,  L,  J,  e.  A  large  quantity  of  ferric  chlo- 
ride will  be  required  to  decompose  the  sodium  sulphite ;  the  resi- 
due in  the  retort  should  have  a  deep  reddish-brown  color. 

Silver  iodide  may  be  decomposed  also  by  fusing  with  sodiunj 
carbonate,  but  not  by  igniting  in  a  current  of  hydrogen,  and  not 
completely  by  zinc  or  iron.  Mercurous  iodide  may  be  easily 
decomposed  by  distilling  with  8  or  10  parts  of  a  mixture  of  1  part 
potassium  cyanide  and  2  parts  quicklime.  For  the  apparatus, 
see  Fig.  88;  ab  is  filled  with  magnesite  (H.  RosEf).  Palladi- 
ous  iodide  may  be  decomposed  by  igniting  in  hydrogen.  Cuprous 
iodide  and  many  other  iodides  may  be  decomposed  by  boiling  with 
potassium  or  sodium  carbonate.  Portions  of  metal,  which  may 
pass  into  the  alkaline  solution,  may  be  thrown  down  by  ammonium 
sulphide,  or  by  acidifying  with  acetic  acid,  and  passing  hydrogen 
sulphide. 

Supplement. 

§  146. 
DETERMINATION  OF  FREE  IODINE. 

The  determination  of  free  iodine  is  an  operation  of  great  impor- 
tance in  analytical  chemistry,  since,  as  BUNSEN^:  first  pointed  out,  it 
is  a  means  for  the  estimation  of  all  those  substances  which,  when 
brought  in  contact  with  potassium  iodide,  separate  from  the  same 
a  definite  quantity  of  iodine  (e.g.,  chlorine,  bromine,  &c.),  or,  when 

*    Zeitschr.f.  analyt.  Chem.,  ix,  208.  .      f  2b.,  n,  1. 

J  Annal.  d.  Chem.  u.  Pharm. ,  LXXXVI,  265. 


§  146.]  IODINE.  543 

boiled  with  hydrochloric  acid,  yield  a  definite  quantity  of  chlorine 
(e.g.,  chromic  acid,  peroxide  of  manganese,  &c.).  By  causing  the 
chlorine  produced  to  act  on  potassium  iodide,  we  obtain  the  equiva- 
lent quantity  of  free  iodine. 

Of  the  various  methods  which  have  been  proposed  for  the  esti- 
mation of  free  iodine,  the  oldest  is  that  of  SCHWARZ.*  It  is  based 
upon  the  following  reaction  :  2Na2S3O3  +  21  =  2NaI  +  NaaS4O6. 
24-832  grm.  pure  crystallized  sodium  thiosulphate  are  dissolved 
to  1  litre.  1000  c.c.  of  the  solution  correspond  to  12 '685,  i.e., 
to  0*1  at.  iodine.  This  solution  is  added  to  the  solution  of  the 
substance  in  potassium  iodide  until  the  fluid  appears  bright  yel- 
low, 3  or  4  c.c.  thin  and  very  clear  starch-paste  are  then  added, 
which  must  produce  blue  coloration,  and  finally  again  sodium 
thiosulphate,  until  the  blue  fluid  is  decolorized. 

This  method,  though  in  itself  excellent,  is  open  to  this  objec- 
tion, that  it  is  difficult  to  obtain  a  solution  of  absolutely  exact  value 
by  weighing  off  sodium  thiosulphate,  as  the  salt  is  not  readily  pro- 
curable in  a  perfectly  pure  and  dry  condition,  and  although  the 
solution  does  not  change  rapidly  or  to  any  great  extent,  it  is  still 
liable  to  gradual  alteration,  especially  under  the  influence  of  light. 

BUNSEN'S  researches  on  the  volumetric  estimation  of  iodine 
cited  above  produced  a  very  important  and  beneficial  effect  on  the 
whole  domain  of  chemical  analysis.  His  process  depends  on  the 
fact  that  when  iodine  comes  in  contact  with  an  aqueous  solution 
of  sulphurous  acid,  a  decomposition  takes  place  in  accordance  with 
the  equation  H2SO3  +  H2O  +  21  =  H2SO4  +  2HI,  provided  the 
solution  does  not  contain  more  than  0*04  to  0*05  per  cent,  of  an- 
hydrous sulphurous  acid.  If  the  solution  is  more  concentrated, 
another  reaction  also  takes  place  to  a  greater  or  less  extent — 
namely,  HaSO4  +  2HI  =  H9SO,  +  H,O  +  21. 

In  this  method,  a  solution  of  iodine  in  potassium  iodide  con- 
taining a  known  quantity  of  free  iodine  is  employed,  and  we  com- 
mence by  determining  the  relation  between  it  and  a  sufficiently 
dilute  solution  of  sulphurous  acid.  In  applying  the  method,  the 
iodine  to  be. estimated  is  dissolved  in  potassium  iodide,  the  stand- 
ard sulphurous  acid  is  added  to  decoloration,  then  thin  starch-paste, 
and  finally  standard  iodine  solution  till  the  blue  color  of  iodide  of 
starch  is  just  visible. 

*Anleit.  zu  Maassamil.  Nachtrage,  1853,  22. 


544  DETERMINATION.  [§  146. 

We  calculate  now  the  c.c.  of  iodine  solution  which  correspond 
to  the  sulphurous  acid  employed,  and  deduct  therefrom  the  c.c.  of 
iodine  added  to  destroy  the  excess  of  sulphurous  &cid.  The 
remainder  gives  the  number  of  c.c.  of  iodine  solution  which 
contain  a  quantity  of  iodine  equal  to  that  in  the  substance  ana- 
lyzed. 

On  account  of  the  rapidity  with  which  solution  of  sulphurous 
acid  changes,  this  method  is  somewhat  inconvenient,  and  has  given 
place  to  the  following,  which  is  now  universally  employed.  It 
retains  the  basis  of  BUNSEN'S  method,  but  substitutes  sodium  thio- 
sulphate for  sulphurous  acid,  employing  the  reaction  of  SCHWARZ'S 
method.  With  F.  MOHR*  I  give  this  "  combined  method "  the 
preference,  because,  first,  we  are  not  bound  to  a  definite  strength 
of  the  thiosulphate  ;  secondly,  the  solution  of  thiosulphate  is  far  less 
.affected  by  the  oxygen  of  the  air  than  sulphurous  acid  ;  and  thirdly, 
it  loses  nothing  by  evaporation.  FINKENER")*  even  says,  that  the 
use  of  thiosulphate  makes  the  method  more  accurate,  his  experi- 
ments having  shown  that  in  using  BUNSEN'S  method  the  results 
differ  if,  on  one  occasion,  we  add  the  sulphurous  acid  to  the 
iodine,  and,  on  another,  the  iodine  to  the  sulphurous  acid. 

a.  REQUISITES  FOR  THE  COMBINED  METHOD. 

a.  Iodine  solution  of  known  strength.  Dissolve  6*2  to  6*3 
:grm.  iodine  with  the  aid  of  about  9  grm.  potassium  iodide  (free 
from  iodic  acid)  to  about  1200  c.c. 

/?.  Solution  of  sodium  thiosulphate.  Dissolve  12*2  to  12*3 
,grm.  of  the  pure  and  dry  salt  to  about  1200  c.c. 

y.  Solution  of  potassium  iodide.  Dissolve  1  part  of  the  salt 
(free  from  iodic  acid)  in  about  10  parts  of  water.  The  solution 
must  be  colorless  and  must  remain  so  immediately  after  the  addi- 
tion of  dilute  sulphuric  or  hydrochloric  acid  (either  must  be  iron- 
free). 

6.  Starch  solution.  Stir  the  purest  starch  powder  gradually 
with  about  100  parts  cold  water  and  heat  to  boiling  with  constant 
stirring.  Allow  to  cool  quietly,  and  pour  off  the  fluid  from  any 
deposit.  The  solution  should  be  almost  clear  and  free  from  all 
lumps.  The  starch  solution  is  best  prepared  fresh  before  each 
series  of  experiments. 

*  Lehrbuchti.  chem.-unalyt.  Titrirmethode,  3.  Aufl.,  256. 

f  H.  ROSE,  Handb.  d.  analyt.  Che?»>,  6.  Aufl.  von  FINKENER,  n,  937. 


§  146.]  IODINE.  545 

5.  PRELIMINARY  DETERMINATIONS. 

a.  Determination  of  the  relation  between  the  Iodine  Solution 
and  ThiosuipJiate  Solution. 

Fill  two  burettes  with  the  solutions.  Hun  20  c.  c.  of  the 
thiosulphate  into  a  beaker,  add  some  water  and  3  or  4  c.  c.  starch 
solution,  then  add  the  iodine  till  a  blue  coloration  is  just  pro- 
duced. If  you  have  added  a  drop  too  much,  run  in  one  or  two 
drops  more  of  the  thiosulphate  and  then  more  cautiously  the 
iodine  solution.  After  a  few  minutes  read  off  the  height  of  the 
fluid  in  both  burettes.  Let  us  suppose  we  had  used  20  c.  c.  thio- 
sulphate to  20-2  c.  c.  iodine. 

ft.  Exact  Determination  of  the  Iodine  in  the  Solution 

This  is  done  immediately  before  each  series  of  analyses  with 
the  aid  of  an  exactly  weighed  quantity  of  pure  and  dry  iodine. 
Experience  has  convinced  me  that  solution  of  iodine  in  potassium 
iodide,  even  when  kept  cool  and  in  the  dark,  is  much  more  liable 
to  change  than  is  usually  supposed.*  The  determination  is  best 
made  as  follows :  The  tubes  shown  in  Fig.  104  are  heated,  al- 
lowed to  cool  in  an  exsiccator,  and  then  weighed.  0*2 
grm.  of  pure,  resublimed  iodine  f  is  then  introduced  into 
the  inner  tube,  the  tube  placed  obliquely  in  a  small 
sand-bath,  heated  until  the  iodine  melts,  and  cooled 
in  a  very  oblique  position,  so  that  it  may  be  held  in 
the  hand.  Now  slip  on  the  outer  tube,  let  the  whole 
cool  in  an  exsiccator,  weigh,  and  thus  ascertain  the 
exact  quantity  of  iodine  in  the  tube.  Now  place  the 
inner  tube  (the  outer  tube  also,  of  course,  if  any  iodine 
adheres  to  it)  in  a  stoppered  bottle  containing  about 
10  c.  c.  potassium-iodide  solution.  As  soon  as  all  the 
iodine  is  dissolved  dilute  with  water,  run  in  sodiurn- 
thiosulphate  solution  from  a  burette  until  decolorization  is  just 
effected,  add  3  or  4  c.  c.  starch  paste,  and  then  iodine  solution 

*I  filled  several  small  well-stoppered  bottles  with  some  solution  of  iodine  in 
potassium  iodide,  the  standard  of  which  had  been  accurately  determined,  and 
placed  them  in.  a  cellar.  Even  in  the  course  of  a  few  weeks  the  standard  had 
altered.  I  now  never  rely  on  the  strength  of  a  solution  of  iodine  unless  I  have 
determined  it  shortly  before. 

f  Regarding  the  preparation  of  absolutely  pure  iodine,  compare  STAS,  ZeitscJif 
f.  analyt.  Chem.,  vi,  419. 


546  DETERMINATION.  [§  146. 

(«,  <*)  until  a  blue  tinge  appears.     Read  off  botli  burettes  and 
calculate  the  iodine  content  of  the  solution  0,  a  as  follows : 

Suppose  we  had  weighed  off  0*15  grm.  iodine  and  used  29 '5 
c.  c.  thiosulphate  and  0'3  c.  c.  iodine  solution. 

From  5,  a,  we  know  that  20  c.  c.  thiosulphate  correspond  to 
20'2  c.  c.  iodine  solution;  29'5  c.  c.  therefore  correspond  to 
29-8  c.  c0 

Now  29-5  c.  c.  thiosulphate  correspond  to  0*15  grm.  iodine 
-\-  0*3  c.  c.  iodine  solution. 

But  29 '5  c.  c.  thiosulphate  also  correspond  to  29*8  c.  c.  iodine 
solution. 

.*.  0*15  grm.  iodine  -f-  0-3  c.  c.  iodine  solution  =  29*8  c.  c. 
iodine  solution. 

.'.   0'15  grm.  iodine  =  29 -5  c.  c.  iodine  solution. 

.•.   1  c.  c.  iodine  solution  =  0-0050847  grm.  iodine. 

The  experiment  just  described  is  repeated  and  the  mean  of 
the  two  results  taken,  provided  they  exhibit  sufficient  uniformity. 

Where  tubes  are  not  at  hand  the  process  may  be  conducted 
in  the  following  manner :  Select  three  watch-glasses,  &,  5,  and  £, 
which  fit  each  other ;  weigh  b  and  c  together  accurately.  Put 
about  0'5  grm.  pure  dry  iodine  into  #,  place  it  on  an  iron  plate, 
and  heat  gently  till  dense  fumes  of  iodine  escape.  Now  cover  it 
with  5  and  regulate  the  heat  so  that  the  iodine  may  sublime  en- 
tirely, or  almost  entirely,  into  Z>.  Next  remove  J  while  still  hot 
and  give  it  a  gentle  swing  in  the  air  to  remove  the  still  uricon- 
densed  iodine  fumes  and  any  traces  of  aqueous  vapor,  cover  it 
with  <?,  allow  to  cool  under  the  desiccator,  weigh,  and  transfer  the 
two  watch-glasses  together  with  the  weighed  iodine  to  a  capacious 
beaker  containing  a  sufficient  quantity  of  potassium-iodide  solu- 
tion to  dissolve  the  whole  of  the  iodine  to  a  clear  fluid.  The 
process  is  then  continued  as  above  detailed. 

y.  Dilution  of  the  Standard  fluids  to  a  Convenient  Strength. 

With  the  aid  of  the  iodine  solution  the  strength  of  which  we  now 
know  exactly,  and  the  solution  of  sodium  thiosulphate  which  stands 
in  a  known  relation  to  the  iodine  solution,  we  might  make  any  deter- 
minations of  iodine.  The  calculation,  although  in  principle  ex- 
tremely simple,  is  yet  somewhat  hampered  by  reason  of  the  long 
decimal  which  expresses  the  quantity  of  iodine  in  1  c.  c.  of  the 
solution.  It  is  therefore  convenient  to  dilute  the  iodine  solution 


§146.]  IODINE.  547 

so  that  1  c.  c.  may  exactly  contain  0*005  grin,  iodine.  This  is 
done  by.  filling  a  litre  flask  therewith  and  adding  the  necessary 
quantity  of  water;  in  our  case  16 '94  c.  c.,  for  5  :  5-0847 :  :  1000: 
1016-94.  If  the  litre  flask  will  hold  above  the  mark  this 
16'94  c.  c.,  it  is  simply  added,  otherwise  it  is  put  into  the  dry 
bottle  destined  to  receive  the  iodine  solution,  the  iodine  solution 
added,  the  whole  shaken  together,  a  portion  of  the  fluid  returned 
to  the  flask,  shaken,  poured  back  into  the  bottle,  and  the  whole 
shaken  again. 

The  solution  of  thiosulphate  may  now  be  diluted  in  a  corre- 
sponding manner.  In  our  case  we  should  have  had  to  add  27-11 
c.  c.  water  to  1 000  c.  c.  of  the  solution,  as  will  be  seen  from  the 
following  consideration  : 

20-2  c.  c.  of  the  original  iodine  solution  correspond  to  20  c.  c. 
of  the  thiosulphate  solution. 

.*.  1000  c.  c.  correspond  to  990'1  c.  c. 

Now  these  1000  c.  c.  were  made  up  to  1016*94  by  addition 
of  water;  if  therefore  we  make  up  990*1  c.  c.  of  the  sodium 
thiosulphate  to  the  same  bulk  by  addition  of  water  we  shall  have 
equivalent  solutions.  Hence,  to  990 -1  c.  c.  we  must  add  26*84 
c.  c.  water,  or  to  1000  c.  c.  27*11  water. 

In  such  cases  of  dilution  I  always  prefer  to  take  exactly  1 
litre  instead  of  an  uneven  number  of  c.  c.,  as  in  measuring  the 
latter  errors  and  inaccuracies  may  readily  occur ;  I  have  therefore 
above  recommended  the  preparation  of  1200  c.  c.  of  the  fluids,  so 
that  after  their  determination  1000  c.  c.  may  be  sure  to  remain. 

<?.   THE  ACTUAL  ANALYSIS. 

Weigh  the  iodine  to  be  determined  in  a  glass-stoppered  tube, 
dissolve  in  potassium-iodide  solution  as  in  J,  /?,  add  thiosulphate 
solution  from  the  burette  till  decoloration  is  just  produced,  then 
3  or  4  c.  c.  starch  solution,  then  iodine  solution  from  a  second 
burette  to  incipient  blueness.  The  substance  contains  the  same 
amount  of  iodine  as  the  c.  c.  of  iodine  solution  corresponding  to 
the  thiosulphate  used  minus  the  c.  c.  of  the  former  used  to  de- 
stroy the  excess  of  the  latter.  Where  the  solutions  are  of  equal 
value>  and  1  c.  c.  corresponds  to  0*005  grin,  iodine,  the  calcula- 
tion is  in  the  highest  degree  simple ;  for  suppose  we  had  used  21 
c.  c.  NaaSaO,  and  1  c.  c.  iodine,  the  quantity  of  iodine  present  is 
0-1  grm. 

21  —  1  =  20,  and  20  X  0*005  =  0*1. 


548  DETERMINATION.  [§  147. 

Where  you  are  analyzing  chromic  acid  or  manganese  dioxide 
by  boiling  with  hydrochloric  acid,  and  passing  the  chlorine  evolved 
into  potassium  iodide,  you  must  allow  the  solution  to  cool  before 
titrating  with  thiosulphate ;  for  at  a  high  temperature  a  portion  of 
the  sodium  tetrathionate  produced  is  converted  into  sodium  sul- 
phate by  the  iodine  (WRIGHT*). 

Free  acid  in  the  iodine  solution  to  be  estimated  is  not  injuri- 
ous ;  when  such  is  present,  however,  the  excess  of  the  thiosulphate 
must  be  titrated  without  delay,  or  the  free  thiosulphuric  acid  may 
be  decomposed  before  the  iodine  is  added. 

d.  KEEPING  OF  THE  SOLUTIONS. 

The  iodine  solution  and  the  thiosulphate  solution  are  kept  in 
glass-stoppered  bottles  in  a  cool,  dark  place. '  But  the  relation 
between  the  two  solutions  must  be  tested  before  each  new  series 
of  experiments,  and  the  iodine  in  the  iodine  solution  must  be  rede- 
termined. 

If  a  fluid  contains  free  iodine  in  presence  of  iodine  in  combina- 
tion, determine  the  former  in  one  portion  by  the  combined  method, 
and  the  total  quantity  in  another  portion.  For  this  purpose  you 
may  either  (1)  add  sulphurous  acid  to  decoloration,  precipitate 
with  silver  nitrate  (§  145,  I.,  a,  a\  digest  the  precipitate  with  nitric 
acid  to  remove  any  silver  sulphite  which  it  may  contain,  filter,  &c. ; 
or  (2)  distil  with  ferric  chloride  as  directed,  §  145,  I. ,5,  e. 

§1*7. 

4.  CYANOGEN,  f 

I.  Determination. 

a.  Gravimetric  .Estimation. — If  you  have  free  hydrocyanic  acid 
in  solution  run  it  into  an  excess  of  solution  of  silver  nitrate,  add  a 
little  nitric  acid,  allow  to  settle  without  warming,  and  determine 
the  precipitated  silver  cyanide  either  by  collecting  on  a  weighed 
filter,  drying  at  100°  and  weighing  (§  115,  3),  or  by  collecting  on 
an  unweighed  filter  and  converting  into  metallic  silver.  The  latter 
operation  is  performed  by  igniting  the  precipitate  in  a  porcelain 

*  ZeitscliT.  f.  analyt.  Chem.,  ix,  482. 

f  With  regard  to  HERAPATH'S  colorimetric  method  which  is  founded  on  the 
intensity  of  the  color  of  a  solution  of  persulphocyanide  of  iron,  compare  (faun. 
Gaz.,  Aug.  1853,  294. 


SI  47.]  CYANOGEN.  649 

crucible  for  J  hour,  or  till  it  ceases  to  lose  weight  (H.  ROSE).  If 
you  wish  to  determine  in  this  way  the  hydrocyanic  acid  in  bitter 
almond  water  or  cherry  laurel  water,  add  ammonia  after  the  addi- 
tion of  the  solution  of  silver  nitrate  till  the  fluid  is  strongly  alka- 
line (it  is  not  necessary  to  dissolve  all  the  silver  cyanide),  and  at 
once  acidify  with  nitric  acid.  When  the  precipitate  has  settled, 
filter.  The  whole  of  the  cyanogen  in  the  fluid  will  have  been  now 
converted  into  silver  cyanide.  (The  cyanogen  was  originally  pres- 
ent partly  as  hydrocyanic  acid,  partly  as  ammonium  cyanide,  but 
principally  as  hydrocyanate  of  benzaldehyd — S.  FELDHAUS.*) 

FELDHAUS  recommends  the  following  proportions :  100  grin, 
bitter  almond  water,  about  1'2  grm.  silver  nitrate,  dissolved  in 
water  and  2  to  3  c.c.  ammonia  sp.  gr.  0*96.  A  portion  of  the  filtrate 
should  be  tested  to  make  sure  that  it  contains  silver  salt  in  excess, 
another  portion  should  be  tested  by  making  it  strongly  alkaline 
with  ammonia,  and  then  acid  again  with  nitric  acid.  If  a  precipi- 
tate is  formed  in  the  latter  case  it  shows  that  the  whole  of  the 
hydrocyanate  of  benzaldehyd  was  not  decomposed,  and  the  precipi- 
tation must  be  repeated.  If  you  want  to  measure  off  a  fluid  con- 
taining hydrocyanic  acid  with  a  pipette,  insert  a  little  tube  with 
soda-lime  between  the  pipette  and  the  flexible  tube  which  you  put 
into  your  mouth. 

b.  LIEBIG'S  Volumetric  Method-)-. — If  hydrocyanic  acid  is  mixed 
with  potassa  to  strong  alkaline  reaction,  and  a  dilute  solution  of 
silver  nitrate  is  then  added,  a  permanent  turbidity  of  silver  cyanide 
— or,  if  a  few  drops  of  solution  of  sodium  chloride  have  been  added, 
of  silver  chloride — forms  only  after  the  whole  of  the  cyanogen  is 
converted  into  double  cyanide  of  silver  and  potassium.  The  first 
drop  of  solution  of  silver  nitrate  added  in  excess  produces  the  per- 
manent precipitate.  1  at.  silver  consumed  in  the  process  corre- 
sponds, therefore,  exactly  to  2  mol.  hydrocyanic  acid  (2KCy  +  Ag 
NO3  =  AgCy.KCy  +  KNO8).  A  decinormal  solution  of  silver 
nitrate,  containing  consequently  10-792  grm.  silver  in  the  litre, 
should  be  used;  1  c.c.  of  this  solution  corresponds  to  0-0054096 
of  hydrocyanic  acid.  In  examining  medicinal  hydrocyanic  acid  5  to 
10  grm.  ought  to  be  used,  but  of  bitter  almond  water  about  50grm.  ; 
if  exactly  5-4096  or  54-096  grm.  are  used,  the  number  of  c.c. 
of  the  silver  solution,  divided  by  10,  or  by  100,  expresses  exactly 

*  Zeitschr.f.  analyt.  Chem.,  in,  34.     \  Annal.  d.  Chem.  u.  Pharm.,  LXXVII,  102. 


£50  DETERMINATION.  [§  147. 

the  percentage  of  hydrocyanic  acid.  Medicinal  hydrocyanic  acid 
is  suitably  diluted  first  by  adding  from  5  to  8  volumes  of  water ; 
bitter  almond  water  also  is  slightly  diluted ;  if  the  latter  is  turbid 
the  end-reaction  will  not  be  sufficiently  distinct,  and  the  gravimetric 
method  is  to  be  preferred. 

LIEBIG  has  examined  hydrocyanic  acid  of  various  degrees  of  dilu- 
tion, and  has  obtained  results  by  this  method  corresponding  exactly 
with  those  obtained  by  a.  SOUCHAY,*  too,  obtained  results  almost 
identical ;  with  pure  dilute  hydrocyanic  acid,  the  gravimetric  results 
were  to  the  volumetric  as  100  to  100*5 — 101 ;  with  clear  or  nearly 
clear  bitter  almond  water  as  100  to  102.  FELDHAUS  (loo.  cit.) 
obtained  very  nearly  similar  results.  The  slightly  higher  results 
of  the  volumetric  process  are  to  be  explained  from  the  fact  that  a 
small  excess  of  silver  solution  is  necessary  to  produce  the  final 
reaction.  The  less  the  amount  of  the  substance  taken  the  greater 
importance  does  this  error  assume.  We  should  also  notice  that  in 
the  bitter  almond  water,  which  contains  ammonium  cyanide,  some 
ammonia  is  set  free  which  has  a  solvent  action  on  the  silver  cyanide. 
In  this  method  it  does  not  matter  whether  the  hydrocyanic  acid 
contains  an  admixture  of  hydrochloric  acid  or  formic  acid.  A 
considerable  excess  of  potassa  must  be  avoided. 

If  it  is  intended  to  determine  potassium  cyanide  by  this 
method,  a  solution  of  that  salt  must  be  prepared  of  known 
strength,  and  a  measured  quantity  used  containing  about  O'l  grm. 
of  the  salt.  Should  it  contain  potassium  sulphide,  a  small  quantity 
of  freshly  precipitated  lead  carbonate  must  be  first  added  and 
the  solution  filtered  before  proceeding  to  the  determination. 

c.  Fordos  and  Gelis^s  Volumetric  Method.^  This  method 
is  founded  on  the  reaction  of  free  iodine  on  potassium  cyanide, 
described  by  SERTJLLAS  and  WOHLER,,  and  which  is  as  follows: 
KCN  +  21  =  KI  +  KIN".  According  to  this,  2  eq.  of  iodine 
correspond  to  1  eq.  of  cyanogen  or  1  eq.  hydrocyanic  acid,  or 
1  eq.  of  potassium  cyanide.  The  iodine  solution  is  best  pre- 
pared according  to  §  146.  If  free  hydrocyanic  acid  is  to  be  deter- 
mined, add  first  some  soda  solution  cautiously  until  an  alkaline 
reaction,  then  add  carbonic-acid  water  to  convert  any  possible 
excess  of  alkali  into  carbonate  (the  fluid  must  not  render  curcuma 
paper  brown),  and  finally  sufficient  iodine  solution  to  just  perrna- 

*  Zeitschr.f.  analyt.  Chem.,  u,  ISO. 

f  Journ.  de  Gliim.  et  de  Pharm.,  xxm,  48;  Journ.  /.  prakt.  Chem.,  LIX,  255. 


§  147.]  CYANOGEN.  551 

nently  tinge  the  colorless  solution  yellowish.  "When  analyzing 
potassium  cyanide  prepare  a  solution  first  of  known  strength  and 
use  a  volume  containing  about  0'05  grm.  of  potassium  cyanide. 
In  this  case,  too,  the  addition  of  carbonic-acid  water  is  necessary. 
The  potassium  cyanide  must  contain  no  potassium  sulphide,  as 
this  vitiates  the  results.  The  method  on  the  whole  gives  good 
results.  Compare  SOTJCHAY  (loo.  cit.)\  it  is  not  applicable  for 
bitter- almond  water,  however. 

II.  Separation  of  Cyanogen  from  the  Metals. 

a.  In  Cyanides  of  the  Alkali  Metals. 

Mix  the  substance  (if  solid,  without  previous  solution  in  water) 
with  excess  of  silver  nitrate  solution,  then  add  water,  finally  nitric 
acid  in  slight  excess,  allow  to  settle  without  warming,  and  deter- 
mine the  silver  cyanide  as  in  L,  a.  The  basic  metals  are  deter- 
mined in  the  filtrate  after  separating  the  excess  of  silver. 

b.  In  Cyanides  and  double  Cyanides,  which  are  completely 
decomposed  by  Silver  Nitrate  and  Nitric  Acid  or  Silver  Nitrate 
and  Ammonia. 

Digest  for  some  time  with  a  dilute  solution  of  silver  nitrate, 
stirring  frequently,*  then  add  nitric  acid  in  moderate  excess,  and 
digest  at  a  gentle  heat,  till  the  foreign  cyanide  is  fully  dissolved 
and  the  silver  cyanide  has  become  pure  and  quite  white.  Then 
add  water  and  filter.  As  a  precautionary  measure  it  is  well  to  test 
the  metal  obtained  by  long  ignition  of  the  silver  cyanide,  whether 
it  is  free  from  those  metals  which  were  combined  with  the  cyano- 
gen. The  filtrate  is  used  for  estimating  the  basic  metals,  the  silver 
being  first  precipitated  with  hydrochloric  acid.  This  method  affords 
us  an  exact  analysis  of  the  double  cyanides  of  potassium  w^ith 
nickel,  copper,  and  zinc  (II.  HOSE). 

W.  WEiTiif  recommends  a  solution  of  silver  nitrate  in  ammo- 
nia for  the  decomposition  of  many  cyanogen  compounds,  such  as 
potassium  ferrocyanide,  Prussian  blue,  and  even  potassium  cobalti- 
cyanide.  He  digests  them  in  sealed  tubes  at  100°  (in  the  case  of 
potassium  cobalticyanide,  150°)  for  4  or  5  hours.  Warm  the  con- 
tents of  the  tube  gently  in  a  dish;  until  the  crystals  of  ammonio- 
cyanide  of  silver  are  dissolved,  filter  off  the  separated  metallic 

*  Double  cyanide  of  nickel  and  potassium  yields  by  this  process  a  mixture  of 
silver  cyanide  with  nickel  cyanide.  Like  double  cyanides  are  similarly  decom- 
posed. -\Zeitschr.f.  analyt.  Chem.,  ix,  379. 


552  DETERMINATION.  [§  147. 

oxide,  wash  it  with  ammonia,  dilute,  and  precipitate  the  silver 
cyanide  by  acidifying  with  nitric  acid.  In  the  filtrate  separate  the 
silver  from  the  alkalies,  &c.  In  respect  to  the  undissolved  oxides 
it  should  be  noted  that  metallic  silver  is  always  mixed  with  the 
ferric  oxide. 

c.  In  Mercuric  Cyanide. 

Precipitate  the  aqueous  solution  with  hydrogen  sulphide ;  the 
mercuric  sulphide  may  be  filtered  without  difficulty  if  a  little 
ammonia  or  hydrochloric  acid  be  added  ;  it  is  determined  accord- 
ing to  §  118,  3.  If  the  compound  is  in  the  solid  condition,  the 
cyanogen  may  be  determined  in  another  portion  by  ignition  with 
cupric  oxide,  the  nitrogen  and  carbonic  acid  being  collected  and 
separated  (comp.  Organic  Analysis). 

H.  ROSE  and  FINKENEK*  have,  after  much  trouble,  succeeded 
in  finding  out  a  method  for  determining  cyanogen  with  precision 
also  in  solutions  of  mercuric  cyanide.  Mix  the  solution  of  the  mer- 
curic cyanide  with  zinc  nitrate  dissolved  in  ammonia.  To  1  part 
of  mercuric  salt  you  may  add  about  2  parts  of  the  zinc  salt.  Add 
to  the  clear  solution  hydrogen  sulphide  water  gradually  till  it  pro- 
duces a  perfectly  white  precipitate  of  zinc  sulphide.  The  precipi- 
tate, wThich  is  a  mixture  of  the  mercuric  and  zinc  sulphides,  settles 
well.  After  a  quarter  of  an  hour  filter  it  off  and  wash  with  very 
dilute  ammonia,  The  filtrate  contains  zinc  cyanide  dissolved  in 
ammonia,  together  with  ammonium  nitrate.  It  does  not  smell  of 
hydrocyanic  acid,  and  consequently  no  escape  of  the  latter  takes 
place.  Mix  it  with  silver  nitrate  and  then  add  dilute  sulphuric  acid 
in  excess.  The  silver  cyanide  is  next  washed  a  little  by  decantation, 
then — to  free  it  from  any  zinc  cyanide  simultaneously  precipitated 
— heated  with  a  solution  of  silver  nitrate,  finally  filtered  off, 
washed,  arid  determined  after  §  147,  I.,  a.  The  precipitated  sul- 
phides may  be  dissolved  in  aqua  regia,  and  the  mercury  precipitated 
as  mercurous  chloride  according  to  §  118,  2.  The  test-analyses  com- 
municated by  ROSE  yielded  excellent  results. 

d.  In  compounds  decomposable  by  Mercuric  Oxide  in  the  Wet 
Way. 

Many  simple  cyanides,  and  also  double  cyanides — both  of  the 
character  of  the  double  cyanide  of  nickel  and  potassium,  and  of 
the  ferro-  or  ferricyanides  (not,  however,  cobalticyanides) — may,  as 

*  Zeitschr.  /.  analyt.  Chem.,  i,  288- 


§  147]  CYANOGEN.  553 

is  well  known,  be  completely  decomposed  by  boiling  with  excess 
of  mercuric  oxide  and  water,  all  cyanogen  being  obtained  as  mer- 
curic cyanide  and  the  metals  passing  into  oxides. 

H.  ROSE  (Loc.  cit.}  has  shown  that  Prussian  blue,  potassium 
ferro-  and  ferricyanide,  more  particularly,  may  be  readily  analyzed 
in  this  manner. 

Boil  a  few  minutes  with  water  and  excess  of  mercuric  oxide  till 
complete  decomposition  is  effected,  add — in  order  to  render  the 
ferric  hydroxide  and  mercuric  oxide  removable  by  filtration — nitric 
acid  in  small  portions,  till  the  alkaline  reaction  has  nearly  disap- 
peared, filter,  wash  with  hot  water,  dry  the  precipitate,  ignite — 
very  gradually  raising  the  heat — under  a  hood  (with  a  good 
draught),  and  weigh  the  ferric  oxide  remaining.  In  the  filtrate 
the  cyanogen  is  determined  according  to  0,  and  any  potassium  that 
may  be  present  is  determined  in  the  filtrate  from  the  silver  cya- 
nide. 

e.  Determination  of  Metals  contained  in  Cyanides  with  decom- 
position and  volatilization  of  the  Cyanogen. 

Of  the  various  means  for  completely  decomposing  compounds 
of  cyanogen,  especially  also  the  double  cyanides,  according  to  H. 
HOSE  (Loc.  cit.}  three  particularly  are  worthy  of  recommendation — 
viz.,  concentrated  sulphuric  acid,  mercuric  sulphate,  and  ammo- 
nium chloride.  The  nitrates  seemed  decidedly  less  suitable  on 
account  of  their  too  violent  action. 

a.  DECOMPOSITION  BY  SULPHURIC  ACID.  All  cyanogen  com- 
pounds, simple  or  double,  are  completely  decomposed  and  con- 
verted into  sulphates  or  oxides,  as  the  case  may  be,  if  treated  in  a 
powdered  condition  in  a  platinum  dish  or  a  capacious  platinum 
crucible  with  a  mixture  of  about  3  parts  concentrated  sulphu- 
ric acid  and  1  part  water,  and  heated  till  almost  all  the  sulphuric 
acid  had  been  expelled.  The  residual  mass  is  then  free  from  cyan- 
ogen. It  is  dissolved  in  water,  if  necessary  with  addition  of 
hydrochloric  acid,  and  the  metals  determined  by  the  usual  methods. 
This  way  is  not  adapted  for  mercuric  cyanide,  as  a  little  of  the 
metal  would  escape  with  the  fumes  of  the  sulphuric  acid. 

/?.  DECOMPOSITION  BY  MERCURIC  SULPHATE.  Of  the  mercuric 
sulphates,  those  suitable  to  our  present  purpose  are  the  normal  and 
the  basic  (Turpeth  mineral).  The  substance  is  mixed  with  (\  parts 
of  the  latter,  heated  in  a  platinum  crucible  gradually,  and  finally 
maintained  for  a  long  time  at  a  red-heat,  till  all  the  mercury  has 


554  DETEKMINATION.  [§  147. 

volatilized,  and  the  weight  of  the  crucible  remains  constant.  If 
alkalies  are  present,  a  little  ammonium  carbonate  is  added  from 
time  to  time  during  the  final  ignition,  in  order  to  convert  the  acid 
sulphates  into  normal.  The  residue  may  usually  be  analyzed  by 
simple  treatment  with  water;  in  the  case  of  potassium  ferro- 
cyanide, for  instance,  the  potassium  sulphate  dissolves  and  pure 
(alkali-free)  ferric  oxide  remains  behind.  The  test-analyses  that 
have  been  communicated  show  excellent  results. 

y.  DECOMPOSITION  BY  AMMONIUM  CHLORIDE.  Mix  the  sub- 
stance with  twice  or  thrice  the  amount  of  this  salt  and  ignite  the 
mixture  moderately  in  a  stream  of  hydrogen  (apparatus,  Fig.  83). 
From  the  cooled  mass  water  extracts  alkali  chloride,  while  the 
reducible  metals  remain  in  the  metallic  state.  The  method  is 
peculiarly  adapted  for  the  analysis  of  double  cyanide  of  nickel  and 
potassium  "and  cobalticyanide  of  potassium,  not  so  for  iron  com- 
pounds, since  the  iron  obtained  is  not  pure,  but  contains  carbon. 

If  one  of  the  methods  described  in  e  is  employed,  the  nitrogen 
and  carbon  (the  cyanogen)  must  be  determined  by  a  combustion, 
if  an  estimation  by  the  loss  is  not  sufficient. 

f.   Determination  of  the  Alkalies,  especially  of  Ammonia,  in 
Soluble  Ferrocyanides. 

.  Mix  the  boiling  solution  with  a  solution  of  cupric  chloride  in 
moderate  excess,  filter  off  the  precipitated  cupric  ferrocyanide, 
free  the  filtrate  from  copper  by  means  of  hydrogen  sulphide,  and 
then  determine  the  alkalies  (REINDEL  *).  In  the  case  of  fixed 
.alkalies  the  object  may  also  be  obtained  by  igniting  with  barium 
thiosulphate  (FROHDE  f). 

g.  Volumetric  Determination  of  Ferro-  and  Ferricyanogen. 

OL.  After  E.  DE  HAEN.  This  method,  devised  in  my  labora- 
tory, is  founded  upon  the  simple  fact  that  a  solution  of  potassium 
ferrocyanide  acidified  with  sulphuric  acid  (and  which  may  accord- 
ingly be  assumed  to  contain  free  hydroferrocyanic  acid)  is  by  ad- 
dition of  potassium  permanganate  converted  into  the  correspond- 
ing ferricyanide.  If  this  conversion  is  effected  in  a  very  dilute 
fluid,  containing  about  0-2  grin,  potassium  ferrocyanide  in  frorq 

*Journ.f.  prakt.  Chem.,  LXV,  452. 
t  Zcitschr.  /.  analyt.  Chem.,  in,  181. 


§  147.]  CYANOGEN.  555 

100  to  200  c.  c.,  the  termination  of  the  reaction  is  clearly  and 
unmistakably  indicated  by  the  change  of  the  originally  pure  yel- 
low color  of  the  fluid  to  reddish-yellow.* 

The  process  requires  two  test-fluids  of  known  strength,  viz. : 

1.  A  solution  of  pure  potassium  ferrocyanide. 

2.  A  solution  of  potassium  permanganate. 

The  former  is  prepared  by  dissolving  20  grin,  perfectly  pure 
and  dry  crystallized  potassium  ferrocyanide  in  water  to  1  litre; 
each  c.c.  therefore  contains  20  mgrm.  The  latter  is  diluted  so  that 
somewhat  less  than  a  buretteful  is  required  for  10  c.c.  of  the  solu- 
tion of  potassium  ferrocyanide. 

To  determine  the  strength  of  the  potassium  permanganate  solu- 
tion in  its  action  upon  the  potassium  ferrocyanide,  measure  off,  by 
means  of  a  pipette,  10  c.c.  of  the  solution  of  potassium  ferrocyanide 
(containing 0'2  grm.),  dilute  with  100  to 200  c.c.  water,  acidify  with 
sulphuric  acid,  place  the  glass  on  a  sheet  of  white  paper,  and  allow 
the  permanganate  to  drop  into  the  fluid,  stirring  it  at  the  same 
time,  until  the  change  from  yellow  to  reddish-yellow  indicates  that 
the  conversion  is  complete.f  Repetitions  of  the  experiment  always 
give  very  accurately  corresponding  results.  If  at  any  time  you 
have  reason  to  suspect  that  the  permanganate  has  suffered  altera- 
tion, recourse  must  be  had  again  to  this  experiment.  If  after 
acidifying  the  potassium  ferrocyanide  with  sulphuric  acid  you  add 
a  trace  of  ferric  chloride  to  produce  a  bluish-green  color,  the  latter 
will  disappear  at  the  end  of  the  reaction,  which  is  thus  rendered 
very  distinct  (GINTL^;). 

To  determine  the  amount  of  real  potassium  ferrocyanide  con- 
tained in  any  given  sample  of  the  commercial  article,  dissolve  5 
grm.  to  250  c.c. ;  take  10  c.c.  of  this  solution,  and  examine  as  just 
directed.  Suppose,  in  determining  the  strength  of  the  permanga- 
nate, you  have  used  20  c.c.,  and  you  find  now  that  19  c.c.  is  suffi- 
cient, the  simple  rule-of-three  sum, 

20: -2::  19:  a; 

•-  Instead  of  the  permanganate  you  may  use  potassium  cbromate.  The  solu- 
tion is  added  till  spots  of  iron  sesquichloride  on  a  plate  are  no  longer  colored 
blue  or  green,  but  brownish.  E.  MEYER,  Zeitschr.  f.  analyt.  Chem.,  vin,  508. 

f  If  you  wish  at  first  for  some  additional  evidence  besides  the  change  of  color, 
add  to  a  drop  of  the  mixture  on  a  plate,  a  drop  of  solution  of  ferric  chloride; 
if  this  fails  to  produce  a  blue  tint,  the  conversion  is  accomplished. 

%  Zeitschr.  f.  analyt.  Chem.,  vi,  446. 


556  DETERMINATION.  [§  147. 

will  inform  you  how  much  pure  potassium  ferrocyanide  0*2  grin,  of 
the  analyzed  salt  contains.  And  even  this  small  calculation  may 
be  dispensed  with  by  diluting  the  permanganate  so  that  exactly  50 
c.  c.  correspond  to  0'2  of  potassium  ferrocyanide,  as,  in  that  case, 
the  number  of  half-c.c.  consumed  expresses  directly  the  percentage 
of  pure  ferrocyanide. 

Instead  of  determining  the  strength  of  the  permanganate  by 
means  of  pure  potassium  ferrocyanide,  which  is  unquestionably 
the  best  way,  one  of  the  methods  given  in  §  112,  2,  may  also  be 
employed ;  bearing  in  mind,  in  that  case,  that  2  mol.  potassium 
ferrocyanide  =  845*256,  2  at.  iron  =  111-8,  and  1  mol.  oxalic 
acid  ==  126-048  are  equivalent  in  their  action  upon  solution  of 
potassium  permanganate. 

The  analysis  of  soluble  ferricyanides  by  this  method  is  effected 
by  reducing  them  to  ferrocyanides,  acidifying,  and  then  proceeding 
in  the  way  described.  The  reduction  is  effected  as  follows  :  Mix 
the  weighed  ferricyanide  with  a  solution  of  soda  or  potassa  in 
excess,  boil  and  add  concentrated  solution  of  ferrous  sulphate 
gradually,  and  in  small  portions,  until  the  color  of  the  precipitate 
appears  black,  which  is  a  sign  that  protosesquioxide  of  iron  has 
precipitated.  Dilute  now  to  300  c.c.,  mix,  filter,  and  proceed  to 
determine  the  ferrocyanide  in  portions  of  50  or  100  c.c.  of  the 
fluid.  As  the  space  occupied  by  the  precipitate  is  not  taken  into 
account  in  this  process,  the  results  are  not  absolutely  accurate  ;  the 
difference  is  so  very  trifling,  however,  that  it  may  safely  be  disre- 
garded. GINTL  (loc.  eit.)  suggests  to  put  the  neutral  or  alkaline 
fluid  in  a  tall  vessel  and  add  a  few  lumps  of  sodium  amalgam  as 
big  as  peas  :  in  ten  minutes  the  reduction  will  be  effected  and  with- 
out the  aid  of  heat. 

Insoluble  ferro-  or  ferricyanides,  decomposable  by  boiling  solu- 
tion of  potassa  (as  are  most  of  these  compounds),  are  analyzed  by 
boiling  a  weighed  sample  sufficiently  long  with  an  excess  of  solu- 
tion of  potassa  (adding,  in  the  case  of  ferricyanides,  ferrous  sul- 
phate), and  then  proceeding  as  directed  above. 

ft.  After  E.  LENSSE^.  Ferricyanides  may  also  be  analyzed 
according  to  the  following  method,  which  too  was  devised  in  my 
laboratory:  The  method  is  based  on  the  fact  that  on  bringing 
together  potassium  ferricyanide,  potassium  iodide,  and  concen- 


§  147.]  CYANOGEN.  557 

trated  hydrochloric  acid,  for  every  eq.  of  potassium  ferricyanide 
1  eq.  of  iodine  are  precipitated,  thus : 

K,Fe(CN).  +  KI  =  K.Fe(UN).+  I. 

On  estimating  the  liberated  iodine  according  to  §  146,  the 
quantity  of  potassium  ferricyanide  is  then  determined.  In  four 
experiments  LENSSEN  obtained  99 -22,  101*7,  102-1,  and  100-5, 
instead  of  100.  The  solution  may  be  diluted  only  after  the  hy- 
drochloric acid  has  been  added.  C.  MOHR  *  obtained  still  more 
accurate  results,  as  he  avoided  the  formation  of  hydroferricyanic 
acid  by  adding  zinc-sulphate  solution,  which  is  not  at  all  decomposed 
by  iodine.  He  directs  adding  potassium  iodide  and  hydrochloric 
acid  in  excess  to  the  diluted  ferricyanide  solution,  then  to  add  an 
excess  of  iron-free  zinc-sulphate  solution,  neutralize  the  free  acid 
with  sodium  bicarbonate  in  slight  excess,  and  to  then  determine 
the  liberated  iodine  according  to  §  146. 

y.  To  determine  potassium  ferrocyanide  in  dyers'  baths, 
which  contain  oxidizable  organic  substances,  and  which,  hence, 
cannot  be  estimated  with  permanganate,  II.  RHEINECK  f  recom- 
mends a  process  based  on  the  fact  that  potassium -ferrocyanide 
solution,  on  gradually  adding  a  solution  of  a  ferric  salt,  whether 
a  mineral  acid  is  added  or  not,  yields  a  clear,  blue  solution  which 
becomes  turbid  and  which,  when  all  the  ferrocyanogen  is  thrown 
down,  forms  a  clear,  colorless  liquid  containing  Berlin  blue  sus- 
pended in  flocculent  form.  Hence  on  adding  a  solution  of  a  fer- 
ric salt  to  equal  volumes  of  a  potassium-ferrocyanide  solution  of 
known  strength  and  of  the  bath,  until  in  both  cases  the  flocculent 
precipitate  forms,  the  unknown  quantity  of  ferrocyanide  may  be 
readily  calculated. 

d.   After  E.  BOHLIG.^ 

In  the  case  of  a  fluid  containing  potassium  ferrocyanide,  and 
also  sulphocyanide  (for  instance,  the  red  liquor  of  the  prussiate 
works),  the  method  given  in  a  cannot  be  employed,  as  the  hydro- 
sulphocyanic  acid  also  reduces  permanganic  acid.  The  following 
method — depending  on  the  precipitation  of  the  ferrocyanogen  with 
solution  of  cupric  sulphate — may  then  be  used;  it  is  accurate 
enough  for  technical  purposes:  Dissolve  10  grm.  pure  cupric  sul- 

*Annal.  d.  Chem.  u.  Pharm.,  cv,  62.        f  Chem.  Centralbl.,  1871,  p.  778. 
%  Polytechn.  Notizblatt,  xvi,  81. 


558  DETERMINATION.  [§  148. 

phate  to  1  litre,  also  4  grm.  pure  dry  potassium  ferrocyanide  to  1 
litre.  Add  to  50  c.c.  of  the  latter  solution  (which  contain  0'2  grm. 
potassium  ferrocyanide)  copper  solution  from  a  burette  to  complete 
precipitation  of  the  ferrocyanogen.  In  order  to  hit  this  point 
exactly,  from  time  to  time  dip  a  strip  of  filter-paper  into  the 
brownish-red  fluid  which  will  imbibe  the  clear  filtrate,  leaving  the 
precipitate  of  copper  ferrocyanide  behind.  At  first  the  moist  strips 
of  paper,  when  touched  with  ferric  chloride,  become  dark  blue,  the 
reaction  gradually  gets  wreaker  and  weaker,  and  finally  vanishes 
altogether.  We  now  know  the  value  of  the  copper  solution  with 
reference  to  its  action  on  potassium  ferrocyanide,  and  can,  there- 
fore, by  its  means  test  solutions  containing  unknown  amounts  of 
ferrocyanogen.  If  alkali  sulphides  are  present,  they  are  first 
removed  by  boiling  with  lead  carbonate.  After  filtering  off  the 
lead  sulphide,  acidify  with  dilute  sulphuric  acid,  and  then  proceed. 

§148. 
5.  SULPHUR. 

I.  Determination. 

To  determine  hydrogen  sulphide  in  a  mixture  of  gases  confined 
over  mercury*  it  may  be  absorbed  by  a  ball  made  of  2  parts  precipi- 
tated lead  phosphate  and  3  parts  plaster  of  Paris.  The  mixture  is 
made  into  a  paste  with  water,  and  pressed  into  a  bullet  mould  in 
which  the  platinum  wire  is  inserted.  The  mould  should  previously 
be  oiled.  The  balls  are  dried  at  100°,  saturated  with  concentrated 
phosphoric  acid,  and  are  then  ready  for  use  (LuDwiaf). 

To  determine  sulphuretted  hydrogen  dissolved  in  water  the 
following  methods  are  in  use  : 

a.  The  method  of  determining  hydrogen  sulphide  volumetri- 
cally  by  solution  of  iodine,  was  employed  first  by  DUPASQUIER  ;  it 
is  very  convenient  and  accurate.  That  chemist  used  alcoholic  solu- 
tion of  iodine.  But  as  the  action  of  the  iodine  upon  the  alcohol 
alters  the  composition  of  this  solution  somewhat  rapidly,  it  is  bet- 
ter to  use  a  solution  of  iodine  in  potassium  iodide.  The  decom- 
position is  as  follows : 

H,S  +  21  =  2HI  +  S 

*  When  this  gas  remains  long  in  contact  with  mercury,  sulphide  of  mercury 
Is  liable  to  be  formed.  f  Annal,  d.  Chem.  u.  Pharm.,  CLXII,  55. 


§148.] 

2  at.  I  =  253-70  correspond,  lience,  to  1  iriol.  II3S  =  34*086. 
However,  this  exact  decomposition  can  be  relied  upon  with  cer- 
tainty only  if  the  amount  of  hydrogen  sulphide  in  the  fluid  does 
not  exceed  0-04  per  cent.(BuNSEN).  Fluids  containing  a  larger  pro- 
portion of  hydrogen  sulphide  must  therefore  first  be  diluted  to  the 
required  degree  with  boiled  water  cooled  out  of  the  contact  of  air. 

The  iodine  solution  of  §  146  may  be  used  for  the  estimation  of 
larger  quantities  of  hydrogen  sulphide ;  for  weak  solutions,  e.g., 
sulphuretted  mineral  water,  it  is  advisable  to  dilute  the  iodine  solu- 
tion 5  times,  so  that  1  c.c.  may  contain  O'OOl  grm.  iodine. 

The  process  is  conducted  as  follows : 

Measure  or  weigh  a  certain  quantity  of  the  sulphuretted  wateiy 
dilute,  if  required,  in  the  manner  directed,  add  some  thin  starch- 
paste,  and  then  solution  of  iodine,  with  constant  shaking  or  stir- 
ring, until  the  permanent  blue  color  begins  to  appear.  The  result 
of  this  experiment  indicates  approximately,  but  not  with  positive 
accuracy,  the  relation  between  the  examined  water  and  the  iodine 
solution.  Suppose  you  have  consumed,  to  220  c.c.  of  the  sulphu- 
retted water,  12  c.c.  of  a  solution  of  iodine  containing  0*000918- 
grm.  iodine  in  the  c.c.*  Introduce  now  into  a  flask  nearly  the 
quantity  of  iodine  solution  required,  add  the  sulphuretted  water 
in  quantity  either  already  determined,  or  to  be  determined,  by 
weight  or  measure  ;f  then  to  the  colorless  fluid  add  thin  starch- 
paste,  and  after  this  iodine  solution  until  the  blue  color  just  begins 
to  show.  By  this  course  of  proceeding,  you  avoid  the  loss  of 
hydrogen  sulphide  which  would  otherwise  be  caused  by  evaporation 
and  oxidation.  In  my  analysis  of  the  Weilbach  water,  256  c.c.  of 
the  water  required,  in  my  second  experiment,  16'26  c.c.  of  iodine 
solution,  which,  calculated  to  the  quantity  of  sulphuretted  water 
used  in  the  first  experiment,  viz.,  220  c.c.,  makes  13'9  c.c.,  or  1'9 
e.c.  more.  But  even  now  the  experiment  cannot  yet  be  considered 
quite  conclusive,  when  made  with  a  solution  of  iodine  so  dilute ;  it 
being  still  necessary  to  ascertain  how  much  iodine  solution  is  required 
to  impart  the  same  blue  tint  to  the  same  quantity  of  ordinary  water 
mixed  with  starch-paste,  of  the  same  temperature,^:  and  as  nearly 
as  possible  in  the  same  condition!  as  the  analyzed  sulphuretted 

*Tlie  numbers  here  stated  are  those  which  I  obtained  in  the  analysis  of  the 
Weilbach  water.  f  Compare  Experiment  No.  82. 

\  Atinal   d.  Chem.  u.  Pharm.,  en,  186. 
§  In  this  connection  I  would  recommend,  in  cases  where  the  sulphuretted 


5(30  DETERMINATION.  [§  148. 

water,  and  to  deduct  tins  from  the  quantity  of  iodine  solution 
used  in  the  second  experiment.  Thus,  in  the  case  mentioned,  I 
had  to  deduct  0*5  c,  c.  from  the  16*26  c.  c.  used.  If  the  instruc- 
tions here  given  are  strictly  followed,  this  method  gives  very 
accurate  results.  (See  Expt.  No.  82.) 

5.   FK.  MOHR'S  method  slightly  modified. 

Add  to  the  sulphuretted  water  a  slight  excess  of  sodium-arsen- 
ite  solution  standardized  against  iodine  solution  (§  127,  5,  a\ 
then  add  hydrochloric  acid  until  the  liquid  is  distinctly  acid. 
Dilute  to  300  c.  c.,  pass  through  a  dry  filter-paper,  make  sure 
that  the  solution  still  contains  sodium  arsenite  by  testing  a  sample 
with  hydrogen -sulphide  water,  and  then  determine  in  100  c.  c. , 
after  adding  powdered  sodium  bicarbonate,  the  remainder  of  the 
arsenous  acid.  Deduct  the  c.  c.  of  iodine  solution  last  used,  mul- 
tiplied by  3  (because  only  100  c.  c.  of  the  300  c.  c.  have  been 
operated  upon),  from  that  corresponding  to  the  entire  quantity  of 
arsenous  acid  used ;  the  remainder  will  express  the  quantity  of 
iodine  solution  equivalent  to  the  hydrogen  sulphide  present.  In 
calculating  it  must  be  remembered  that  here  2  eq.  of  iodine  cor- 
respond to  3  eq.  of  hydrogen  sulphide,  since  1  eq.  of  AsaO, 
decomposes  3  eq.  of  HaS  on  the  one  hand,  forming  AsaS,  and 
3HaO,  and  requires  on  the  other  hand  2  eq.  of  iodine  for  its  con- 
version into  arsenic  acid. 

Yery  dilute  hydrogen-sulphide  solutions  cannot  be  estimated 
by  this  method,  as  the  arsenic  sulphide  formed  in  them  takes  a 
long  time  to  deposit,  and  a  very  small  portion  always  remains  in 
solution.* 

c.  Mix  the  sulphuretted  fluid  with  an  excess  of  solution  of 
sodium  arsenite,  add  hydrochloric  acid,  allow  to  deposit,  and  deter- 
mine the  arsenous  sulphide  as  directed  in  §  127,  4.  The  results  are 
accurate,  unless  the  solution  is  very  dilute,  in  which  case  the  slight 
solubility  of  arsenous  sulphide  occasions  loss.  (See  Expt.  No.  82.) 

In  an  analysis  of  the  Weilbach  waters,  this  method  hence 
gave  0-006621  and  0-006604  per  1000,  whereas  water  taken 

water  contains  bicarbonate  of  soda,  to  add  to  the  ordinary  water  an  equal  quan- 
tity of  this  salt,  as  its  presence  has  a  slight  influence  on  the  appearance  of  the 
final  reaction. 

*  Hydrogen-sulphide  water  containing  0'003  grin.  HaS  in  a  litre  gave  with  a 
solution  of  arsenous  acid  in  hydrochloric  acid  a  precipitate  that  could  be  filtered 
off  only  after  12  hours. 


§  148.]  SULPHUK.  561 

from  the  well  at  the  same  time,  and  titrated  with  iodine,  gave 
O -007025  IIaS  per  1000.  Instead  of  arsenous  acid  the  precipi- 
tation may  be  effected  by  means  of  cupric  acetate  together  with 
a  little  acetic  acid,  or  by  'means  of  a  silver  solution,  and  the  sul- 
phur determined  in  the  precipitated  copper  sulphide  as  barium 
sulphate,  according  to  §  148,  II,  or  the  silver  may  be  determined 
in  the  silver  sulphide  in  the  metallic  state.  When  copper  salts  are 
used  in  very  dilute  solutions,  the  results  are  also  too  low ;  whether 
this  is  also  the  case  with  the  silver  solution  I  cannot  say,  from 
lack  of  personal  experience.  The  silver  solution  best  adapted  for 
the  purposes  is  recommended  by  LYTE  *  to  be  prepared  by  dissolv- 
ing silver  chloride  in  sodium-thiosulphate  solution  and  adding  a  few 
drops  of  ammonia.  In  an  analysis  of  water  containing  iron  sul- 
phate, LYTE  f  threw  down  the  hydrogen  sulphide  with  freshly  pre- 
cipitated lead  sulphate,  filtered,  washed,  and  extracted  the  lead  sul- 
phate with  hot  ammonium-acetate  solution,  converted  the  residual 
lead  sulphide  into  lead  sulphate  by  oxidation  with  nitric  acid,  etc., 
and  then  weighed  the  sulphate. 

In  the  case  of  mineral  waters  the  method  a  is  always  to  be 
preferred,  unless  thiosulphates  should  be  present  and  impair  its 
accuracy. 

d.  If  the  hydrogen  sulphide  is  evolved  in  the  gaseous  state,  and 
large  quantities  are  to  be  determined,  the  best  way  is  to  conduct  it 
first  through  several  bulbed  U-tubes  (Fig.  103),  containing  an 
alkaline  solution  of  sodium  arsenite,  then  through  a  tube  connected 
with  the  exit  of  the  last  U-tube,  which  contains  pieces  of  glass 
moistened  with  solution  of  soda,  to  mix  the  fluids  afterwards,  and 
proceed  as  in  b.  If,  on  the  other  hand,  we  have  to  determine 
small  quantities  of  hydrogen  sulphide  contained  in  a  large  amount 
of  air,  etc.,  it  is  well  to  pass  the  gaseous  mixture  in  separate  small 
bubbles  through  a  very  dilute  solution  of  iodine  in  potassium 
iodide  of  known  volume  and  strength,  which  is  contained  in  a  long 
glass  tube  fixed  in  an  inclined  position  and  protected  from  sun- 
light. The  free  iodine  remaining  is  finally  estimated  by  means 
of  a  solution  of  sodium  thiosulphate  (§  146);  the  difference  gives 
us  the  quantity  of  iodine  which  has  been  converted  by  hydrogen 
sulphide  into  hydriodic  acid,  and  consequently  corresponds  to  the 

*  Compt.  rend.,  XLIII,  765.  f  Zeilschr.  f.  analyt.  Chem.,  v,  441. 


562  DETERMINATION".  [§  148. 

amount  of  the  hydrogen  sulphide  present.  The  volume  of  the 
gaseous  mixture  may  be  known  by  measuring  the  water  which 
has  escaped  from  the  aspirator  used.  The  arrangement  of  the 
absorption  tube  is  the  same  as  is  figured  in  connection  with  the 
Determination  of  Carbonic  Acid  in  Air.  The  thin  glass  tube 
conducting  the  gas  into  the  absorption  tube,  however,  must  not 
be  provided  with  an  india-rubber  elongation. 

From  my  own  experiments*  it  appears  that  hydrogen  sul- 
phide, whether  in  small  or  large  quantities,  may  be  also  estimated 
by  the  increase  in  weight  of  absorption  tubes.  We  have  only  to 
take  care  that  the  mixture  of  gases  is  first  thoroughly  dried  by 
passing  over  calcium  chloride.  To  take  up  the  hydrogen  sulphide 
we  use  U-tubes,  five-sixths  filled  with  copper  sulphate  on  pumice, 
one-sixth  at  the  exit  containing  calcium  chloride.  To  prepare  the 
pumice  with  copper  sulphate,  proceed  as  follows :  Treat  60  grin, 
pumice  in  lumps  the  size  of  peas  in  a  small  porcelain  dish  with  a 
hot  concentrated  solution  of  30  or  35  grm.  copper  sulphate,  dry 
the  whole  with  constant  stirring,  place  the  dish  in  an  air  or  oil 
bath  of  the  temperature  of  150°  to  160°,  and  allow  to  remain 
therein  four  hours.  A  tube  containing  14  grm.  of  this  prepared 
pumice  will  absorb  about  0*2  grm.  hydrogen  sulphide.  It  is  well 
always  to  employ  two  such  tubes.  If  the  prepared  pumice  is 
dried  at  a  lower  temperature  it  takes  up  much  less  of  the  gas,  if 
dried  at  a  higher  temperature  the  gas  is  decomposed  and  sulphur- 
ous acid  is  formed.  This  method  is  more  completely  detailed 
under  the  Analysis  of  Black  Ash. 

Finally,  small  quantities  of  hydrogen  sulphide  mixed  with  other 
gases  may  be  estimated  by  passing  through  bromine  water  and  con- 
verting into  sulphuric  acid. 

II.  Separation  and  Determination  of  Sulphur  in  Sulphides. 

A.   METHODS  BASED  ON  THE  CONVERSION  OF  THE  SULPHUR  INTO 

SULPHURIC  ACID. 

1.  'Methods  in  the  Dry  Way. 

a.  Oxidation  ~by  Alkali  Nitrates  (applicable  to  all  compounds 
of  sulphur).  If  the  sulphides  do  not  lose  any  sulphur  on  heating, 
mix  the  pulverized  and  weighed  substance  with  6  parts  of  anhy- 

*  Zeitschr.f.  analyt.  Chem.,  x,  75 


^  148.]  SULPHUR.  563 

drous  sodium  carbonate  and  4  of  potassium  nitrate,  with  the  aid 
of  a  rounded  glass  rod,  wipe  the  particles  of  the  mixture  which 
adhere  to  the  rod  carefully  off  against  some  sodium  carbonate,  and 
add  this  to  the  mixture.  Heat  in  a  platinum  or  porcelain  crucible 
(which,  however,  is  somewhat  affected  by  the  process),  at  a  grad- 
ually increased  temperature  to  fusion  ;*  keep  the  mass  in  that  state 
for  some  time,  then  allow  it  to  cool,  heat  the  residue  with  water, 
filter  the  fluid,  boil  the  residue  with  a  solution  of  pure  sodium  car- 
bonate, filter,  wash,  remove  all  nitric  acid  from  the  filtrate  by 
repeated  evaporation  with  pure  hydrochloric  acid,  and  determine 
the  sulphuric  acid  as  directed  in  §  132.  The  metal,  metallic  oxider 
or  carbonate,  which  remains  undissolved,  is  determined,  according 
to  circumstances,  either  by  direct  weighing  or  in  some  other  suit- 
able way.  In  the  presence  of  lead,  before  filtering,  pass  carbonic 
acid  tlirough  the  solution  of  the  fused  mass,  to  precipitate  the 
small  quantity  of  that  metal  which  has  passed  into  the  alkaline 
solution. 

Should  the  sulphides,  on  the  contrary,  lose  sulphur  on  heat- 
ing, the  finely  powdered  compound  is  mixed  with  4  parts  sodium 
carbonate,  8  parts  nitre,  and  24  parts  pure  and  perfectly  dry 
sodium  chloride,  and  the  process  otherwise  conducted  as  already 
given. 

b.  Oxidation  by  Potassium  Chlorate.  The  oxidation  of 
metallic  sulphides  by  a  mixture  of  potassium  chlorate  and  sodium 
carbonate  has  been  repeatedly  recommended.  It  is  advantageous- 
in  so  far  that  the  sulphuric  acid  in  the  melt  may  be  more  readily 
converted  into  barium  sulphate  than  when  nitrates  are  present; 
on  the  other  hand,  it  is  dangerous  because,  when  the  mixture  is 
used  in  the  proportions  usually  recommended — 1  part  sulphide, 
3  parts  potassium  chlorate,  and  3  parts  sodium  carbonate  (or  4 
parts  sodium-potassium  carbonate) — many  sulphides,  e.g. ,  fahl- 
erz,  antimony  sulphide,  etc.,  afford  violent  explosions. f  Also 
with  many  sulphides,  like  iron  pyrites  and  copper  pyrites  (Fu. 
MOHR),  the  decomposition  is  not  complete.  Great  caution  must 
hence  be  exercised  in  using  potassium  chlorate  in  this  method. 

*  If  gas  not  free  from  sulphur  is  used  for  heating,  some  sulphur  is  likely  to 
be  absorbed  (PRICE,  Journ.  Chem.  Soc.  (2),  n,  51).  If  a  platinum  crucible  is 
used,  do  not  raise  the  heat  more  than  is  necessary  or  the  crucible  will  be 
attacked. 

•\  Annal.  d.  Chem.  u.  Pharm.,  cvn,  128. 


564  DETERMINATION.  [§  148. 

II.  EOSE  recommends  taking  6  to  8  parts  of  sodium  carbonate  and 
1  part  of  potassium  chlorate  to  1  part  of  substance. 

I.  Oxidation  ly  Chlorine  Gas  (after  BERZELIUS  and  II.  EOSE, 
especially  suitable  for  sulphosalts  of  complicated  composition). 

The  following  apparatus  (Fig.  105),  or  one  of  similar  con- 
struction, is  used.  Corks  should  be  used,  not  india-rubber  stop- 


Fig.  105. 

pers,  and  wherever  there  is  an  india-rubber  connection  the  glass 
tubes  should  be  close  to  each  other. 

A  is  a  chlorine-evolution  flask,*  B  contains  concentrated  sul- 
phuric acid,  and  C  calcium  chloride.  The  sulphide  to  be  decom- 
posed is  placed  in  the  bulb-tube  D,  the  straight  tube  of  which 
should  be  rather  narrow  and  somewhat  inclined,  to  prevent  the 
heavy  fumes  of  the  chlorine  compound  from  returning.  .S'is  the 
receiver  containing  water  (or,  if  antimony  is  present,  a  solution  of 
tartaric  acid  in  diluted  hydrochloric  acid),  F  is  a  U-tube  contain- 
ing water,  and  G  conducts  the  escaping  chlorine  into  a  carboy 
containing  moist  calcium  hydroxide. 

When  the  apparatus  is  arranged  weigh  off  the  substance  in  a 
narrow  glass  tube  sealed  at  one  end,  and  carefully  transfer 


*  18  parts  of  salt  mixed  with  15  parts  finely  powdered  manganese  dioxide  are 
treated  with  a  perfectly  cold  mixture  of  45  parts  sulphuric  acid  and  21  parts  of 
•water.  On  shaking,  chlorine  is  evolved,  and  when  the  evolution  slackens,  it 
may  be  promoted  by  a  gentle  heat. 


§  148.]  STLPIIUR.  565 

from  this  to  the  bulb  D  in  the  manner  shown  in  Fig.  106,  so  that 
no  part  of  the  substance  is  allowed  to  get 
into  the  ends  of  the  bulb-tube.  When 
the  apparatus  is  filled  with  chlorine,  con- 
nect D  and  (7,  and  allow  the  chlorine  to 
act  on  the  sulphide,  and  at  first  without 

applying  heat.  When  no  further  change  is  observed,  and  the 
receiver  E  is  completely  filled  with  chlorine,  heat  the  bulb  D 
gently,  and  take  care  to  keep  the  tube  0  warm  also,  in  order  to 
prevent  it  from  being  closed  up  by  the  sublimate  from  a  volatile 
chloride.  The  sulphide  is  completely  decomposed  by  the  chlo- 
rine, the  metals  being  converted  into  chlorides,  part  of  which 
remain  in  the  bulb  and  part  (the  volatile  ones,  like  antimony, 
arsenic,  and  mercury  chlorides)  distil  over  into  the  receiver.  The 
sulphur  unites  with  the  chlorine  to  form  sulphur  chloride,  which 
flows  into  the  receiver  E,  where,  coming  into  contact  with  water, 
it  decomposes,  hydrochloric  and  thiosulphuric  acids  being  formed 
and  sulphur  precipitating.  The  thiosulphuric  acid  in  turn  decom- 
poses into  sulphur  and  sulphurous  acid,  and  this  last  is  converted 
by  the  action  of  the  chlorine  in  ^into  sulphuric  acid.  The  final 
result  of  the  decomposition  is  hence  sulphuric  acid,  with  more  or 
less  precipitated  sulphur.  As  the  separation  of  sulphur  renders 
troublesome  the  further  treatment  of  the  contents  of  the  receiver, 
the  separation  is  usually  prevented  by  slowly  heating,  so  that  only 
small  portions  of  sulphur  chloride  reach  the  fluid  (saturated  with 
chlorine)  in  E.  The  operation  is  concluded  when  no  more  products- 
(excepting  perhaps  a  little  ferric  chloride,  the  complete  expulsion 
of  which  need  not  be  waited  for)  distil  over  from  the  bulb.  Then 
heat  the  bulb- tube  from  D  to  0  in  a  manner  to  drive  all  the 
sulphur  chloride  and  volatile  metallic  chlorides  into  E,  or  at  least 
to  the  end  of  the  bulb-tube. 

Let  the  apparatus  stand  undisturbed  for  a  short  time  longer, 
then  cut  off  the  tube  under  the  bend  0,  and  close  the  separated 
end,  containing  usually  a  part  of  the  volatile  chlorine  compounds, 
with  a  smooth  cork,  or  by  inverting  over  it  a  glass  tube  sealed  at 
one  end  and  moistened  within.  Let  the  whole  now  stand  for  24 
hours  to  allow  the  volatile  metallic  chlorides  to  absorb  moisture 
and  thus  become  soluble  in  water  without  generating  heat.  The 
chlorides  in  the  cut-off  end  are  dissolved  in  diluted  hydrochloric 


566  DETERMINATION.  [§  148. 

acid,  the  tube-end  rinsed  out,  and  the  solution  added  to  the  con- 
tents of  the  tubes  E  and  F\  a  very  gentle  heat  is  applied  until  the 
free  chlorine  is  expelled,  and  the  fluid  is  then  allowed  to  stand  until 
the  sulphur,  if  any  is  present,  has  solidified.  The  sulphur  is  filtered 
off  on  a  weighed  filter,  washed,  dried,  and  weighed.  The  filtrate  is 
precipitated  with  barium  chloride  (§  132),  by  which  operation  the 
amount  of  that  portion  of  the  sulphur  is  determined  which  has  been 
converted  into  sulphuric  acid.  The  fluid  filtered  from  the  barium 
sulphate  contains,  besides  the  excess  of  barium  chloride  added,  also 
the  volatile  metallic  chlorides,  which  latter  are  finally  determined 
in  it  by  the  proper  methods,  which  will  be  found  in  Section  Y. 

The  chloride  remaining  in  the  bulb-tube  is  either  at  once 
wc-ighed  as  such  (silver  chloride,  lead  chloride),  or  where  this  is 
impracticable — as  in  the  case  of  copper,  for  instance,  which  remains 
partly  as  cuprous,  partly  as  cupric  chloride — it  is  dissolved  in  water, 
hydrochloric  acid,  nitrohydrochloric  acid,  or  some  other  suitable 
solvent,  and  the  metal  or  metals  in  the  solution  are  determined  by 
the  methods  already  described,  or  which  will  be  found  in  Section 
V.  To  be  enabled  to  ascertain  the  weight  of  the  bulb-tube  con- 
taining silver  chloride,  it  .is  advisable  to  reduce  the  chloride  by 
hydrogen  gas,  and  then  dissolve  the  metal  in  nitric  acid. 

In  cases  where  you  have  only  to  estimate  the,  sulphur,  say  in 
substances  containing  also  sulphuric  acid,  O.  LINDT*  recommends 
conducting  the  chloride  of  sulphur  and  the  volatile  metallic 
chlorides  into  pure  solution  of  soda,  when  decomposition  immedi- 
ately takes  place,  producing  sodium  sulphide,  sodium  thiosulpliate, 
sodium  chloride,  and  hypochlorite.  When  the  decomposition  is 
over,  continue  passing  the  chlorine  for  two  hours  through  the  soda, 
evaporate  then  to  dryness,  ignite  the  residue  cautiously  to  destroy 
the  sodium  chlorate,  dissolve  in  water,  and  estimate  the  sulphuric 
acid  according  to  §  13i5. 

c.   Oxidation  ~by  Oxide  of  Mercury  (after  BUNSENJ. 
•  This  method,  which  will  be  found  in  detail,  §188,  is  particu- 
larly suited  to  the  estimation  of  sulphur  in  volatile  compounds,  or 
in  substances  which  when  heated  lose  sulphur. 

*Zeitschr.f.  analyt.  Chem.,  iv,  870. 


§  148.]  SULPHUR.  567 

2.  Methods  in  the  Wet  Way. 

a.  Oxidation  of  the  Sulphur  l»j  Acids  yielding  Oxygen,  or  ~by 
Halogens* 

a.  Weigh  the  finely  pulverized  sulphide  in  a  small  glass  tube 
sealed  at  one  end,  and  drop  the  tube  into  a  tolerably  capacious 
strong  bottle  with  glass  stopper,  which  contains  red  fuming  nitric 
acid  (perfectly  free  from  sulphuric  acid  f )  in  more  than  sufficient 
quantity  to  effect  the  decomposition  of  the  sulphide.  Immediately 
after  having  dropped  in  the  tube,  close  the  bottle.  When  the  action, 
which  is  very  impetuous  at  first,  has  somewhat  abated,  shake  the 
bottle  a  little ;  as  soon  as  this  operation  ceases  to  cause  renewed 
action,  and  the  fumes  in  the  flask  have  condensed,  take  out  the 
stopper,  rinse  this  with  a  little  nitric  acid  into  the  bottle,  and  then 
heat  the  latter  gently. 

aa.  The  whole  of  the  Sulphur  has  been  oxidized,  the  Fluid  is 
perfectly  clear :  J  Evaporate  with  some  sodium  chloride,  towards 
the  end  adding  pure  hydrochloric  acid  repeatedly,  cooling  the  dish 
each  time  before  adding  the  acid.  Dilute  with  much  water,  and 
determine  the  sulphuric  acid  as  directed  §  132.  Make  sure  that  the 
precipitate  is  pure  ;  if  it  is  not,  purify  it  according  to  §  132.  Separate 
the  bases  in  the  filtrate  from  the  excess  of  the  barium  salt  by  the 
methods  given  in  Section  Y. 

bb.  Undissolved  Sulphur  floats  in  the  Fluid:  Add  potassium 
chlorate  in  small  portions,  or  strong  hydrochloric  acid,  and  digest 
some  time  on  a  water-bath.  This  process  will  often  succeed  in  dis- 
solving the  whole  of  the  sulphur.  Should  this  not  be  the  case,  and 
the  undissolved  sulphur  appear  of  a  pure  yellow  color,  dilute  with 
water,  collect  on  a  weighed  filter,  wash  carefully,  dry,  and  weigh. 
After  weighing,  ignite  the  whole,  or  a  portion  of  it,  to  ascertain 
whether  it  is  perfectly  pure.  If  a  fixed  residue  remains  (consisting 

*  In  presence  of  lead,  barium,  strontium,  calcium,  tin,  and  antimony,  method 
b  is  preferable  to  a. 

f  To  test  for  sulphuric  acid  in  nitric  or  hydrochloric  acid,  it  is  necessary  to 
evaporate  on  a  water-bath  nearly  to  dryness  and  take  up  with  water  before  add- 
ing barium  chloride.  When  the  acid  cannot  be  got  pure,  determine  the  sul- 
phuric acid  and  allow  for  it. 

%  This  can  of  course  be  the  case  only  in  absence  of  metals  forming  insoluble 
salts  with  sulphuric  acid.  If  such  metals  are  present,  proceed  as  in  bb,  as  it  is 
in  that  case  less  easy  to  judge  whether  complete  oxidation  of  the  sulphur  has 
been  attained. 


568  DETERMINATION.  [§  148.. 

commonly  of  quartz,  gangue,  &c.,  but  possibly  also  of  lead  sul- 
phate, barium  sulphate,  &e.),  deduct  its  weight  from  that  of  the 
impure  sulphur.  In  the  filtered  fluid  determine  the  sulphuric  acid 
as  in  aa,  calculate  the  sulphur  in  it,  and  add  the  amount  to  that  of 
the  undissolved  sulphur.  If  the  residue  left  upon  the  ignition  of 
the  undissolved  sulphur  contains  an  insoluble  sulphate,  decompose 
this  as  directed  in  §132,  and  add  the  sulphur  found  in  it  to  the 
principal  amount. 

In  the  presence  of  bismuth,  the  addition  of  potassium  chlorate 
or  of  hydrochloric  acid,  is  not  advisable,  as  chlorine  interferes  with 
the  determination  of  bismuth. 

/?.  Mix  the  finely  pulverized  metallic  sulphide  in  a  dry  flask, 
by  shaking,  with  powdered  potassium  chlorate  (free  from  sulphuric 
acid),  and  add  moderately  concentrated  hydrochloric  acid  in  small 
portions.  Cover  the  flask  with  a  watch-glass,  or  with  an  inverted 
small  flask.  After  digestion  in  the  cold  for  some  time,  heat  gently,, 
finally  on  the  water-bath,  until  the  fluid  smells  no  longer  of  chlo- 
rine. Proceed  now  as  directed  in  <r,  aa,  or  bb,  according  as  the 
sulphur  is  completely  dissolved  or  not.  In  the  latter  case  you  must 
of  course  immediately  dilute  and  filter.  The  oxidation  of  the  sul- 
phur may  be  usually  effected  more  quickly  and  completely  by 
warming  with  nitric  acid  of  1-36  sp.  gr.  on  a  water-bath,  and  add- 
ing potassium  chlorate  in  small  portions.  Compare  STOKER,*  PEAR- 
SON, and  BowDiTCH.f 

y.  Aqua  regia  is  also  frequently  used.  J.  LEFORT^:  recommends 
a  mixture  of  1  part  strong  hydrochloric  acid  and  3  parts  strongest 
nitric  acid.  Complete  conversion  of  sulphur  into  sulphuric  acidr 
however,  is  rarely  effected  by  aqua  regia. 

d.  Bromine  may  also  be  used.  Pyrites  or  blende  is  digested  at 
a  gentle  heat  with  water,  and  bromine  gradually  added.  If  the  sul- 
phides have  been  prepared  in  the  wet  way,  good  bromine  water  is 
sufficient  to  oxidize  them.  P.  WAAGE§  prefers  bromine  to  all  other 
wet  agents,  and  advises  its  purification  by  distillation  in  an  appa- 
ratus from  which  all  caoutchouc  connections  are  excluded. 

b.  Oxidation  of  the  Sulphur  by  Chlorine  in  Alkaline  Solution, 
after  EIVOT,  BEUDANT,  <m^DAGuiN.||  (Suitable  also  for  determining 
the  sulphur  in  the  crude  article.) 

*  Z :'itsdir.f.  analyt.  Chem.,  ix,  71.  f  76.,  ix,  82.  J  7&.,  ix,  81. 

§  Ib  ,  x,  206. 

I  Compt.  rend.,  1835,  865  ;  Jvurn.  f.  prakt.  Chem.,  LXT,  134, 


§  148.]  SULPHUR.  569 

Heat  the  very  finely  pulverized  sulphide  or  crude  sulphur  for 
several  hours  with  solution  of  potassa  free  from  sulphuric  acid 
(which  dissolves  free  sulphur,  as  well  as  the  sulphides  of  arsenic 
and  antimony),  and  then  conduct  chlorine  into  the  fluid.  This 
speedily  oxidizes  the  sulphur;  the  sulphuric  acid  formed  combines 
with  the  potassa  to  sulphate,  which  dissolves  in  the  fluid,  whilst 
the  metals  converted  into  oxides  remain  undissolved.  Filter,  acid- 
ify the  alkaline  filtrate,  and  precipitate  the  sulphuric  acid  by  barium 
chloride  (§  132).  Arsenic  and  antimony  pass  into  the  alkaline 
solution  in  the  form  of  acids,  but  not  so  lead,  which  is  converted 
into  binoxide,  and  remains  completely  undissolved.  This  method 
is,  therefore,  particularly  suitable  in  presence  of  lead  sulphide.  In 
presence  of  iron  sulphide,  potassium  sulphate  is  formed  at  first, 
and  ferric  hydroxide,  which,  if  the  action  of  the  chlorine  is  allowed 
to  continue,  begins  to  be  converted  into  potassium  ferrate.  As 
soon,  therefore,  as  the  fluid  commences  to  acquire  a  red  tint  the 
transmission  of  chlorine  must  be  discontinued,  and  the  fluid  gently- 
heated  for  a  few  moments  with  powdered  quartz,  to  decompose  the 
ferric  acid. 

It  occasionally  happens,  more  particularly  in  presence  of  sand,. 
iron  pyrites,  cupric  oxide,  &c.,  that  the  process  is  attended  with 
impetuous  disengagement  of  oxygen,  which  almost  completely  pre- 
vents the  oxidizing  action  of  the  chlorine.  However,  this  acci- 
dent may  be  guarded  against  by  reducing  the  substance  to  the 
very  finest  powder. 

c.  Regarding  the  method  of  CLOEZ  and  GUIGNET  (oxidation 
by  potassium  permanganate),  see  the  Analysis  of  Gunpowder  in 
the  Special  Part. 

B.  METHODS  BASED  ON  THE  CONVERSION  OF  THE  SULPHUR  INTO- 
HYDROGEN   SULPHIDE,   OR  A  METALLIC  SULPHIDE. 

a.  The  determination  of  the  sulphur  in  the  sulphides  of  the 
metals  of  the  alkalies  and  alkaline  earths  soluble  in  water  is  best 
effected — provided  they  are  free  from  excess  of  sulphur — by  I. , 
5,  or  c.  In  the  absence  of  acids  of  sulphur  you  may  also  convert 
the  sulphur  into  sulphuric  acid  by  bromine  water.  The  bases  are 
conveniently  estimated  in  a  separate  portion,  which  is  decomposed 
by  evaporation  with  hydrochloric  or  sulphuric  acid,  or — when 
none  but  alkali  metals  are  present — by  ignition  with  5  parts  of 
ammonium  chloride  in  a  porcelain  crucible.  If  the  compounds 


570  DETERMINATION.  [§  148. 

contain  excess  of  sulphur,  they  should  be  oxidized  either  by  chlo- 
rine in  alkaline  solution,  or  treated  according  to  B,  c,  or  C.  If 
they  contain  thiosulphate  or  sulphite,  proceed  according  to  §  168. 

b.  The  sulphur  contained  in  alkaline  liuids  as  monosulphide  or 
hydrosulphate  of  the  sulphide  may  also  be  determined  directly  by 
volumetric  analysis,  by  means  of  a  standard  ammoniacal  silver  or 
copper    solution.      In  using    the   former,  mix  the  solution  with 
ammonia,  heat,  and  add  the  standard  fluid  till,  on  filtering  off  a 
small  portion  and  adding  silver  solution,  a  mere  opalescence  is 
produced  (.LESTELLE*).     In  using  the  copper  solution,  mix  the 
fluid  to  be  tested  with  ammonia,  heat  to  50°  or  60°,  and  add  the 
standard  solution,  frequently  shaking  and  boiling  till  no  further 
precipitation  of  CuO.    5CuS  is  produced,  and  the  solution  begins  to 
be  blue  (VERSTRAET  f).    To  make  a  standard  copper  solution  1  c.  c. 
of  which  shall  equal  O'Ol,  Na.,8,   dissolve  9-763  pure  copper  in 
40  grin,  nitric  acid,  boil,  add  180  to  200  c.  c.  ammonia  and  water 
to  1  litre.     These  methods  are  well  adapted  for  technical  pur- 
poses, for  the  estimation  of  sulphide  in  soda  lyes,  for  instance.    It 
need  hardly  be  added  that  precipitated  silver,  copper,  or  lead  sul- 
phide (if  you  have  used  a  solution  of  oxide  of  lead  in  potassa)  may 
be  estimated  gravimetrically. 

c.  If  all  the  sulphur  can  be  expelled  from  the  substance  in  the 
form  of  sulphuretted  hydrogen  by  heating  with  hydrochloric  acid, 
the  sulphide  may  be  heated  in  a  small  flask  with  the  concentrated 
acid  to  complete  decomposition  and  expulsion  of  the  hydrogen 
sulphide,  the  latter  being    determined    according    to  I.     If  the 
substance  is  a  liquid,  the  hydrogen-sulphide  apparatus  shown  on 
page  49  i  may  be  used  for  the  expulsion  of  the  carbonic-acid  gas. 
In  this  case,  however,  the  tube  h  is  replaced  by  a  small  upright  con- 
denser (see  Special  Part  under  "Analysis  of  Black  Ash").     In 
the  case  of  polysulphides,  the  sulphur  separated  in  the  evolution 
flask  is  collected  on  a  filter  dried  at  100°,  washed,  dried  first  at 
70°,  then  for  a  short  time  at  100°,  and  weighed. 

C.  METHOD  BASED  ON  THE  SEPARATION  AND  WEIGHING  OF  THE 

SULPHUR. 

M.    MORTREUX  J  recommends  the  following  process  for  esti- 
mating sulphur  in  alkali  polysulphides:    Extract  10  grin,    with 
*  Zeitschr.f.  analyt.  Chem.,  n,  94.  f  lb.,  iv,  216.  \  Ib.,  i,  390. 


§149.]  NITRIC    ACID.  571 

boiled  water,  make  up  the  filtrate  to  100  grm.  (or  c.  c.)  with  the 
washings  of  the  residue,  and  transfer  10  grm.  (or  c.  c.) — represent- 
ing the  soluble  constituents  of  1  grm.  substance — to  a  burette 
provided  with  a  glass  cock  and  of  40  to  50  c.  c.  capacity ;  the 
lower  point  should  be  narrow  and  cut  off  obliquely.  Now  add, 
shaking  the  stoppered  burette  occasionally,  sufficient  of  a  solution  of 
1  part  of  iodine  dissolved  in  5  parts  of  potassium  iodide  and  50  parts 
water  to  just  decolorize  the  solution,  and  until  a  portion  of  the 
fluid  ceases  to  brown  paper  saturated  with  iron  sulphate  and  dried. 
Now  add  8  to  10  c.  c.  carbon  disulphide,  stopper,  and  shake, 
keeping  the  finger  on  the  stopper.  Keep  the  burette  inverted  for 
a  while,  then  place  upright,  and  allow  almost  all  of  the  solution  of 
sulphur  in  the  carbon  disulphide  to  flow  into  a  weighed  dish ;  in- 
troduce a  fresh  portion  of  carbon  disulphide  into  the  burette, 
mix,  allow  to  run  out,  and  repeat  the  operation  once  more. 
After  evaporating  the  carbon  disulphide  weigh  the  residual 
sulphur. 

Third  Group. 

NITRIC    ACID CHLORIC    ACID. 

§  149. 

1.  NITRIC  ACID. 
I.  Determination. 

Pree  nitric  acid  in  a  solution  containing  no  other  acid  is  deter- 
mined most  simply  in  the  volumetric  way  by  neutralizing  with  a 
dilute  solution  of  soda  or  ammonia  of  known  strength  (comp.  Spe- 
cial Part,  "Acidimetry  ").  The  following  method  also  effects 
the  same  purpose  :  Mix  the  solution  with  baryta  water  until  the 
reaction  is  just  alkaline,  evaporate  slowly  in  the  air,  nearly  to  dry- 
ness,  dilute  the  residue  with  water,  filter  the  solution  which  has 
ceased  to  be  alkaline,  wash  the  barium  carbonate  formed  by  the 
action  of  the  carbonic  acid  of  the  atmosphere  upon  the  excess  of 
the  baryta  water,  add  the  washings  to  the  filtrate,  and  determine 
in  the  fluid  the  barium  as  directed  in  §  101.  Calculate  for  each 
1  at.  barium  2  mol.  nitric  acid.  Lastly,  free  nitric  acid  may  also 
be  determined  in  a  simple  manner  by  supersaturating  with  am- 
monia, evaporating  in  a  weighed  platinum  dish,  drying  the  resi- 
due at  110°  to  120°,  and  weighing  the  NH4NO,  (SCHAFFGOTSCH). 


572  DETERMINATION.  [ 

Of  course,  the  results  can  be  accurate  only  then  when  the  ammo- 
nia employed  leaves  no  residue  when  evaporated  from  platinum. 

II.   Separation  of  nitric  acid  from  the  basic  radicals  ^ 
and  determination  of  the  acid  in  nitrates. 

The  determination  of  combined  nitric  acid  is  an  important  and 
at  times  difficult  problem,  the  solution  of  which  has  engaged  the 
attention  of  many  chemists  of  late.  I  would  advise  that,  after 
selecting  the  method  it  is  intended  to  use,  it  be  tried  repeatedly 
on  weighed  quantities  of  a  pure  nitrate  in  order  to  become  per- 
fectly familiar  with  the  method,  and  to  acquire  the  skill  without 
which  accuracy  cannot  be  expected  in  the  frequently  complicated 
processes.  Of  the  great  number  of  methods  proposed  I  shall 
confine  myself  to  describing  only  the  simplest  and  best. 

a.  Methods  based  on  the  decomposition  of  Nitrates  in  the 
Dry  Way. 

of.  In  anhydrous  metallic  nitrates  which  leave  upon  ignition  a. 
metallic  oxide  of  known  and  definite  composition,  the  nitric  acid 
may  be  determined  by  ignition  and  calculation  from  the  weight  of 
the  residue. 

/3.  In  the  case  of  nitrates  the  residue  of  which  on  ignition 
has  no  constant  composition,  or  by  the  ignition  of  which  the  cru- 
cible is  much  attacked  (alkali  and  alkali-earth  nitrates),  fuse  the 
substance  (which  must  be  anhydrous  and  also  free  from  organic 
and  other  volatile  bodies)  with  a  non-volatile  flux,  and  estimate 
the  nitric  acid  from  the  loss.  The  following  have  been  proposed 
as  fluxes  :  Borax  glass,  by  SCHAFFGOTSCH  *  (3  parts  borax  glass  to 
1  part  nitrate) ;  potassium  bichromate,  by  PERSOZ  f  (2  parts  of 
the  bichromate  to  1  part  nitrate),  and  silicic  acid,  by  REICH.  J 
All  three  give  satisfactory  results  when  careful  regard  is  paid  to 
the  peculiarities  of  the  individual  fluxes. §  Silicic  acid  is  the  best 
flux,  as  it  may  be  readily  procured,  and  the  execution  is  the  most 
easy  and  the  most  certain  to  succeed.  I  shall  describe  the  method 
in  its  application  to  potassium  or  sodium  nitrate. 

Fuse  the  latter  at  a  low  temperature,  pour  out  on  to  a  warm. 

*Pogg.  Annal,  LVIT,  260. 

}  Rep.  de  chim.  appliquee,  1861,  253;  Zeitschr.  /.  analyt.  Ghem.,  T,  85. 
\Berg-  und  Uuttenmdnnische  Zeitschrift,  1861,  No.  21;  Zeitschr.  /.  analyt.. 
,  i,  86.  §  Zeitschr.  f.  analyt.  Ghem.,  i,  181. 


§  149.]  NITRIC   ACID.  573 

porcelain  dish,  powder,  and  dry  again  before  weighing.  Now 
transfer  to  a  platinum  crucible  2  to  3  grm.  powdered  quartz,  ignite 
well,  and  weigh  after  cooling.  Add  about  0*5  grin,  of  the  salt 
prepared  as  above,  mix  well,  and  convince  yourself  by  the  balance 
that  nothing  has  been  lost  during  mixing.  The  covered  crucible 
is  then  exposed  to  a  low  red  heat  (just  visible  by  day)  for  half  an 
hour  and  weighed,  after  cooling,  with  the  cover.  The  loss  of 
weight  represents  the  quantity  of  NaO6.  Sulphates  or  chlorides 
are  not  decomposed  at  the  given  temperature ;  if  a  higher  heat  be 
applied,  the  latter  may  volatilize.  The  action  of  reducing  gases 
must  be  avoided.  The  test  analyses,  communicated  by  REICH  (loo. 
cit.\  as  well  as  those  performed  in  my  own  laboratory,*  gave  very 
satisfactory  results. 

1).  Method  'based' on  the  distillation  of  Nitric  Acid. 

All  nitrates  may  be  decomposed  by  distillation  with  moderately 
dilute  sulphuric  acid.  The  nitric  acid  passing  into  the  receiver 
may  then  be  determined  according  to  I,  volumetrically  or  gravi- 
metrically.  This  process  was  originally  proposed  by  GLADSTONE  f, 
but  was  later  carefully  studied  by  II.  ROSE  and  FINKENER  J.  1 
to  2  grm.  of  the  nitrate  should  be  treated  with  a  cooled  mixture 
of  1  volume  concentrated  sulphuric  acid  and  2  volumes  water. 
For  1  grm.  nitre  take  5  c.  c.  sulphuric  acid  and  10  c.  c.  water. 
The  distillation  may  be  performed  either  with  a  thermometer  at 
160°  to  170°  in  a  paraffin-  or  sand-bath  (duration  of  the  distilla- 
tion for  1  to  2  grrn.  nitre,  3  to  4  hours)  or  in  vacuo,  with  the  use 
of  a  water-bath.  The  latter  process  is  the  better.  In  the  former, 
the  neck  of  the  tubulated  retort  (which  is  drawn  out  and  bent 
down)  is  connected  writh  a  bulbed  U-tube§  containing  a  measured 
quantity  of  standard  soda  or  potassa  solution  (§  215).  The  dis- 
tillation in  vacuo  may  be  conducted,  without  the  use  of  an  air- 
pump,  according  to  FINKENER,  as  follows :  Transfer  the  measured 
quantity  of  water  and  concentrated  sulphuric  acid  to  the  tubulated 
retort,  arid  the  necessary  quantity  of  standard  potassa  or  soda  solu- 
tion, diluted  to  30  c.  c.,  to  a  flask  with  a  narrow  neck  of  about 
200  c.c.  capacity.  Then,  by  means  of  an  india-rubber  tube,  con- 

*  Zeitschr.  f.  analyt.  Chem.,  i,  184.  f  Jo  urn.  f.  prakt.  Chem.,  LXIV,  442. 

IZeitschr.f.  analyt.  Chem.,  i,  309. 

§  The  bulbed  U-tube  will  be  found  figured  in  §  185. 


574  DETERMINATION.  [§  149. 

nect  the  flask  with  the  retort  air-tight,  so  that  the  drawn-out  point 
of  the  latter  may  extend  to  the  body  of  the  flask,  and — with  tub- 
ulure  open — heat  the  contents  of  the  retort  and  of  the  flask  to 
boiling.  "When  the  air  has  been  expelled  from  the  apparatus  by 
long  boiling,  transfer  the  salt  (weighed  in  a  small  tube)  to  the 
retort  through  the  tubulure,  close  the  latter  immediately,  and 
at  the  same  time  take  away  the  lamp.  The  retort  is  then  heated 
on  a  water-bath,  the  flask  being  kept  cool.  The  quantity  of 
nitric  acid  that  has  passed  over  is  finally  ascertained  by  determin- 
ing the  still  free  alkali  with  standard  acid.  If  it  is  suspected  that 
all  the  nitric  acid  has  not  been  driven  into  the  receiver  by  one 
distillation,  you  may — by  heating  the  flask  and  cooling  the  retort 
— distil  the  water  back  into  the  latter,  and  then  the  distillation 
from  the  retort  may  be  repeated.  The  distillate  thus  obtained  is 
always  free  from  sulphuric  acid,  hence  the  results  are  very  exact. 
The  base  remains  as  sulphate  in  the  retort.  In  the  presence  of 
chloride  add  to  the  contents  of  the  retort  a  sufficiency  of  dissolved 
silver  sulphate,  or — when  much  chloride  is  present — moist  silver 
oxide.  The  nitric  acid  is  then  obtained  entirely  free  from 
chlorine. 

c.  Methods  based  on  the  decomposition  of  Nitrates  ~by  Alka- 
lies and  Alkali  Earths. 

a.  Nitrates  of  metals  which  are  completely  precipitated  by 
alkali  hydroxides  or  carbonates — provided  basic  salts  are  not  pre- 
cipitated at  the  same  time — may  be  analyzed  by  simple  boiling 
with  an  excess  of  standard  potassa  or  soda  or  their  carbonates. 
After  cooling,  dilute  to  J  or  •£•  litre,  mix,  allow  to  settle,  draw  off 
a  portion  of  the  supernatant  clear  fluid,  determine  the  free  alkali 
remaining  in  it,  and  calculate  therefrom  the  amount  which  ha& 
been  converted  into  nitrate.  This  method  was  used  by  LANGER 
and  WAWNIEKIEWICZ,  *  but  was,  however,  already  previously  known. 
HAYES  obtained  with  silver  and  bismuth  nitrates  good  results ;  but 
with  rnercurous  nitrate  (using  sodium  carbonate)  the  results  wrere 
not  so  satisfactory. f  If  the  method  is  applied  to  ammonium 
nitrate,  heat  must  be  applied  after  adding  the  alkali,  until  all  the 
ammonia  is  expelled.  That  the  method  is  applicable  only  when 
no  other  acid  is  present  need  scarcely  be  mentioned. 

*  Annal.  d.  Chem.  u.  Pharm.,  cxvir,  230. 
|H.  ROSE,  Zeitschrift  f.  analyt.  Chem.,  i,  306. 


*  >  X 

or  THE  \ 

UNIVERSITY    } 

OF 

. 
§149.]  NITRIC   ACID.  575 

fi.  Iii  nitrates  from  which  the  basic  metals  are  precipitated  by 
barium  or  calcium  hydroxides  or  their  carbonates  (or  by  barium 
sulphydrate,  recently  precipitated  and  free  from  barium  thiosul- 
phate  —  GLAUS*),  the  nitric  acid  may  be  estimated  with  great 
accuracy  —  if  no  other  acids  are  present  —  by  filtering,  after  pre- 
cipitation has  been  effected,  warm  or  cold,  passing  carbonic  acid 
through  the  filtrate,  if  necessary,  till  all  the  barium  is  precipitated, 
warming,  filtering,  and  determining  the  barium  in  the  filtrate  by 
sulphuric  acid.  1  at.  of  the  barium  corresponds  to  1  mol.  nitric 
anhydride  (NaO5).  In  case  of  bismuth-nitrate,  boil  after  adding 
the  barium  hydroxide  until  the  separated  oxide  is  perfectly  yellow. 
(HUGE;  LuDDECKEf). 

y.  In  many  nitrates  the  bases  of  which  are  precipitable  by 
hydrogen  sulphide,  the  nitric  acid  may  be  determined  accord- 
ing to  GIBBS  by  adding  to  the  salt  in  solution  about  its  own  weight 
of  some  neutral  organic  salt,  e.g.,  Rochelle  salt,  and  throwing 
down  the  metal  by  H2S.  The  filtrate  and  washings  are  brought 
to  a  definite  bulk,  and  the  free  acid  is  determined  in  aliquot  por- 
tions alkalimetrically.J 

d.  Methods  based  upon  the  decomposition  of  Nitric  Acid  ~by 
Ferrous  Chloride. 

a.  PELOUZE  If  was  the  first  to  utilize  the  action  of  free  nitric 
acid  on  ferrous  chloride  in  the  determination  of  nitric  acid.  The 
decomposition  is  as  follows  : 


6FeCla  +  2KX03  +  8HC1  =  3Fe,Cl.  +  2KC1  +  4HaO  +  NaOa. 

PELOTJZE  used  a  known  quantity  of  ferrous  chloride  in  excess, 
and  titrated  the  excess  with  potassium  permanganate.  The 
method  used  by  him,  and  given  in  the  foot-note,  §  affords  variable 
results,  sometimes  good,  sometimes  not  reliable  ;  on  this  point  all 

*  Zeitsehr.  /.  analyt.  Chem.,  I,  372.  \lb.,  vi,  233. 

|  Amer.  Jour.  Sci.t  XLIV,  209. 

If  Journ.  /.  prakt.  Chem.  XL,  324. 

§  Dissolve  2  grm.  piano  wire  in  80  to  100  c.  c.  pure  concentrated  hydrochloric 
acid  with  the  aid  of  heat,  in  a  150-c.  c.  flask  the  cork  of  which  is  fitted  with  a 
glass  tube.  Then  add  1'  2  grm.  of  the  potassium  nitrate,  or  an  equivalent  quan- 
tity of  another  nitrate  to  be  analyzed,  stopper,  and  rapidly  heat  to  boiling. 
After  five  or  six  minutes  pour  the  fluid,  which  has  again  cleared,  into  a  larger 
flask,  dilute  with  much  water,  and  estimate  the  ferrous  chloride  present  with. 
permanganate. 


576  DETERMINATION.  [§  149. 

who  used  the  method  agree  (compare  FK.  MOHR,*  and  ABEL  and 
BLOXAM  f).  The  numerous  experiments  made  in  my  own  labor- 
atory also  confirm  this.  The  reasons  for  the  lack  of  accuracy  of 
the  method  are  as  follows : 

a.  Action  of  the  air  on  the  nitric  oxide  in  the  flask  in  the 
presence  of  the  aqueous  vapor  therein,  whereby  nitric  acid  is 
regenerated ;  this  is  the  principal  cause  of  inaccuracy. 

l>.  Incomplete  expulsion  of  the  nitric  oxide  from  the  liquid, 
in  consequence  of  which  more  permanganate  is  reduced  than  cor- 
responds to  the  ferrous  chloride  present;  this  is  to  be  appre- 
hended only  in  dilute  solutions. 

c.  Escape  of  nitric  acid  before  it  has  acted  upon  the  ferrous 
chloride;    this  is  to  be  apprehended  when  the  liquid  is  boiled 
rapidly  after  adding  the  nitrate,  and  when  the  excess  of  ferrous 
chloride  is  comparatively  small. 

d.  Occasionally  loss  of  iron  from  incautious  boiling ;   this  is  to 
be  apprehended  when  a  part  of  the  ferrous  chloride  deposits  in 
solid  form  on  the  sides  of  the  vessel  above  the  fluid. 

I  have  succeeded  in  so  modifying  the  process  as  to  avoid  all 
these  sources  of  error  and  to  obtain  results  which,  so  far  as  relia- 
bility arid  accuracy  are  concerned,  a.re  perfectly  satisfactory.  My 
process  is  as  follows : 

Select  a  tubulated  retort  of  about  200  c.  c.  capacity,  with  a 
long  neck,  and  fix  it  so  that  the  latter  is  inclined  a  little  upwards. 
Introduce  into  the  body  of  the  retort  about  1*5  grm.  fine  piano- 
forte wire,  accurately  weighed,  and  add  about  30  or  40  c.  c.  pure 
fuming  hydrochloric  acid.  Conduct  now  through  the  tubulure,  by 
means  of  a  glass  tube  reaching  only  about  2  cm.  into  the  retort, 
hydrogen  gas  washed  by  solution  of  potassa,  or  pure  carbonic  acid, 
and  connect  the  neck  of  the  retort  with  a  U-tube  containing  some 
water.  Place  the  body  of  the  retort  on  a  water-bath,  and  heat 
gently  until  the  iron  is  dissolved.  Let  the  contents  of  the  retort 
cool  in  the  current  of  hydrogen  gas  or  carbonic  acid ;  increase  the 
latter,  and  drop  in,  through  the  neck  of  the  retort,  into  the  body, 
a  small  tube  containing  a  weighed  portion  of  the  nitrate  under 
examination,  which  should  not  contain  more  than  about  0*2  erm. 

*  LehrbucJi  der  Titrirmeihode,  i,  216. 

f  Quart.  Journ.  of  the  Chem.  Soc.,  ix,   97 ;    also  Journ.  f.  prakt.  Chem., 
I.XIX,  262. 


§  149.]  NITRIC   ACID.  577 

of  NaO5.  After  restoring  the  connection  between  the  neck  and. 
the  U-tube,  heat  the  contents  of  the  retort  in  the  water-bath  for 
about  a  quarter  of  an  hour,  then  remove  the  water-bath,  heat  with 
the  lamp  to  boiling,  until  the  fluid,  to  which  the  nitric  oxide  had 
imparted  a  dark  tint,  shows  the  color  of  ferric  chloride,  and  con- 
tinue boiling  for  some  minutes  longer.  Care  must  be  taken  to 
give  the  fluid  an  occasional  shake,  to  prevent  the  deposition  of  dry 
salt  on  the  sides  of  the  retort.  Before  discontinuing  boiling, 
increase  the  current  of  hydrogen  or  carbonic-acid  gas,  so  that  no  air 
may  enter  through  the  U-tube  when  the  lamp  is  removed.  Let  the 
contents  cool  in  the  current  of  gas,  dilute  copiously  with  water,  and 
determine  the  iron  still  present  as  ferrous  chloride  volumetrically 
by  potassium  dichromate  or  permanganate  —  335-4  of  iron  con- 
verted by  the  nitric  acid  from  ferrous  to  ferric  chloride  correspond 
to  108*08  (NaOB).  My  test-analyses  of  pure  potassium  nitrate 
gave  100-1  —  100-03  —  100-03,  and  100-05,  instead  of  100.* 
[The  iron  remaining  as  ferric  chloride  may  also  be  determined  by 
sodium  thiosulphate.] 

fi.  Since  we  have  learned  to  titrate  ferric  salts  di- 
rectly with  accuracy,  it  is,  as  a  rule,  more  convenient  and 
exact  not  to  estimate  (as  in  a)  the  residual  ferrous  salt,  but  to 
determine  the  ferric  salt  produced,  as  first  pointed  out  by  C.  D. 


I  would  recommend  the  following  method  as  the  best.  J  Besides 
the  requisites  for  titrating  ferric  chloride  by  means  of  stannous 
chloride  (p.  327),  there  is  required  a  solution  prepared  by  dissolving 
100  grin,  ferrous  sulphate  as  free  from  ferric  salt  as  possible  in  150 
to  200  c.  c.  of  hydrochloric  acid  (sp.  gr.  1-1  —  1-2)  by  the  aid  of 
heat  in  a  500-c.  c.  flask,  and  finally  filling  the  flask  to  the  mark 
with  fuming  hydrochloric  acid,  and  shaking.  As,  however,  a 
solution  absolutely  free  from  ferric  salt  cannot  be  obtained,  esti- 
mate according  to  the  method  given  on  page  327  how  much  stan- 
nous chloride  solution  is  required  to  reduce  the  ferric  chloride 
present  in  50  c.  c.  of  the  solution  made  as  a~bove.  It  is  advisable 

*  Annal.  de  Chem.  u.  Pharm.,  cvi,  217. 

\Journ.f-  prakt.  Chem.,  LXXXI,  421. 

i  It  is  particularly  convenient  when  several  analyses  are  to  be  made  ;  when 
only  one  or  two  estimations  are  to  be  made,  however,  the  iron  wire  may  be  dis- 
solved in  hydrochloric  acid  as  in  a. 


578  DETERMINATION.  [§  149^ 

to  heat  the  solution  in  an  atmosphere  of  carbon  dioxide,  and  to 
titrate  it  either  immediately  before  or  directly  after  the 
analysis. 

Place  the  weighed  nitrate  (the  quantity  must  be  such  as  to  con- 
tain not  more  than  0*2  grin,  nitric  acid)  in  a  long-necked  flasK, 
provided  with  a  doubly- perforated  stopper  carrying  two  glass 
tubes.  One  of  these  reaches  to  the  body  of  the  flask,  while  the 
other  but  just  enters  it.  Through  the  former  pass  in  a  current  of 
carbon  dioxide,  and  when  all  the  air  has  been  displaced  introduce 
50  c.  c.  of  the  hydrochloric-acid  solution  of  ferrous  sulphate ;  con- 
tinue to  pass  in  the  carbon  dioxide  for  some  time  longer,  then  heat, 
at  first  gently  for  some  time,  and  then  gradually  to  boiling,  until 
the  liquid  has  lost  its  blackish  color  and  exhibits  the  pure  color  of 
ferric  chloride,  and  until  the  escaping  gas,  passed  into  dilute  starch 
paste  containing  a  little  potassium  iodide,  ceases  to  exhibit  the 
blue  color  of  starch  iodide.  Now  remove  the  stopper  from  the 
flask,  rinse  off  the  longer  tube  if  necessary,  dilute  the  residue 
with  double  its  volume  of  water,  and  estimate  the  ferric  chloride 
as  on  p.  327.  The  cooling  for  determining  the  slight  excess  of 
stannous  chloride  with  iodine  is  best  conducted  in  a  current  of  car- 
bon dioxide.  From  the  stannous-chloride  solution  used  altogether 
deduct  first  the  small  excess  ascertained  by  the  iodine  solution, 
and  then  the  small  quantity  corresponding  to  the  ferric  salt  pre- 
sent in  50  c.  c.  of  the  acid  ferrous-sulphate  solution.  The  re- 
mainder gives  the  iron  in  the  ferric  salt  produced ;  and,  when 
multiplied  by  0*32224,  gives  the  nitric  acid.  This  factor  is  ob- 
tained thus :  6  eq.  of  iron  (335-4) :  1  eq.  of  N2OB  (108 -08) :  :  ferric 
iron  present  :  x. 

It  will  be  seen  that  it  is  best,  once  for  all,  to  multiply  the- 
known  quantity  of  iron  in  the  ferric-chloride  solution  used  for 
standardizing  the  stannous-chloride  solution  by  the  above  factor, 
and  to  mark  the  product  on  the  label  as  the  quantity  of  nitric 
acid  corresponding  to  10  c.  c.  of  ferric -chloride  solution.  If  no 
standardized  ferric-chloride  solution  is  at  hand,  the  stannous-chlo- 
ride solution  may  be  standardized  directly  against  nitric  acid  by 
adding  a  weighed  quantity  of  pure  potassium  nitrate  to  50  c.  c.  of 
the  acid  ferrous-sulphate  solution,  and  then  determining  the  fer- 
ric chloride  formed  according  to  the  method  given  above.  The 


§  149.]  NITRIC  ACID.  679 

results  are  perfectly  satisfactory  if  the  process  is  properly  carried 
out,  and  the  estimations  succeed  each  other  immediately.* 

y.  SCHLOSING'S  method,  f  which  was  employed  more  espe- 
cially for  estimating  nitric  acid  in  tobacco,  affords  the  important 
advantage  that  it  ^may  be  employed  in  the  presence  of  organic 
matter.  Numerous  experiments  have  shown  this  method  to  be 
thoroughly  satisfactory.  It  is  conducted  in  the  apparatus  shown 
in  Fig.  107. 


Fig.  107. 

The  dissolved  nitrate  is  introduced  into  the  flask  A,  the 
drawn-out  neck  of  which  is  connected  by  means  of  a  rubber  tube, 
a,  with  a  narrow  glass  tube,  &;  c  is  another  rubber  tube,  15  cm. 
long,  and  connected  with  ~b.  The  solution  of  the  salt,  which 
must  be  neutral  or  alkaline,  is  boiled  down  to  a  small  volume, 
the  aqueous  vapor  completely  expelling  all  the  air  from  A  and 
the  tubes,  c  is  then  immersed  in  a  solution  of  ferrous  chloride 
in  hydrochloric  acid  contained  in  a  glass  vessel,  the  lamp  removed, 
and  the  receding  regulated  by  compressing  the  tube  c  with  the 
fingers.  When  the  iron  solution  is  almost  entirely  absorbed  a 
little  hydrochloric  acid  is  allowed  to  recede,  in  separate  portions, 
three  or  four  times,  in  order  to  entirely  free  the  tube  from  fer- 
rous chloride,  this  being  absolutely  necessary.  Before  the  air  can 
force  its  way  into  the  tubes  c  is  closed  by  means  of  an  iron  com- 
pression-cock, and  its  end  immersed  into  the  mercury  in  the 
trough  and  brought  up  under  the  bell  B.  The  lamp  is  again 

*  Zeit&clir.  /.  analyt.  Chem.,  i,  38.     HOLLAND  gives  a  method  in  which  the 
use  of  an  indifferent  gas  is  unnecessary.      Zeitschr.  f.  analyt.  Chem.,  vin,  452  ; 

.  News,  xvn,  219. 
\Annal.  de  Chem.  3  ser.,  XL,  479  ;   Journ.f.  prakt.,  Chem.,  LXII,  142. 


580  DETERMINATION.  [§  149. 

placed  under  A,  in  order  to  let  the  reaction  proceed,  and  the 
compression-cock,  immediately  replaced  by  the  pressure  of  the 
fingers,  this  compression  being  in  turn  relieved  as  soon  as  a  press- 
ure is  felt  from  within.  The  reaction  is  ordinarily  at  an  end  in 
eight  minutes,  when  c  is  removed,  from  under  B.  B  is  a  small 
bell- jar,  made  from  an  adapter,  and  must  have  a  capacity  three 

to  four  times  the  volume  of  gas  to 
be  received.  In  cases  where  the 
evolution  of  gas  is  impetuous  it  is 
at  times  necessary  to  immerse  it  in 
the  trough  in  order  to  better  con- 
108.  dense  the  vapors.  The  upper  part 

of  B  is  drawn  out  as  shown  in  Fig.  108,  in  order  to  facilitate  its 
ready  insertion  into  the  rubber  tube  and  also  the  breaking  off  of 
its  point.  The  bell  is  first  filled  with  water  in  order  to  expel  all 
the  air,  and  then  with  mercury ;  well-boiled  milk-of-lime  is  then 
introduced  by  means  of  a  curved  pipette.  The  nitric  oxide 
entering  B  is  thus  freed  from  the  slightest  trace  of  acid  vapor. 
The  nitric  oxide  is  now  to  be  transferred  into  the  flask  (7,  there 
to  be  reconverted  into  nitric  acid  by  means  of  oxygen.  The 
flask  C  contains  a  little  water ;  it  is  connected  by  means  of  the 
rubber  tube  d  with  the  glass  tube  <?,  the  other  end  of  which  car* 
ries  the  narrow  rubber  tube,  /,  10  cm.  long. 

The  water  in  C  is  now  heated  to  boiling,  and  all  the  air  ex- 
pelled from  the  flask  and  tubes  by  the  aqueous  vapor; /'is  then 
connected  with  the  tip  of  the  bell-jar  B,  which  has  previously  been 
slightly  scratched  with  a  diamond,  and  the  tip  is  then  broken  off. 
At  first  the  aqueous  vapor  condenses  in  the  bell- jar,  and  expels 
at  the  same  tim^  the  small  quantity  of  milk-of-lime  remaining  in 
the  tip.  On  now  removing  the  lamp,  a  return  current  is  soon 
established,  which  drives  the  nitric  oxide  into  C.  If  this  pro- 
ceeds too  rapidly, /is  compressed  by  the  fingers.  As  soon  as  the 
milk-of-lime  in  the  bell-jar  has  nearly  reached  the  rim  of  /,  the 
latter  is  closed  by  a  compression-cock.  20  to  30  c.  c.  of  pure 
hydrogen  gas  are  now  introduced  into  the  bell-jar,  in  order  to 
insure  the  transferral  of  the  last  traces  of  nitric  oxide  into  (7,  to 
be  absorbed  like  the  rest,  /is  now  closed  with  the  compression- 
cock,  its  end  removed  from  the  tip  of  the  bell- jar  and  connected 
instead  with  the  glass  tube  h  of  the  oxygen  jar  D ;  the  cock,  r, 


§  149.]  NITRIC   ACID.  581 

of  this  is  now  opened,  and  then  the  compression-cock,  thus  allow- 
ing oxygen  to  enter  c.  As  soon  as  the  object  of  the  operation  is 
effected,  r  is  closed,  h  and  f  separated,  and,  after  waiting  fifteen 
minutes,  the  regenerated  free  nitric  acid  is  estimated  by  means  of 
very  dilute  soda  lye  (§  215). 

The  success  of  the  method  depends  materially  upon  the  com- 
plete expulsion  of  air  from  a  and  c.  The  test  experiments  made 
by  SCHLOSING,  as  well  as  the  analyses  made  in  my  own  labora- 
tory, yielded  highly  satisfactory  results.*  Similarly  satisfactory 
results  were  also  obtained  by  K.  FRUHLING  f  and  H.  GROUVEN, 
as  well  as  by  E.  SCHULZE.  \  "When  the  quantity  of  nitric  acid  is 
small,  it  is  advantageous  to  use  a  larger  quantity  of  ferrous 
chloride. 

It  is  quite  evident  that  the  apparatus  described  above  may  be 
variously  modified,  while  the  principle  of  its  action  is  still  retained. 
For  instance,  SCHLOSIXG  recommended  a  slightly  modified  appa- 
ratus for  the  estimation  of  very  small  quantities  of  nitric  acid  (less 
than  0*01  grm).  FRUHLING  and  GROUVEN  (loc.  cit.),  also  devised 
various,  though  unimportant,  modifications.  The  most  decided 
modification  is  that  proposed  by  E.  REICHARDT.§  In  this  the  mer- 
cury trough  is  dispensed  with,  and  the  nitric  oxide  is  received  in 
a  vessel  filled  with  soda-lye,  after  all  the  air  in  the  apparatus 
has  been  expelled  by  hydrogen.  Since,  however,  it  is  difficult  to 
obtain  this  gas  perfectly  free  from  oxygen,  the  results  obtained 
with  this  modification  are  apt  to  be  too  low. 

The  modification  proposed  by  F.  SCHULZE,  and  described  by 
H.  WULFERT,  ||  differs  from  the  other  modifications  in  that  the 
nitric  oxide  is  first  collected  in  a  bell-jar  filled  with  mercury  and 
provided  with  a  glass  cock,  then  conducted  into  a  measuring- 
tube,  and  its  volume  measured.  If  a  foreign  gas  is  also  present, 
its  volume  is  also  ascertained  by  absorbing  the  nitric  oxide  with 
ferrous- chloride  solution.  In  his  experiments,  however,  WUL- 
FERT  never  found  more  than  0-33  c.  c.  unabsorbable  gas.  The 
results  obtained  were  very  satisfactory,  even  with  very  small  quan- 
tities of  nitric  acid,  and  in  the  presence  of  considerable  organic 
matter. 

*  Zeitschr.f.  analyt.  CJiem.,  I,  39.  §76.,  ix,  24. 

\Landwirthschaftl.  Versuchsstat.,  ix,  14  and  150.  ||/6.,  ix,  400. 

I  Zeitschr.f.  analyt.  Chem.,  vi,  384. 


582 


DETERMINATION. 


149. 


d.   SCHULZE'S  Method  *  modified  by 

The  solution  containing  the  nitrate  is  concentrated  if  necessary 
to  a  volume  of  about  50  c.  c.  and  introduced  into  the  flask  A, 
which  should  have  a  capacity  of  about  200  c.  c.  This  flask 
(Fig.  109)  is  provided  with  a  rubber  stopper,  through  which  pass 


Fig.  109. 

two  bent  tubes,  a  1}  c  and  efg.  The  first  is  drawn  out  to  a  point 
(not  too  small)  at  0,  and  projects  through  the  stopper  about 
2  cm. ;  the  second  terminates  without  diminution  of  size  exactly 
at  the  lower  surface  of  the  stopper.  These  two  tubes  are  con- 
nected by  rubber  tubes  (bound  with  thread)  at  c  and  g  with  the 
glass  tubes  c  d  and  g  h.  A  rubber  tube  is  drawn  over  the  lower 
end  of  g  h  to  protect  it  from  fracture.  JS  is  a  glass  vessel  con- 
taining 1'0-per  cent,  soda  solution.  A  measuring  tube  graduated 
in  0*1  c.  c.,  of  not  too  great  diameter,  filled  with  previously  boiled 


*  Zeitschr.  f.  analyt.  Chem.,  1870,  400. 

f  Anleitung  zur  Untersuchung.  von  Wasser,  von  W.  KUBEL,  Zweite  Auflage 
von  F.  TIEMANN,  Braunchsweig,  Fr.  Vieweg  u.  Sohn.,  1870,  s.  55. 


§  149.J  NITRIC    ACID.  583 

soda  solution,  is  supported  so  that  its  open  end  is  under  the  sur- 
face of  the  liquid  in  I>. 

The  solution  of  the  nitrate  in  the  flask  is  further  concentrated 
by  boiling,  and  finally  the  lower  end  of  the  tube  efg  h  is  brought 
into  the  soda  solution  so  that  a  part  of  the  steam  escapes  through  it. 
After  a  few  minutes  the  rubber  tube  at  g  is  pressed  together  with 
the  fingers;  if  the  air  has  been  completely  displaced  from  the 
flask  by  boiling,  the  soda  solution  will  rise  suddenly  in  the  tube  as 
in  a  vacuum,  and  a  slight  blow  against  the  finger  will  be  perceptible. 
In  this  case  the  rubber  tube  at  g  is  closed  with  a  clamp  and 
the  steam  is  allowed  to  escape  through  a  b  o  d  until  only  10  c.  c. 
of  fluid  remain  in  the  flask.  The  lamp  is  now  removed  and  the 
rubber  tube  at  c  is  closed  with  a  clamp,  and  the  tube  c  d  filled  by 
a  jet  of  water.  If  an  air  bubble  remains  in  the  rubber  tube  at  c, 
it  must  be  removed  by  pressure  with  the  fingers.  The  graduated 
measuring  tube  is  now  brought  over  the  upcurved  end  of  the  evo- 
lution tube  efg  h  so  that  the  end  rises  in  it  2-3  cm.  The  flask 
must  next  be  allowed  to  stand  a  few  minutes  until  a  partial  vacuum  is 
produced  in  it,  which  is  manifested  by  a  contraction  of  the  rubber 
tubes  at  c  and  g.  A  nearly  saturated  solution  of  ferrous  chloride 
is  poured  into  a  small  beaker,  the  upper  part  of  which  is  marked 
so  as  to  show  the  space  occupied  by  20  c.c.  ;  two  other  beakers 
must  also  be  at  hand  partly  filled  with  concentrated  hydrochloric 
acid.  The  tnbecd  is  now  dipped  into  the  ferrous- chloride  solution, 
and  the  clamp  at  c  is  loosened  until  15-20  c.c.  are  drawn  into  the 
flask.  The  ferrous  chloride  remaining  in  the  tube  is  next  removed 
by  drawing  in  a  small  quantity  of  hydrochloric  acid  in  two  suc- 
cessive portions.  Small  bubbles  may  frequently  be  observed  at 
#,  occasioned  by  evolution  of  hydrochloric  gas  caused  by  dimin- 
ished pressure  in  the  flask.  They  disappear  almost  completely  so 
soon  as  the  pressure  rises. 

Heat  is  applied,  at  first  very  gently,  until  the  rubber  tubes  at  o 
and  g  are  slightly  expanded ;  then  the  rubber  tube  at  g  is  held  com- 
pressed by  the  fingers,  the  clamp  being  removed,  until  the  pressure 
becomes  stronger,  when  the  gas  is  allowed  to  pass  over  to  the  grad- 
uated tube.  Toward  the  end  of  the  operation  heat  is  increased  and 
distillation  continued  until  the  volume  of  gas  in  the  measuring 
tube  no  longer  increases.  The  hydrochloric  gas,  abundantly 
evolved  in  the  last  part  of  the  process,  is  absorbed  with  violence 
by  the  soda  solution  with  a  peculiar  clattering  sound ;  there  is  nd 


584  DETERMINATION.  [§  149. 

danger,  however,  of  breaking  the  evolution  tube  if  care  has  been 
taken  to  enclose  the  lower  end  with  a  rubber  tube  as  above  directed. 

The  measuring  tube  is  brought  into  a  large  cylinder  containing; 
cold  water,  best  of  15-18°  C.,  and  by  means  of  some  suitable  fix- 
ture held  wholly  submerged  in  the  same.  The  transfer  is  effected 
with  the  help  of  a  small  porcelain  dish  filled  with  soda  solution. 

After  15-20  minutes,  the  temperature  of  the  water  in  the^ 
cylinder  is  ascertained  with  a  sensitive  thermometer,  and  the  state 
of  the  barometer  is  also  observed.  Then  the  tube  is  taken  hold 
of  at  the  upper  end  with  a  strip  of  paper  or  cloth,  in  order  to  avoid 
imparting  heat  to  it  by  direct  contact  of  the  hand,  and  drawn 
up  perpendicularly  so  far  that  the  level  of  the  fluids  within  and 
without  it  exactly  coincide,  and  the  volume  of  the  gas  is  read  off. 
From  the  data  thus  obtained,  the  volume  which  the  dry  gas  would 
occupy  at  0°  C.  and  760  mm.  bar.  pressure  is  to  be  computed.  (See 
pp.  160,  161,  on  Calculation  of  Analyses.)  1  c.c.  ]STaO3  at  0°  C. 
and  760  mm.  bar.  pressure  corresponds  to  0'002415  grm.  N2O6. 

A  condition  indispensable  for  the  success  of  the  operation  is. 
the  complete  expulsion  of  air  from  the  apparatus  in  the  beginning. 
When  an  abundant  quantity  of  nitric  acid  is  present  in  the  sub- 
stance, enough  to  produce  about  80  c.c.  nitrogen  dioxide  is  a  suit- 
able .quantity  to  use  for  its  determination,  and  a  somewhat  larger 
quantity  of  ferrous  chloride  and  hydrochloric  acid  than  above  indi- 
cated may  be  used.  An  unnecessary  amount  of  these  reagents 
should,  however,  be  avoided,  since  it  is  difficult  to  boil  a  small  quan- 
tity of  nitrogen  dioxide  out  of  a  large  volume  of  liquid. 

This  method  is  easy  to  carry  out  and  gives  satisfactory  results, 

e.  Methods  lased  on  the  conversion  of  Nitric  Acid  into 
Ammonia. 

On  heating  a  nitrate  in  an  alkaline  fluid  in  which  nascent 
hydrogen  is  being  evolved  in  sufficient  quantity,  all  the  nitric  acid 
of  the  nitrate  is  converted  into  ammonia,*  from  the  volume  of 
which  the  quantity  of  nitric  acid  may  be  accurately  determined. 
FK.  SCHULZE  f  was  the  first  to  base  on  this  principle  a  method  for 
the  estimation  -of  nitric  acid,  and  he  was  soon  followed  by 


*The  conversion  takes  place  in  acid  solution  also,  but  is  then  only  partial 
(L.  GMELIN;  MARTIN). 

f  Chem.  CentralbL,  1861,  657  and  833. 


§  149.J  .  NITEIC  ACID.  585 

W.  WOLF,*  HARCOURT^  and  SIEWERT. :f  Later  on  BUNSEN,§ 
and  also  HAGER,  |  modified  both  the  methods  and  the  apparatus. 
SCHULZE  effected  reduction  with  platinized  zinc ;  W.  WOLF,  HAR- 
COURT, and  SIEWERT  with  zinc  and  iron  filings;  BU.NSEN  with  a 
zinc-iron  spiral.  Zinc  and  iron  appear  to  afford  the  most  satis- 
factory results,  therefore  I  shall  first  describe  HARCOURT' s  process, 
in  which  an  aqueous  potassa  solution  is  used,  and  then  describe 
SIEWERT' s  method,  in  which  an  alcoholic  potassa  solution  is  em- 
ployed. If  organic  substances  are  present,  these  methods  do  not 
afford  good  results  (FRUHLIN&T).  The  reliability  of  the  results, 
however,  is  questioned  even  when  organic  substances  are  absent. 
While  the  test-analyses  afforded  HARCOURT  and  SIEWERT  uniformly 
good  results,  WOLF  (loo.  cit.)  states  that  the  three  following  con- 
ditions are  essential  for  the  success  of  the  method:  1.  The  con- 
version of  nitric  acid  into  ammonia  must  take  place  in  the  cold 
(on  heating,  while  the  hydrogen  is  being  evolved,  some  ammonia 
is  lost,  most  probably  from  the  escape  of  nitrogen  as  such). 

2.  A  copious  and  uniform  evolution  of  hydrogen  is  necessary, 
and  is   best    secured  by  using  zinc  in    conjunction   with  iron. 

3.  The  potassa  or  soda  must  be  dissolved  in  not  less  than  7  or 
more  than  8  parts  of  water.     It  will  be  observed  that  these  con- 
ditions are  at  direct  variance  with  the  directions  given  by  HAR- 
COURT.     FINKENER  *'*  rejects  all  the  methods  based  on  the  above 
principle,  because,  although  all  the  nitric  acid  is  decomposed,  yet 
all  the  nitrogen  is  not  converted  into* ammonia.     I  have  not  studied 
the  methods  sufficiently  to  give  a  decided  opinion,  but  I  must  say 
that  in  my  laboratory  the  methods  of  HARCOURT  and  of  SIEWERT 
have  generally  given  good  results. 

HARCOURT  employs  the  apparatus  shown  in  Fig.  110.  Bring 
the  tube  e  into  a  vertical  position  by  turning  it  half  round  in  the 
tubulure ;  then  run  into  it  from  a  burette  standard  acid  (more 
than  is  sufficient  to  fix  the  ammonia)  into  d,  add  a  little  litmus 

*  Chem.  CentralbL,  1862,  379;  also  Journ.  f.  prakt.  Chem.,  LXXXIX,  93,  and 
Zeitechr.f.  analyt.  Chem.,  n,  401. 

f  Journ.  of  the  Chem.  Soc.,  xv,  385;  also  Zeitschr.f.  analyt.  Chem.,  ir,  14. 

\Annal.  d.  Chem.  u.  Pharm.,  cxxv.  293. 

§Zeitschr.f.  analyt.  Chem.,  x,  414. 

1/6.,  x,  334. 

\Landwirthschaftl.  Versuchsstat.,  vin,  473. 

**  H.  KOBE,  Handb.  d.  analyt.  Chem. ,  6  Aufl.  von  FINKENER,  n,  829. 

• 


586  DETERMINATION.  [§  149. 

tincture,  turn  the  tube  e  back  to  its  horizontal  position,  and  let  a 
little  more  of  the  standard  acid  run  into  the  bulbs.      Now  remove 


Fig.  110. 

the  flask  #,  while  its  stopper  carrying  the  glass  tube,  and  also  the 
small  flask  J,  containing  a  little  water,  are  allowed  to  retain  their 
position  on  the  sand-bath  unchanged.  Into  a  introduce  about  50 
grm.  of  finely  granulated  zinc  and  about  25  grm.  of  iron  filings 
(purified  by  first  sifting  and  then  heating  in  a  current  of  hydro- 
gen), add  the  weighed  quantity  of  nitrate  (for  instance  0*5 
potassium  nitrate),  20  c.  c.  of  water,  and  20  c.  c.  of  potassa  solu- 
tion of  sp.  gr.  1*3.  That  part  of  the  sand-bath  c,  directly 
under  #,  is  now  heated  until  the  contents  of  a  boil.  When  the 
bubbles  of  air  and  hydrogen  pass  quietly  through  the  bulbs  £,  a 
loss  of  ammonia  is  not  to  be  feared.  As  soon  as  distillation  begins, 
place  the  lamp  so  that  also  the  contents  of  the  flask  l>  boil  gently. 
In  this -manner  the  fluid  is  by  one  operation  distilled  twice,  and  the 
traces  of  potassa  carried  over  from  a  are  completely  retained  in  J. 
The  end  of  each  exit-tube,  as  a  further  precaution,  is  drawn  out 
and  bent  upwards  in  the  form  of  a  hook.  The  distillation  re- 
quires from  1  to  2  hours.  It  may  be  stopped  when  the  hydro- 
gen, which  is  evolved  more  freely  as  the  potassa  solution  becomes 
more  concentrated,  has  passed  through  the  bulb-tube  e  for  5  or  10 
minutes  regularly.  As  soon  as  the  fluid  e  has  receded  to  d  on  the 
cooling  of  the  apparatus,  remove  the  rubber  stopper  from  the  small 
tubulure/*,  and  pass  a  stream  of  water  through  the  condenser  in 
order  to  rinse  the  last  traces  of  ammonia  into  the  receiver.  Once 


§  149.]  NITRIC    ACID.  587 

more  bring  the  tube  e  to  a  vertical  position  by  a  half-turn,  rinse  it 
out  with  water,  then  remove  it,  and  close  the  tubulure  of  the  re- 
ceiver with  a  cork.  Finally  remove  the  receiver,  and  rinse  off  the 
outside  of  the  lower  end  of  the  condenser,  and  proceed  to  titrate 
the  residual  free  acid.  The  metals  remaining  in  a  need  only  be 
washed  with  water,  diluted  acid,  and  again  with  water,  in  order 
.  to  render  them  serviceable  for  a  second  determination.  Metals 
which  have  been  once  used  evolve  hydrogen  more  slowly 
than  do  bright  zinc  and  recently  ignited  iron,  but  the  evolu- 
tion of  ammonia  proceeds  equally  well  in  both  cases.  Metallic 
chlorides  and  sulphates  have  no  influence  on  the  result.  If 
lead  is  present  it  appears  advisable  to  add  some  potassium 
sulphate. 

SIEWEET  employed  for  every  gramme  of  saltpetre  4  grm. 
iron  filings  and  8  to  10  grin,  zinc  filings,  and  also  16  grin,  potas- 
sium hydroxide  and  100  c.  c.  alcohol  of  0-825  sp.  gr.  By  the 
iise  of  alcohol  the  danger  of  the  boiling  fiuid  receding  is  avoided. 
The  apparatus  used  by  him  consists  of  a  300-  to  350-c.  c. 
flask  with  an  evolution  tube  connected  with  the  two  flasks./?  and  C, 
arranged  as  shown  in  Fig.  111.  These  flasks  have  a  capacity 
of  150  to  200  c.  c.  each  and  contain 
standard  acid.  The  connecting  tube  b 
is  cut  off  obliquely  at  both  ends;  c 
serves  for  the  introduction  of  a  strip  of 
litmus  paper  during  the  operation,  and 
after  the  latter  is  complete,  for  the 
transference  of  the  liquid  from  one 
flask  to  the  other  at  will.  After  put- 
ting the  apparatus  together,  the  disen- 
gagement of  gas  may  be  allowed  to  pro- 
ceed first  in  the  cold,  or  it  may  be 

Tj^J  _       1  "1  "I 

assisted  from  the  beginning  by  the  aid 

of  a  small  flame.  After  half  an  hour  the  ammonia  formed  begins 
to  pass  over  in  proportion  as  the  alcohol  distils  off.  As  soon  as  the 
latter  has  completely  disappeared  from  the  evolution  flask,  apply 
heat  very  cautiously  (in  order  to  drive  off  the  last  traces  of  am- 
monia) until  steam  appears  in  the  evolution  tube ;  or  10  to  15  c.  c. 
of  alcohol  are  rapidly  introduced  once  or  twice  into  the  evolution 
flask  and  distilled  off. 


588 


DETERMINATION. 


[§  149. 


f.  Method  of  estimating  nitric  acid  from  the  loss  of  hydro- 
gen, after  FK.  SCHULZE.* 

On  dissolving  aluminium  in  potassa  lye,  a  potassium-alumin- 
ium compound  is  formed  and  hydrogen  is  evolved,  the  quantity 


Fig.  112. 

evolved  corresponding  to  the  weight  of  the  aluminium  dissolved. 
If  a  nitrate  is  added  to  the  mixture  evolving  the  hydrogen,  less 
hydrogen  is  obtained  than  were  no  nitrate  present,  since  part  of 
the  nascent  hydrogen  serves  to  convert  the  ]STaO5  of  the  nitrate 

*Zeitsc7ir.f.  anatyt.  Chem.,  n,  300. 


§  149.]  NITRIC   ACID.  589 

into  ammonia  (KaO6  +  16H  —  2OTI3  +  5H9O),  and  the  loss  of 
hydrogen  is,  of  course,  proportional  to  the  quantity  of  Na06  con- 
verted into  ammonia.  Since,  according  to  FR.  SCHULZE,  this 
conversion  is  complete  when  the  process  is  conducted  slowly 
(FINKENER,  *  however,  contradicts  this),  and  since  a  small  quan- 
tity of  N2O6  is  able  to  effect  a  relatively  large  deficit  of  hydrogen, 
this  method  can  be  applied  for  the  accurate  determination  of  even 
small  quantities  of  nitric  acid.  The  method  cannot,  according  to 
E.  SCHULZE,  f  be  used  if  organic  matter  is  present,  because  then 
the  results  are  inaccurate.  In  such  a  case  the  substance  must  first 
undergo  the  following  preliminary  treatment:  Heat  with  dilute 
potassa  lye  until  all  ammonia  is  expelled,  add  a  concentrated  solution 
of  pure  potassium  permanganate  until  the  fluid  retains  a  red  color 
even  on  continuous  boiling  during  10  minutes,  then  add  a  little 
formic  acid  to  decompose  the  excess  of  permanganate  present, 
filter,  wash,  concentrate  the  filtrate,  neutralize  accurately  with 
dilute  sulphuric  acid,  and  then  subject  the  fluid  so  obtained,  and 
concentrated  by  evaporation,  if  necessary,  to  the  treatment  to  be 
described  below  (FRANZ  SCHULZE^:). 

I  shall  first  describe  the  apparatus  §  used,  resembling  KNOP'S 
azotometer  || ,  and  then  the  process. 

The  flask  A  (Fig.  112)  has  a  capacity  of  about  50  c.  c.  Into 
its  neck  is  ground  airtight  the  tube  ./?,  expanded  above  into  a 
bulb.  A  glass  rod,  <?,  is  ground  to  fit  the  lower  opening  of  B, 
which  it  closes  perfectly ;  it  passes  through  the  cork  d,  and  is  so 
long  that,  when  the  cork  is  slid  up  to  the  end  of  0,  fluid  may  be 
introduced  into  J?  by  means  of  a  pipette.  For  measuring  the 
gases  there  is  used  a  tube,  C\  divided  into  (VI  c.  c.  and  connected 
by  means  of  a  rubber  tube  with  a  tube,  D,  of  similar  size,  but 
plain.  The  tubulure  f  of  the  latter  bears  an  arrangement  for 


*H.  ROSE,  Handb.  d.  analyt.  CJiem.,6.  Aufl.  von  FINKENER,  n,  829. 

f  Zeitschr.f.  analyt.  Chem.,  vi,  379. 

\Zeitschr.f.  Chem.  (N.  F.).  iv,  296;  ZeitscJir.f.  analyt.  Chem.,  vn,  390. 

§  Instead  of  this,  RUMPF'S  may  be  used  (Zeitschr.  f.  analyt.  Chem.,  vi,  399). 

I  Chem.  Centralbl.,  I860,  244.  The  original  KNOP  apparatus  differs  from  the 
modification  here  given  in  that  the  tube  D  5s  not  provided  with  the  lateral 
tubulure.  The  removal  of  water  from  D  is  effected  by  suction  into  a  flask.  In 
RAUTENBEKG'S  modification  the  tubes  C  and  D  are  placed  in  a  cylinder  filled 
with  water,  in  order  to  enable  the  operator  to  better  regulate  the  temperature  of 
the  gases  and  estimate  them. 


590  DETERMINATION.  [§  149, 

allowing  water  to  run  off,  as  shown  ;   the  upper  end  of  the  tube  O 
is  connected  by  means  of  a  rubber  tube  ~k  with  the  glass  tube  A, 
which  in  turn  is  fitted  into  the  cork  inserted  into  the  tubulure  a. 
To  perform  a  series  of  experiments,  a  rather  large  quantity  of 
aluminium  filings  is  required  from  which  any   iron  present  has 
been  removed  by  means  of  a  magnet.      The  first  thing  to  be 'clone- 
is  to  determine   the  weight  of  hydrogen  evolved   by  a  weighed 
quantity  of  this  aluminium  powder  on  dissolving  in  potassa  lye. 
This    preliminary  experiment  is  absolutely    indispensable,   since 
every  kind  of  aluminium  behaves  differently  in  this  respect.      To 
carry  out  this  experiment,  introduce  a  weighed  quantity,  say  0-075 
grin.,  of  aluminium  powder  into  A  and  add  to  it  a  little  water. 
On  the  other  hand,   introduce  exactly  5  c.  c.  potassa  lye  into  B 
and  place  this  on  A,  airtight.      Now  pour  water  into  D  until  its 
level  stands  exactly  at  the  zero  point  in   C,  and  connect  A  with 
the    measuring-tube    by   inserting   the  tube   h   into    the    rubber 
tube  p.     After  again   making  sure  that    the  water-level  in    0 
and  1)  is  at  the  same  height,  and  that  it  stands  at  zero  in  (7,  note 
the  temperature  of  the  room,  and  place  A  in  a  beaker  of  water 
having  the  same  temperature.     Water  is  now  allowed  to  run  out 
from  n  until  the  level  of  water  in  D  stands  exactly  at  a  certain 
height,  say  at  30  c.  c. ,  and  the  water  has  fallen  in  C  to  about  the 
mark  1.      If  both  levels  remain  unchanged  for  some  time,  and 
you  are  sure  that  all  the  parts  of  the  apparatus  are  airtight,  raise 
the  glass  rod  C  slightly  to  allow  the  lye  in  B  to  flow  into  A.     As, 
on  account  of  the  lower  level  of  fluid  in  D,  the  air  in  A  is  under 
less  pressure  than  is  the  air  in  B,  or  the  free  atmosphere,  care 
must  be  taken  to  again  close  the  opening  e  airtight  the  moment 
the  fluid  in  B  has  almost  run  out,   and  when  only  just  enough 
remains  to  prevent  free  communication  between  A  and  B.     The 
volume  of  liquid  originally  present  in  B  (in  this  case  5  c.   c.)  is 
subsequently  to  be  deducted  from  the  volume  of  gas  in  C.     In 
the  proportion  in  which  the  aluminium  dissolves,  and  the  hydrogen 
is  evolved,  the  level  in  C  sinks,  while  that  in  D  rises,  arid  renders 
necessary  the  withdrawal  of  more  water  through  n,  so  that  both 
levels  may  remain  about  the  same.      When  the  evolution  of  gas 
has  entirely  ceased,  and  you  are  sure  that  the  water  in  which 
the  flask  A   stands,   as  well   as    the    air,    are    still  at  the  same 
temperature  as  at  first,  bring  the  level  in  D  to  the  exact  height  in 


§  149.]  NITKIC    ACID.  591 

6',  so  that  the  tension  of  gas  in  A  and  C  may  correspond  exactly 
to  the  atmospheric  pressure,  and  then  read  off  the  height  of  the 
water  in  C.  This  reading,  minus  the  number  of  c.  c.  of  fluid 
which  flowed  from  B  into  A,  expresses  the  number  of  c.  c. 
of  hydrogen  evolved  by  the  aluminium  dissolved  under  the  pre- 
vailing circumstances  of  atmospheric  pressure,  temperature,  and 
tension  of  aqueous  vapor.  Reduce  the  measured  volume  to  the 
dry  condition  at  0°  and  760  mm.  (§  198),  calculate  the  weight  of 
this  volume  (1000  c.  c.  —  0*08988  grm.)of  hydrogen,  and  ascer- 
tain the  weight  of  aluminium  required  to  evolve  1  grm.  of  hydro- 
gen by  dividing  the  weight  of  the  aluminium  used  by  the  weight 
of  the  volume  of  hydrogen  found.  SOHULZE  found  this  quotient 
to  be  in-one  case  10 '5042,  i.e.,  this  weight  of  aluminium  evolved 
1  grm.  hydrogen  (9-16  grm.  absolutely  pure  aluminium  evolve 
1  grrn.  hydrogen).  Since  16  eq.  of  hydrogen  (16-128)  corre- 
spond to  1  eq.  of  K"SO.  (108-08),  16-128  X  10-5042=  169-4117 
grm.  of  the  aluminium  in  question  correspond  to  108-08  grm. 
N,0, 

Suppose,  now  we  know  the  exact  value  of  our  aluminium, 
we  desire,  on  some  occasion,  to  make  a  determination  of  NaO6, 
using  the  aluminium.  We  begin  first  by  calculating  how  many 
c.  c.  of  hydrogen  a  certain  weight  aluminium,  say  0*05  grm., 
will  afford  on  the  day  the  determination  is  about  to  be  made, 
i.e.,  at  the  temperature  and  atmospheric  pressure  prevailing  on 
that  particular  day,  and  which,  it  is  assumed,  will  remain  con- 
stant throughout  the  process  (to  best  insure  this  a  room  in  which 
the  temperature  remains  constant  is  chosen).  Let  us  assume  we 
obtained  58*4  c.  c.  as  the  number  of  c.  c.  of  hydrogen  correspond- 
ing to  0-05  grm.  of  aluminium.  Introduce  the  fluid,  the  nitric  acid 
of  which  is  to  be  determined,  and  the  volume  of  which  may  be 
about  20  c.  c.,  into  the  flask  A,  add  a  weighed  quantity  of  alumin- 
ium powder  sufficient  to  insure  at  least  2  parts  being  present  for 
every  1  part  of  NaOB,  connect  the  apparatus  as  detailed  above, 
and  allow  the  potassa lye  to  flow  into  A,  at  first  by  drops.  To  insure 
total  conversion  of  K2O6  into  ammonia,  in  which  case  the  hydrogen 
deficit  will  correspond  to  the  N3O6,  the  solution  of  the  aluminium 
must  be  so  conducted  that  for  at  least  one  hour  an  evolution  of 
hydrogen  is  scarcely  observable,  and  that  from  3  to  4  hours  are 
required  for  the  completion  of  the  process.  After  making  sure 


592  DETERMINATION. 


L8 


that  the  barometer  and  thermometer  stand  as  at  the  beginning  of 
the  experiment,  read  off.  Let  us  assume,  taking  one  of  SOHULZE'S 
experiments  as  an  example,  that  0.15  grm.  of  the  aluminium 
powder  and  a  definite  quantity  of  potassium  nitrate  had  been  taken, 
and  had  afforded  95 '6  c.  c.  hydrogen.  How  much  NQO6  was  pres- 
ent? 0*15  grm.  aluminium  would  have  evolved  3  X  58*4  = 
175*2  c.c.  hydrogen;  but  we  obtained  only  95*6  c.  c.,  hence  the 
hydrogen  deficit  would  be  175*2  —  95*6  —  79*6  c.  c.,  which 
according  to  the  proportion 

58-4:  0-05:  :  79*6:  ce  =  0*06815  aluminium; 
and  further : 

169*4117  :  108*08  : :  0-06815  :  a?,  •=  0*04348  ]STaO6. 
(The  0*083  grin,   of  nitre  added  by  SCHULZE  contained  0*0443 

N,6,). 

g.  Methods  in  which  the  Nitrogen  of  the  Nitric  Acid  is  sep- 
arated and  measured  in  the  gaseous  fortn. 

These  methods  are  more  particularly  suitable  for  analyzing 
nitrates  which  are  decomposed  by  ignition  into  oxide  or  metal  and 
oxides  of  nitrogen  ;  they  will  be  found  in  the  Section  on  the  Ulti- 
mate Analysis  of  Organic  Bodies  in  §  185.  MAEIGNAC  employed 
them  to  analyze  mercurous  nitrates.  BROMEIS  analyzed  nitrite, 
<fec.,  of  lead  by  a  similar  method,  recommended  by  BUNSEN.  *  In 
cases  where  it  is  intended  to  determine  the  water  of  the  analyzed 
nitrate  in  the  direct  way  such  methods  are  almost  in dispen sable. f 

If  the  nitrogen  evolved  on  igniting  a  nitrate  with  finely  divided 
copper  is  to  be  estimated  gravimetrically,  the  method  recommended 
by  GIBBS  \  may  be  employed. 

h.  The  methods  employed  in  determining  the  small  quantities 
of  nitric  acid  occurring  in  natural  waters  will  be  described  under 
Water  Analysis. 


*Annal.  d.  Chem.  u.  Pharm.,  LXXII,  40. 

f  See  also  GIBBS,  Am.  Journ.  Set.,  xxxvn,  350. 

j  Zeitschr.f.  analyt.  Chem.,  in,  393. 


§  150.]  CHLORIC   ACID.  593 

§  150. 
2.   CHLORIC  ACID. 

I.  Determination. 

Free  chloric  acid  in  aqueous  solution  may  be  determined  by 
converting  it  into  hydrochloric  acid  by  the  agency  of  nascent 
hydrogen  (II.,  5),  and  determining  the  acid  formed,  as  directed  in 
§  141 ;  or  by  saturating  with  solution  of  soda,  evaporating  the 
fluid,  and  treating  the  residue  as  directed  in  II.,  a  or  c. 

II.  Separation  of  Chloric  Acid  from  the  Bases  and 
Determination  of  the  Acid  in  Chlorates. 

a.  After  BUNSEN.*  When  warm  hydrochloric  acid  acts  upon 
chlorates,  the  latter  are  reduced ;  as  this  reduction  is  not  attended 
with  separation  of  oxygen,  the  following  decompositions  may  take 
place : 

C12O5  j  £]]2o    C12O5  j  3C12O   C12O5  j  ^[2°    C1205  (  g^0     C1205    j  12C1 

Which  of  these  products  of  decomposition  may  actually  be  formed, 
whether  all  or  only  certain  of  them,  cannot  be  foreseen.  But  no 
matter  which  of  them  may  be  formed,  they  all  of  them  agree  in 
this,  that,  in  contact  with  solution  of  potassium  iodide,  they  liber- 
ate for  every  2  mol.  chloric  acid  (HC1O,),  or  1  mol.  ClaO6  in  the 
chlorate,  12  at.  iodine.  1522 '2  of  iodine  liberated  correspond 
accordingly  to  150*9  C1,O5.  The  analytical  process  is  conducted 
as  described  in  §  142,  1. 

The  test-analysis  made  by  BUNSEN  gave  good  results.  Accord- 
ing to  FINKENER  f  however,  too  little  iodine  is  precipitated  in  this 
method,  hence  he  advises,  in  order  to  obtain  correct  results,  to 
boil  33  c.  c.  hydrochloric  acid,  66  c.  c.  water,  10  grm.  potassium 
iodide,  and  1  c.  c.  aqueous  solution  of  sulphurous  acid  five  minutes 
in  a  current  of  carbonic  acid,  allowing  to  cool  in  the  current  of 
gas,  and  to  then  add  this  solution  to  the  chlorate  in  a  stoppered 

*  Annal.  d  Chem.  u.  Pharm.,  86,  282. 

f  H.  ROSE,  Ilandbuch  der  analyt.  Chem.,  6.  Aufl.  von  FINKENER,  n,  612. 


594  DETERMINATION.  [§  150.. 

flask.  The  flask  should  have  been  previously  filled  with  carbonic- 
acid  gas,  then  filled  with  the  acid  solution,  tightly  stoppered, 
heated  for  15  minutes  in  a  water-bath,  allowed  to  become  perfectly 
cold,  shaken,  the  solution  then  diluted  and  the  iodine  liberated 
estimated. 

b.  Heat  the  weighed  chlorate  with  an  excess  of  a  solution  of 
ferrous  sulphate  in  hydrochloric  acid,  and  estimate  the  ferric  chlo- 
ride formed.     The  process  is  conducted  according  to  the  rules 
given   under  §  149,  II,  d,  ft.     12  eq.  of  Fed,  raised  to  Fe,Cl. 
correspond  to  1  eq.  of  ClaO6. 

c.  The  conversion  of  chloric  acid  (and  generally  all  oxygen 
compounds  of  chlorine,   excepting  perchloric  acid)  may  also  be 
accomplished  in  very  dilute  solution  by  nitrous  acid  or  a  nitrite, 
preferably  by  neutral  lead  nitrite  (H.   TOUSSAINT  *) .     Mix  the< 
dilute  aqueous  solution  of  the  chlorate  with  a  slight  excess  of  lead- 
nitrite  solution,  f  acidulate  with  nitric  acid,  warm,  and  con  vert  the 
hydrochloric  acid  formed  into  silver  chloride  as  in  §  141 ,  I,  a. 

If  it  is  intended  to  apply  this  principle  to  the  volumetric  esti- 
mation of  chloric  acid,  introduce  a  very  dilute  solution  of  potas- 
sium chlorate  of  .known  strength  into  a  stoppered  flask,  add  an 
excess  of  silver  nitrate,  acidulate  strongly  with  nitric  acid,  heat  in 
a  water-bath,  and  then  add,  under  frequent  shaking  (which  pro- 
motes the  deposition  of  the  silver  chloride),  the  solution  of  lead 
nitrite  until  a  drop  no  longer  causes  a  precipitate  of  silver  chloride. 
The  lead-nitrite  solution  being  thus  standardized,  it  may  be  em- 
ployed for  estimating  the  chloric  acid  of  solutions  of  unknown 
strength.  The  author's  test-analyses  gave  good  results,  which, 
were  corroborated  in  my  laboratory. 

d.  The  reduction  of  chloric  ncid  may  also  be  simply  effected  by 
ferrous  hydroxide.     Add  a  sufficient  quantity  of  ferrous  sulphate 
to  the  solution  of  the  alkali  chlorate,  supersaturate  strongly  with 
chlorine-free  potassa  lye,  boil  for  a  long  time,  filter  off  the  ferroso- 
ferric  hydroxide,  wash,  acidulate  the  filtrate  with  nitric  acid,  and 

*  Annal.  d.  Chem.  u.  Pharm.,  cxxxvn,  114;  Zeitschr.  f.  analyt.  CJiem.,  v, 
210. 

fTo  prepare  this,  boil  together  1  part  plumbic  nitrate,  \\  part  lead,  and  50 
parts  water  for  a  long,  time,  when  tetrabasic  lead  nitrite  (PbfOHjNOaPbO) 
precipitates  as  a  white  powder  on  rapidly  cooling  the  at  first  yellow,  then 
colorless,  solution.  Suspend  the  nitrite  in  water  and  pass  in  carbonic  acid  until 
the  basic  salt  is  entirely  decomposed.  The  filtered  solution  may  be  preserved 
for  a  long  time  in  completely  filled  bottles. 


§  150.]  CHLORIC   ACID.  595 

precipitate  the  chlorine  with  silver  solution  (§  141, 1,  a. — C.  STEE- 
LING *).  According  to  my  investigations  it  is  advisable  to  bring  the 
filtrate  up  to  250  c.  c.,  and  to  then  test  a  portion  of  it  for  chloric 
acid  by  adding,  first,  sulphuric  acid  to  acidity,  then  a  very  small 
quantity  of  indigo  solution,  and  finally  a  little  sulphurous  acid. 
When  the  indigo  is  no  longer  decolorized,  and  you  are  certain  that 
all  the  chlorate  in  the  solution  has  been  converted  into  chloride, 
proceed  to  estimate  the  chlorine  as  silver  chloride  in  an  aliquot 
part  of  the  250  c.  c. 

e.  After  SESTINI.  f     To  the  concentrated  aqueous  solution  of 
the  weighed  chlorate  add  a  piece  of  zinc  and  then  some  pure 
'dilute  sulphuric  acid,  and  allow  to  stand  for  some  time  (with  0*1 
grm.  potassium  chlorate  half  an  hour  is  sufficient).  -By  the  nascent 
hydrogen  evolved  the  chloric  acid  is  converted  into  hydrochloric 
acid,  which,  after  removal  and  rinsing  of  the  zinc,  is  determined 
according  to  §  141.     To  use  the  volumetric  method  (§  141,  J,  «), 
the  sulphuric  acid  is  first  precipitated  with  barium  nitrate,  then  the 
zinc  and  excess  of  barium  with  sodium  carbonate ;  the  liquid  is 
filtered  and  neutralized,  then  potassium  chromate  is  added,  and 
finally  standard  silver  solution. 

f.  The  basic  radicals  are  determined  with  advantage  in  a  sepa- 
rate portion,  by  converting  the  chlorate  either  by  very  cautious 
ignition,  or  by  warming  with  hydrochloric  acid,  into  chloride. 

The  estimation  of  hypochlorous  acid  will  be  described  in  the 
Special  Part,  article  "  Chlorimetry." 

*  Zeitschr.  /.  analyt.  Chem.,  vi,  32. 
f/6.,  i,  500. 


SECTION   T. 
• 

SEPAEATION    OF   BODIES. 

§  151- 

WHEN  only  one  basic  or  one  acid  radical  is  present,  the  method 
of  its  determination  has  been  considered  in  the  previous  Section. 
"When  more  than  one  basic  or  more  than  one  acid  radical  is  p res- 
sent,  the  methods  of  separating  and  determining  them  will  be 
described  in  the  present  Section. 

The  separation  of  bodies  may  be  effected  in  three  ways :  viz.,  a, 
by  direct  analysis  •  &,  by  indirect  analysis  •  c,  by  estimation  ly 
difference. 

By  direct  analysis,  we  understand  the  actual  separation  of  rad- 
icals or  elements.  Thus,  we  separate  potassium  from  sodium  by 
platinic  chloride;  copper  from  tin  by  nitric  acid;  arsenic  from 
iron  by  hydrogen  sulphide  ;  iodine  from  chlorine  by  palladious 
nitrate  ;  carbon  from  potassium  nitrate  by  water,  &c.,  &c.  In 
direct  analysis  we  render  one  body  insoluble,  while  the  others 
remain  in  solution,  or  vice  versa,  or  we  volatilize  one  body,  leav- 
ing the  others  behind,  or  we  effect  actual  separation  in  some  other 
manner.  This  is  the  mode  of  analysis  most  frequently  employed. 
It  generally  deserves  the  preference  where  choice  is  permitted. 

We  term  an  analysis  indirect  if  it  does  not  effect  the  actual  sep- 
aration of  the  bodies,  but  causes  certain  changes  which  enable  us 
to  calculate  their  quantity.  Thus,  the  quantity  of  potassium  and 
sodium  in  a  mixture  of  compounds  of  the  two  may  be  determined 
by  converting  them  into  chlorides,  weighing  the  latter,  and  deter- 
mining the  chlorine  (§152,  3). 

Finally,  if  we  weigh  two  bodies  together,  determine  one  of  them, 
and  subtract  its  weight  from  that  of  the  two,  we  shall  find  the 
weight  of  the  other  body.  In  this  case  the  second  body  is  said  to 
be  estimated  ~by  difference.  Thus,  aluminium  may  be  determined 
when  its  oxide  is  mixed  with  ferric  oxide,  by  weighing  the  mix- 
ture and  determining  the  iron  volu metrically. 

596 


[§  151..  SEPARATION   OF   BODIES.  597 

Indirect  analysis  and  estimation  by  difference  maybe  employed 
in  an  exceedingly  large  number  of  cases ;  but  their  use  is  as  a  rule 
only  to  be  recommended  where  good  methods  of  true  separation 
are  wanting.  The  special  cases  in  which  they  are  preferable  to 
direct  analysis  cannot  be  all  foreseen ;  those  alone  are  pointed 
out  which  are  of  more  frequent  occurrence.  As  regards  the  calcu- 
lations required  in  indirect  analysis,  I  have  given  general  direc- 
tions under  "  the  Calculation  of  Analysis  ;"  wherever  it  appeared 
judicious,  I  have  added  the  necessary  directions  to  the  description 
of  the  method  itself. 

I  have  retained  our  former  subdivision  into  groups,  and,  as  far 
as  practicable,  systematically  arranged,  first,  the  general  separation 
of  all  the  bodies  belonging  to  one  group  from  those  of  the  preced- 
ing groups ;  secondly,  the  separation  of  the  individual  bodies  of  one 
group  from  all  or  from  certain  bodies  of  the  preceding  groups ; 
and  finally,  the  separation  of  bodies  belonging  to  one  and  the  same 
group  from  each  other.  I  think  I  need  scarcely  observe  that  the 
general  methods  which  serve  to  separate  the  whole  of  the  bodies  of 
one  group  from  those  of  another  group  are  also  applicable  to  the 
separation  of  every  individual  body  of  the  one  group  from  one  or 
several  bodies  of  the  other  group.  It  must  not  be  understood  that 
the  more  special  methods  are  necessarily  in  all  cases  preferable  to 
the  more  general  ones.  As  a  rule,  it  must  be  left  to  individual 
chemists  to  decide  for  themselves  in  each  special  case  which  method 
should  be  adopted.  "With  respect  to  the  general  methods  for  sepa- 
rating one  group  from  another,  I  would  observe  that  those  adduced 
appeared  to  me  more  adapted  to  the  purpose  than  others,  but 
still  there  may  be  others  that  are  equally  suitable,  and  in  special 
cases  even  more  so.  A  wide  field  is  here  open  to  the  ingenuity 
of  the  analyst. 

The  methods  given  for  the  separation  of  both  basic  and  acid 
radicals  are  generally  based  upon  the  supposition  that  they  are  in 
the  form  of  free  acids  or  bases,  or  in  the  form  of  salts  soluble  in 
water.  Wherever  this  is  not  the  case,  special  mention  is  made  of 
the  circumstance. 

From  among  the  host  of  proposed  methods,  I  have,  as  far  as 
practicable,  chosen  those  which  have  been  sanctioned  by  experience 
and  are  distinguished  for  accurate  results.  In  cases  where  two 
methods  were  on  a  par  with  each  other  as  regards  these  two  points, 
I  have  either  given  both  or  selected  the  more  simple  one.  Methods 


598  SEPAKATION   OF   BODIES.  {§  151. 

which  experience  has  shown  to  be  defective  or  fallacious  have  been 
altogether  omitted.  I  have  endeavored  to  point  out,  so  far  as  pos- 
sible, the  particular  circumstances  under  which  either  the  one  or 
the  other  of  several  methods  deserves  the  preference. 

Where  the  accuracy  of  an  analytical  method  has  been  estab- 
lished already,  in  Section  IV.,  no  furtlmr  statements  are  made  on 
the  subject  here.  Paragraphs  of  former  Sections  deserving  par- 
ticular attention  are  referred  to  in  parentheses. 

The  extension  of  chemical  science  introduces  almost  every  day 
new  analytical  methods  of  every  description,  which  are,  rightly  or 
wrongly,  preferred  to  the  older  methods ;  the  present  time  may 
therefore  be  looked  upon  in  this,  as  in  so  many  other  respects,  as  a 
period  of  transition,  in  which  the  new  strives  more  than  ever  to 
overcome  and  supplant  the  old.  I  make  this  remark  to  show  the 
impossibility  of  always  adding  to  the  description  of  a  method  an 
opinion  of  its  usefulness  and  accuracy,  and  also  to  point  out  the 
importance,  under  such  circumstances,  of  a  proper  systematic 
arrangement.  I  have  in  this  Section  generally  arranged  the  vari- 
ous analytical  methods  upon  the  bases  of  their  scientific  principles, 
firmly  persuaded  that  this  will  greatly  tend  to  facilitate  the  study 
of  the  science,  and  will  lead  to  endeavors  to  apply  known  princi- 
ples to  the  separation  of  other  bodies  besides  those  to  which  they 
are  already  applied,  or  to  apply  new  principles  where  experience 
has  proved  the  old  ones  fallacious,  and  the  methods  based  on  them 
defective. 

I  conclude  these  introductory  remarks  with  the  important  cau- 
tion to  the  student  never  to  look  upon  a  separation  as  successfully 
accomplished  before  he  has  convinced  him  self  that  the  weighed  pre- 
cipitates, <&c.,  are  pure  and  more  particularly  free  from  those 
bodies  from  which  it  was  intended  to  separate  them. 


.[§  152.  BASES   OF   GROUP   I.  599 

I.  SEPARATION  OF   THE   BASIC    RADICALS  FROM  EACH  OTHER. 

First  Group. 

POTASSIUM SODIUM AMMONIUM (LITHIUM) .  * 

§  152. 

INDEX.     The  numbers  refer  to  those  in  the  margin. 

Potassium  from  sodium,  1,  2,  6. 

ammonium,  3,  4,  5. 
Sodium  from  potassium,  1,  2,  6. 

ammonium,  3,  4,  5. 
Ammonium  from  potassium,  4,  5. 
sodium,  3,  4,  5. 
(Lithium  from  the  other  alkalies,  7,  8,  9.) 

1.  Methods  based  upon  the  different  degrees  of  Solubility 
in  Alcohol,  of  Sodium  Platinic  Chloride,  and  Potassium 
Platinic  Chloride. 

a.  POTASSIUM  FROM  SODIUM. 

It  is  an  indispensable  condition  in  this  method  that  the  1 
two  alkalies  should  exist  in  the  form  of  chlorides.  If,  there- 
fore, they  are  present  in  any  other  form,  they  must  be  first  con- 
verted into  chlorides,  which  in  most  cases  may  be  effected  by 
evaporation  with  hydrochloric  acid  in  excess ;  in  the  case  of 
nitrates,  the  evaporation  with  hydrochloric  acid  must  be 
repeated  4 — 6  times  till  the  weight  of  the  gently  ignited  mass 
•ceases  to  diminish.  In  presence  of  sulphuric  acid,  phosphoric 
acid,  and  boric  acid,  this  simple  method  wTill  not  answer.  For 
the  methods  of  separating  the  alkalies  from  the  two  latter  acids 
and  converting  them  into  chlorides,  see  §§  135  and  130.  The 
presence  of  sulphuric  acid  being  a  circumstance  of  rather  fre- 
quent occurrence,  the  way  of  meeting  this  contingency  is  given 
below  (2). 

Determine  the  total  quantity  of  the  sodium  chloride  and 
potassium  chloride  f  (§§  97,  98),  dissolve  in  the  least  quantity 

*  Regarding  the  separation  of  ca3sium  and  rubidium  from  the  other  alkalies, 
I  refer  to  the  "  Analysis  of  Mineral  Waters  "  in  the  Special  Part. 

f  Never  take  the  weight  of  the  alkali  chlorides  without  convincing  yourself 
of  their  purity  by  dissolving  them  in  water,  which  should  give  a  clear  solution, 
and  testing  the  solution  with  ammonia  and  ammonium  carbonate,  which  must 
throw  down  no  precipitate.  It  may  be  thought,  perhaps,  that  a  matter  so  simple 
need  not  be  mentioned  here  ;  still  I  have  found  that  neglect  in  this  respect  is 
by  no  means  uncommon. 


600  SEPAKATION.  [§  152. 

of  water,  and  add  to  the  fluid  in  a  porcelain  dish  an  excess  of  a 
strong  aqueous  solution  of  platinic  chloride  as  neutral  as  pos- 
sible. Enough  platinum  solution  should  be  added  to  convert 
the  sodium  as  well  as  the  potassium  into  platinochloride.  It 
is  best  to  use  a  solution  of  known  strength  and  to  calculate 
roughly  how  much  should  be  added.  Evaporate  on  the  water- 
bath  nearly  to  dryness  (the  water  in  the  bath  should  never 
actually  boil,  and  the  sodium  platinic  chloride  should  not  lose 
its  water  of  crystallization),  treat  the  residue  with  alcohol  of 
from  0-86  to  0*87  sp,  gr.,  cover  the  dish  with  a  glass  plate,  and 
allow  to  stand  a  few  hours,  with  occasional  stirring.  If  the  super- 
natant fluid  is  not  deep  yellow,  this  is  a  proof  that  the  quantity  of 
platinic  chloride  used  is  insufficient.  When  the  precipitate  has 
settled,  pour  off  the  clear  fluid  through  a  filter  (preferably  an 
asbestos  filter,  §  97,  4,  a)  and  examine  the  precipitate  most 
minutely,  if  necessary,  with  the  aid  of  a  microscope.  If  it  is 
a  heavy  yellow  powder  (sufficiently  magnified,  small  octahe- 
dral crystals)  it  is  the  pure  potassium-platinic  chloride.*  Then 
transfer  it — best  with  the  aid  of  the  filtrate — to  the  filter, 
wash  it  with  alcohol  of  0'86  to  0*87  sp.  gr.,  and  proceed 
according  to  §  97,  4,  a.  (Instead  of  weighing  the  double 
chloride  or  the  platinum  obtained  from  it,  you  may  ignite  gen- 
tly in  hydrogen,  extract  the  potassium  chloride  with  water, 
and  weigh  this  or  titrate  the  chlorine  in  it  by  §  141,  I.,  &,  a). 
If,  on  the  contrary,  white  saline  particles  (sodium  chloride) 
are  to  be  seen  mixed  with  the  yellow  crystalline  powder,  pla- 
tinic chloride  has  been  wanting,  the  whole  of  the  sodium  chlo- 
ride not  having  been  completely  converted  into  sodium  platinic 
chloride.  In  this  case  the  precipitate  in  the  dish  must  be 
treated  with  some  water,  till  all  the  sodium  chloride  is  dis- 
solved, a  fresh  portion  of  platinic  chloride  is  added,  the  whole 
evaporated  nearly  to  dryness,  and  the  above  examination 
repeated.  The  quantity  of  the  sodium  is  usually  estimated 
by  subtracting  from  the  united  weight  of  the  sodium  chloride 
and  potassium  chloride  the  weight  of  the  latter,  calculated 
from  that  of  the  potassium  platinic  chloride. 

To  make  quite  sure  that  the  potassium  has  completely  sep- 


*  If  small  tesseral  crystals  are  visible  of  a  dark  orange-yellow  color,  and 
relatively  large  size,  and  appearing  transparent  by  transmitted  light,  then  the 
double  chloride  contains  lithium  platinic  chloride  (JKNZSCH,  Pogg.  Ann.,  civ, 
102). 


§  152.]  BASES   OF   GROUP  I.  601 

arated,  it  is  advisable  to  add  to  the  filtrate  some  water,  some 
more  platinic  chloride,  and  if  the  quantity  of  sodium  is  only 
small,  also  some  sodium  chloride ;  evaporate  on  the  water-bath 
nearly  to  dryness,  at  a  temperature  not  exceeding  75°  (BISCHOF), 
and  treat  the  residue  in  the  manner  just  described.  In  order 
to  diminish  the  solvent  action  of  the  alcohol  on  the  potassium 
platinic  chloride,  J  ether  may  be  now  mixed  with  it.  Should 
this  operation  again  leave  a  small  undissolved  residue  of  potash 
sium  platinic  chloride,  it  is  filtered  off,  best  on  a  separate  filter, 
and  first  washed  with  alcohol  and  ether.  As,  however,  this 
remainder  of  the  double  salt  is  generally  impure,  dissolve  it  on 
the  filter  with  boiling  water,  evaporate  with  a  few  drops  of  pla- 
tinic chloride,  treat  the  residue  with  alcohol,  and  if  any  potas- 
sium salt  remains,  determine  it  either  with  the  principal  quan- 
tity or  by  itself. 

If  you  are  not  satisfied  with  an  indirect  estimation  of 
the  sodium,  one  of  the  following  direct  methods  may  be 
employed,  a.  Evaporate  the  filtrate  till  the  spirit  has  gone  off, 
dilute,  digest  the  solution  with  small  pure  iron  filings  till  the 
platinum  is  all  thrown  down,  filter,  add  chlorine  water  till  the 
ferrous  is  converted  into  ferric  chloride,  precipitate  with  ammo- 
nia, filter  off  the  ferric  hydroxide,  and  determine  the  sodium 
chloride  in  the  filtrate,  ft.  Evaporate  the  filtrate,  finally  in  a 
porcelain  crucible,  to  dryness,  heat  the  residue  to  low  redness 
in  a  current  of  hydrogen>  extract  with  water,  and  determine 
the  sodium  chloride  in  the  solution.  For  small  quantities  of 
fluid  this  method  will  be  found  convenient,  y.  A.  MITSCHER- 
LICH  recommends  to  mix  the  filtrate  with  sulphuric  acid,  evapo- 
rate to  dryness,  ignite  the  residue,  extract  the  sodium  sul- 
phate with  water,  and  determine  it  according  to  §  98, 1.  These 
methods,  of  course,  yield  the  sodium  salt  in  a  pure  condition 
only  when  the  separation  of  the  potassium  has  been  perfect. 
They  present  the  advantage  that  the  sodium  salt  is  brought 
under  one's  eyes  and  may  be  tested  after  weighing. 

Should  the  solution  contain  sulphuric  acid,  it  may  be  in     2 
presence  of  hydrochloric  acid  or  of  some  volatile  acid,  convert 
the  alkalies  first  into  normal  sulphates  (§§  97,  98),  and  weigh 
them  as  such.     For  the  estimation  of  the  potassium,  one  of  the 
two  following  methods  may  be  used  : 

a.  First  convert  the  sulphates  into  chlorides  and  then  pro- 


602  SEPAEATIOTT.  [§  152. 

ceed  as  above.  For  this  purpose  barium  salts  were  formerly 
employed,  or,  better,  an  alcoholic  solution  of  strontium  chloride. 
The  barium  sulphate,  however,  carries  down  considerable  quan- 
tities of  alkali  salt,  and  the  strontium  sulphate  noticeable 
quantities ;  hence  the  employment  of  these  reagents,  more  par- 
ticularly barium,  cannot  be  recommended.  H.  ROSE  advises 
repeated  ignition  of  the  alkali  sulphates  with  ammonium 
chloride  till  the  weight  remains  constant;  this  process  is  simple 
and  well  adapted  for  small  quantities ;  no  loss  of  alkali  need  be 
feared  if  the  heat  is  not  unnecessarily  raised.  L.  SMITH  advises 
the  use  of  lead  salts.  Dissolve  the  alkali  sulphate,  precipitate 
with  pure  neutral  lead  acetate,  avoiding  a  large  excess,  add 
some  alcohol,  filter,  precipitate  the  excess  of  lead  with  sulphuric 
acid,  and  evaporate  to  dryness  with  addition  of  sulphuric  acid. 
This  method,  when  carefully  conducted,  yields  excellent  results. 

/?.  Precipitate  the  potash  directly  out  of  the  solution  of  the 
sulphates.  R.  FINKENER*  gives  the  following  process  :  To  the 
rather  dilute  solution  of  the  salts  in  a  capacious  porcelain  dish 
add  platinic  chloride  in  quantity  more  than  sufficient  to  throw 
down  all  the  potassium,  evaporate  on  a  water-bath  down  to  a 
few  c.c.,  allow  to  cool,  add,  at  first  in  small  quantities,  20 
times  the  volume  of  a  mixture  of  2  parts  absolute  alcohol-  and 
1  part  ether,  with  stirring ;  filter  after  a  short  time,  and  wash 
the  precipitate  with  alcohol  and  ether  till  the  washings  are 
colorless.  If,  when  the  alcohol  and  ether  are  first  added,  a 
strong  aqueous  solution  of  sodium  sulphate  separates,  add  some 
hydrochloric  acid  till  the  fluids  mix.  Dry  the  precipitate  con- 
sisting of  potassium  platinic  chloride  and  sodium  sulphate, 
heat  with  the  filter  in  a  porcelain  crucible  till  the  filter  is  car- 
bonized, then  in  a  current  of  hydrogen  to  scarcely  visible 
redness  extract  the  residue  with  hot  water,  ignite  the  platinum 
in  the  air,  weigh  and  calculate  from  the  weight  the  quantity  of 
potassium. 

The  separation  of  potassium  from  sodium  by  platinic 
chloride  gives  results  which  are  fully  satisfactory,  and  at  all 
events  far  more  exact  than  any  method  depending  on  another 
principle  ;  provided  that  the  platinum  solution  is  pure  and  the 
operations  have  been  carefully  performed  in  accordance  with 
the  directions.  If  you  have  any  occasion  to  doubt  the  perfect 


II.  ROSE,  Handbuch  der  analyt.  Chem.,  6.  Aufl.  von  FINKENER,  n,  923. 


§   152.]  BASES    OF   GROUP  I.  603 

purity  of  the  weighed  double  salt,  you  may  always  dissolve  it 
in  boiling  water,  evaporate  with  addition  of  a  little  platinum 
solution,  and  re  weigh  the  salt  thus  purified. 

Where  a  series  of  analyses  is  being  made,  the  potassium  in 
the  potassium-platinic  chloride  may  be  volu metrically  estimated. 
For  this  purpose  triturate  it  with  double  its  quantity  of  pure 
sodium  oxalate  (free  from  chlorides),  heat  the  mixture  in  a 
platinum  crucible  to  fusion,  leach  the  residue  with  water, 
neutralize  the  filtrate  nearly  with  acetic  acid,  determine  the 
chlorine  in  the  alkali  chloride  with  decinormal  silver  solution 
(§  141,  I.,  5,  <*),  and  calculate  1  eq.  of  potassium  for  3  eq. 
chlorine.  If  the  quantities  of  potassium-platinic  chloride  are 
very  small,  moisten  with  a  cencentrated  solution  of  neutral 
potassium  oxalate,  dry,  ignite  in, a  covered  crucible,  and  pro- 
ceed as  above.  The  separated  platinum,  if  weighed,  will 
afford  a  good  control  (F.  MOHR  *). 

b.  AMMONIUM  FKOM  SODIUM. 

The  process  is  conducted  exactly  as  in  #,  when  the  alka-  3 
lies  are  present  as  chlorides.  See  also  §  99,  2.  If  potassium 
also  is  present,  the  precipitate  produced  by  platinic  chloride  is  a 
mixture  of  ammonium  platinic  chloride  and  potassium  platinic 
chloride ;  in  which  case  the  weighed  precipitate  is  cautiously 
ignited  for  a  sufficient  length  of  time,  but  not  too  strongly, 
until  the  ammonium  chloride  is  expelled,  the  gentle  ignition 
continued  in  a  stream  of  hydrogen  or  with  addition  of  oxalic 
acid,  the  residue  extracted  with  water,  a  few  drops  of  hydro- 
chloric acid  added  if  oxalic  acid  was  employed,  and  the  potas- 
sium chloride  in  the  solution  determined  as  directed  §  97,  3. 
The  weight  found  is  calculated  into  potassium  platinic  chloride, 
and  the  result  deducted  from  the  weight  of  the  whole  precipi- 
tate :  the  difference  gives  the  animmonium  platinic  chloride. 
The  weighing  of  the  separated  platinum  affords  a  good  control. 
The  method  is  seldom  employed,  as  that  given  in  V.  yields  more 
exact  results. 

*  Zeitschr.  f.  analyt.  Chem.,  xn,  137. 


604  SEPARATION.  [§  152. 

2.  Methods  based  upon  the  Volatility  of  Ammonium 
Salts  and  Ammonia. 

AMMONIUM  FKOM  POTASSIUM  AND  SODIUM. 

a.  The  salts  of  the  alkalies  to  he  separated  contain  the  same     4 
volatile  acid,  and  admit  of  the  total  expulsion  of  their  water  hy 
drying  at  100°,  without  losing  ammonia  (e.g.,  the  chlorides). 

Weigh  the  total  quantity  of  the  salts  in  a  platinum  crucible, 
and  heat,  with  the  lid  on,  gently  at  .first,  but  ultimately  for 
some  time  to  faint  redness ;  let  the  mass  cool,  and  weigh.  The 
decrease  of  weight  gives  the  quantity  of  the  ammonium  salt. 
If  the  acid  present  is  sulphuric  acid,  you  must,  in  the  first 
place,  take  care  to  heat  very  gradually,  as  otherwise  you  will 
suffer  loss  from  the  decrepitation  of  ammonium  sulphate  ;  and, 
in  the  second  place,  bear  in  mind  that  part  of  the  sulphuric 
acid  of  the  ammonium  sulphate  remains  with  the  fixed  alkali 
sulphates,  and  that  you  must  accordingly  convert  them  into 
normal  salts,  by  ignition  in  an  atmosphere  of  ammonium  car- 
bonate, before  proceeding  to  determine  their  weight  (compare 
§§  97  and  98).  Ammonium  chloride  cannot  be  separated  in 
this  manner  from  fixed  alkali  sulphates,  as  it  converts  them, 
upon  ignition,  partly  or  totally  into  chlorides. 

b.  Some  one  or  other  of  the  conditions  given  in  "  a "  is  not 
fulfilled. 

If  it  is  impracticable  to  alter  the  circumstances  by  simple  5 
means,  so  as  to  make  the  method  a  applicable,  the  fixed  alkalies 
and  the  ammonium  must  be  determined  separately  in  different 
portions  of  the  substance.  The  portion  in  which  it  is  intended 
to  determine  the  potassium  and  sodium  is  gently  ignited  until 
ammonium  is  .completely  expelled.  The  fixed  alkalies  are  con- 
verted, according  to  circumstances,  into  chlorides  or  sulphates, 
and  treated  as  directed  in  1,  2,  or  6.  The  ammonium  is  esti- 
mated in  another  portion  according  to  §  99,  3. 

3.  Indirect  Methods. 

Of  course,  a  great  many  of  these  may  be  devised ;  but  the     6 
following  is  the  only  one  in  general  use. 

POTASSIUM  FKOM  SODIUM. 

Convert   both   alkalies   into   neutral  sulphates  or  chlorides 


§  152.]  BASES    OF   GROUP   I.  605 

(§§  97  and  98),  and  weigh  as  such ;  estimate  the  sulphuric  acid 
(§  132)  or  chlorine  (§  141);  and  from  the  amount  of  this  cal- 
culate the  quantities  of  the  sodium  and  potassium  (see  "  Calcu- 
lation of. Analysis,"  §  200*). 

The  indirect  method  of  determining  sodium  and  potassium 
is  applicable  only  in  the  analysis  of  mixtures  containing  toler- 
ably large  quantities  of  both  bases ;  but  where  this  is  the  case, 
the  process  answers  very  well,  affording  also,  more  particularly, 
the  advantage  of  expedition,  if  the  chlorine  in  the  weighed 
chlorides  is  titrated  (§  141,  I.,  &). 

Supplement  to  the  First  Group. 
SEPARATION  OF  LITHIUM  FROM  THE  OTHER  ALKALIES. 

Lithium  may  be  separated  from  potassium  and  sodium  in  the     7 
indirect  way,  and  by  two  direct  methods: 

a.  Treat  the  nitrates  or  the  chlorides,  dried  at  120°,  with  a 
mixture  of  equal  volumes  of  absolute  alcohol  and  anhydrous 
ether,  digest  at  least  for  24  hours,  with  occasional  shaking  (the 
salts  must  be  completely  disintegrated),  decant  rapidly  on  to  a 
filter  covering  the  funnel,  and  treat  the  residue  again  several 
times  with  smaller  portions  of  the  mixture  of  alcohol  and  ether. 
Determine,  on  the  one  part,  the  undissolved  potassium  and 
sodium  salts ;  on  the  other,  the  dissolved  lithium  salt,  by  dis- 
tilling the  fluid  off,  and  converting  the  residue  into  sulphate. 
This  method  is  apt  to  give  too  much  lithium,  as  the  potassium 
and  sodium  salts,  especially  the  chlorides,  are  not  absolutely 
insoluble  in  a  mixture  of  alcohol  and  ether.  The  results  may 
be  rendered  more  accurate  by  treating  the  impure  lithium  salt, 
obtained  by  distilling  off  the  ether  and  alcohol,  once  more  with 
alcohol  and  ether,  with  addition  of  a  drop  of  nitric  or  hydro- 
chloric acid,  adding  the  residue  left  to  the  principal  residue, 
and  then  converting  the  lithium  salt  into  sulphate.  If  the 
salts,  which  it  is  intended  to  treat  with  alcohol  and  ether,  have 
been  ignited,  however  so  gently,  caustic  lithia  is  formed — in 
the  case  of  the  chloride  by  the  action  of  water — and  lithium 
carbonate  by  attraction  of  carbonic  acid  ;  in  that  case  it  is  neces- 
sary, therefore,  to  add  a  few  drops  of  nitric  or,  as  the  case  may 

*  Other  methods  are  given  by  STOLE  A  (Zeitschr.  f.  analyt.  Chem.,  n,  397) 
and  MOHR  (lb.,  vn,  173). 


606  SEPARATION.  [§  152. 

be,  hydrochloric  acid,  in  the  process  of  digestion.  The  separa- 
tion of  the  alkali  chlorides  by  means  of  ether- alcohol  was  first 
proposed  by  KAMMELSBERG  *  and  later  on  recommended  by 
jENZscH.f  This  method,  however,  can  yield  only  approximate 
results,  as  the  lithium  salt  obtained  on  evaporating  the  alcoholic 
extract  is  found  by  spectroscopic  examination  to  be  always  im- 
pure (DlEHL^). 

If  we  have  to  separate  the  sulphates,  they  must  be  converted 
into  nitrates  or  chlorides  before  they  can  be  subjected  to  the 
above  method.  This  conversion  is  best  effected  by  means  of 
lead  salts,  see  2.  Ignition  with  ammonium  chloride  does  not 
answer  for  lithium  sulphate,  nor  can  the  sulphuric  acid  be 
removed  by  barium,  or  strontium,  as  the  precipitated  sulphates 
would  contain  lithium  (DIEHL§). 

1).  Weigh  the  mixed  alkalies,  best  in  form  of  sulphates,  and  8 
then  determine  the  lithium  as  phosphate  according  to  §  100. 
If  the  quantity  of  lithium  is  relatively  very  small,  convert  the 
weighed  sulphates  into  chlorides  (7),  separate,  in  the  first  place, 
the  principal  amount  of  the  potassa  and  soda  by  means  of  alco- 
hol (§  100),  and  then  determine  the  lithium  (MAYER  |). 

c.  When  exact  results  are  required,  the  indirect  method  is  9 
to  be  preferred.  Proceed  first  according  to  $,  evaporate  the 
spirituous  solution  of  the  lithium  chloride  containing  the  remain- 
der of  the  other  chlorides  to  dryness,  heat  moderately,  weigh, 
dissolve  in  water,  estimate  the  chlorine,  and  calculate  therefrom 
the  lithium  and  sodium  or  potassium.  BUNSEN  1"  also  applied 
the  method  to  the  indirect  estimation  of  lithium  in  presence  of 
potassium  and  sodium  by  removing  the  silver  from  the  filtrate, 
and  separating  the  potassium  with  platinum.  But  I  must  here 
point  out,  that  according  to  JENZSCH**  the  potassium  double  salt 
will  contain  lithium  apparently  in  the  form  of  the  platino- 
chloride  of  potassium  and  lithium. 

The  sulphuric  acid  in  weighed  quantities  of  the  sulphates 
of  lithium,  and  of  potassium  and  sodium,  cannot  be  determined 
as  barium  sulphate  (see  end  of  7). 

*  Pogg.  AnnaL,  LXVI,  79.  f  Ib.,  civ,  105. 

\Annal.  d.  Chem.  u.  Pharm.,  cxxi,  97.  §  Ib.,  cxxi,  98. 

I  Ib.,  xcvin,  193. 

*f\Annal.  d.  Chem.  u.  Pharm.,  cxxn,  348.  **  Pogg.  Annal.,  civ,  102. 


§  153.]  BASES   OF   GROUP  II.  607 

The  separation  of  lithium  from  ammonium  may  be  effected 
like  that  of  potassium  and  sodium  from  ammonium  (4  and  5). 


Second  Group. 

BARIUM STRONTIUM CALCIUM MAGNESIUM. 

I.  SEPARATION  OF  THE  BASIC  RADICALS  OF  THE  SECOND  GROUP  FROM 

THOSE    OF    THE    FlRST. 

§153. 

INDEX.     (The  numbers  refer  to  those  in  the  margin.) 
Barium  from  potassium  and  sodium,  10,  12. 

ammonium,  11. 
Strontium  from  potassium  and  sodium,  10,  13. 

ammonium,  11. 
Calcium  from  potassium  and  sodium,  10,  14. 

ammonium,  11. 

•  Magnesium  from  potassium  and  sodium,  15-26. 

ammonium,  11. 

A..   General  Method. 

1.  THE  WHOLE  OF  THE  ALKALI-EARTH  METALS  FROM  Po- 
TASSIUM  AND  SODIUM. 

Principle  on  which  the  method  is  based :  Ammonium  car-  10 
bonate  precipitates,  from   a   solution  containing  ammonium 
chloride,  only  barium,  strontium,  and  calcium. 

Mix  the  solution,  in  which  the  metals  are  assumed  to  be 
contained  in  the  form  of  chlorides,  with  a  sufficient  quantity  of 
ammonium  chloride  to  prevent  the  precipitation  of  the  magne- 
sium by  ammonia ;  dilute  rather  considerably,  add  some  ammo- 
nia, the^n  ammonium  carbonate  in  slight  excess,  let  the  mixture 
stand  covered  for  an  hour  in  a  moderately  warm  place,  filter,  • 
and  wash  the  precipitate  with  water  to  which  a  few  drops  of 
ammonia  have  been  added. 

The  precipitate  contains  the  barium,  strontium,  and  cal- 
cium ;  the  filtrate  the  magnesium  and  the  alkalies.  So  at 
least  we  may  assume  in  cases  where  the  highest  degree  of 
accuracy  is  not  required.  Strictly  speaking,  however,  the 
solution  still  contains  exceedingly  minute  traces  of  calcium 
and  somewhat  more  considerable  traces  of  barium,  as  the  car- 


608  SEPARATION.  [§  153. 

bonates  of  these  two  metals  are  not  absolutely  insoluble  in 
a  fluid  containing  ammonium  chloride;  the  precipitate  also 
may  contain  possibly  a  little  ammonium  magnesium  carbonate. 
Treat  the  precipitate  according  to  §  154,  and  the  filtrate — in 
rigorous  analyses — as  follows  :  Add  3  or  4  drops  (but  not  much 
more)  of  dilute  sulphuric  acid,  then  ammonium  oxalate,  and 
let  the  fluid  stand*  again  for  12  hours  in  a  warm  place.  If  a 
precipitate  forms,  collect  this  on  a  small  filter,  wash,  and  treat 
on  the  filter  with  some  dilute  hydrochloric  acid,  which  dis- 
solves the  calcium  oxalate,  and  leaves  the  barium  sulphate 
undissolved.  Since  a  little  magnesium  oxalate  may  have  sepa- 
rated with  the  former,  add  some  ammonia  to  the  hydrochloric 
solution,  filter  after  the  precipitate  has  settled,  and  mix  the 
filtrate  with  the  principal  filtrate. 

Evaporate  the  fluid  containing  the  magnesium  and  the  alka- 
lies to  dryness,  and  remove  the  ammonium  salts  by  gentle  igni- 
tion in  a  covered  crucible,  or  in  a  small  covered  dish  of  platinum 
or  porcelain.*  In  the  residue,  separate  the  magnesium  from 
the  alkalies  by  one  of  the  methods  given  15 — 24. 

2.  THE  WHOLE  OF  THE  ALKALI-EARTH  METALS  FROM  AM-  11 
MONIUM. — The  same  principle  and  the  same  process  as  in  the 
separation  of  potassium  and  sodium  from  ammonium  (4  and  5). 

B.  Special  Methods. 

SINGLE  ALKALI-EARTH  METALS  FROM  POTASSIUM  AND  SO- 
DIUM. 

1.  BARIUM  FROM  POTASSIUM  AND  SODIUM. 

Precipitate  the  barium  with  dilute  sulphuric  acid  (§  101, 1,  ^),  12 
evaporate  the  filtrate  to  dryness,  and  ignite  the  residue,  with 
addition  towards  the  end  of  ammonium  carbonate  (§  97,  1  and 
§  '98,  1).  Take  care  to  add  a  sufficient  quantity  of  sulphuric 
acid  to  convert  the  alkalies  also  completely  into  sulphates.  In 
exact  analyses,  in  order  to  save  the  alkali  salts  adhering  to  the 

*  This  operation  effects  also  the  removal  of  the  small  quantity  of  sulphuric 
acid  added  to  precipitate  the  traces  of  barium,  as  sulphates  of  the  alkalies  are 
converted  into  chlorides  upon  ignition  in  presence  of  a  large  proportion  of 
ammonium  chloride. 


§  153.]  BASES    OF   GROUP   II.  609 

barium  sulphate,  remove  the  dry  barium  sulphate  from  the 
filter,  heat  it  with  a  sufficient  quantity  of  pure  strong  sulphu- 
ric acid  to  dissolve  it  completely,  allow  to  cool,  dilute  largely, 
collect  the  barium  sulphate  (now  almost  absolutely  pure)  on  the 
•first  filter,  ignite,  and  weigh.  Evaporate  the  filtrate  in  a  plati- 
num dish,  drive  off  the  sulphuric  acid,  and  estimate  the  traces 
of  the  alkalies. 

This  method  is,  on  account  of  its  greater  accuracy,  prefer- 
able to  the  one  in  A,  in  cases  where  the  barium  has  to  be  sepa- 
rated only  from  one  of  the  two  fixed  alkalies;  but  if  both  alka- 
lies are  present,  the  other  method  is  more  convenient,  since  the 
alkalies  are  then  obtained  as  chlorides. 

2.  STRONTIUM  FKOM  POTASSIUM  AND  SODIUM. 

Strontium  may  be  separated  from  the  alkalies  like  barium,  13 
t>y  means  of  sulphuric  acid  (§  102,  1,  «);   but  this  method  is 
not  preferable  to  the  one  in  10,  in  cases  where  the  choice  is 
permitted  (comp.  §  102). 

3.  CALCIUM  FROM  POTASSIUM  AND  SODIUM. 

Precipitate  the  calcium  with  ammonium  oxalate  (§  103,  2,  14 
&,  or),  evaporate  the  nitrate  to  dryness,  and  determine  the  alka- 
lies in  the  ignited  residue.  In  determining  the  alkalies,  dis- 
solve the  residue,  freed  by  ignition  from  the  ammonium  salts, 
in  water,  filter  if  necessary,  acidify  the  filtrate,  according  to  cir- 
cumstances, with  hydrochloric  acid  or  sulphuric  acid,  and  then 
evaporate  to  dryness ;  this  treatment  of  the  residue  is  neces- 
sary, because  ammonium  oxalate  partially  decomposes  chlorides 
of  the  alkali  metals  upon  ignition  with  formation  of  alkali  car- 
bonates, except  in  presence  of  a  large  proportion  of  ammonium 
chloride.  The  results  are  still  more  accurate  than  in  A,  except 
where  ammonium  oxalate  has  been  used,  after  the  precipitation 
by  ammonium  carbonate,  to  remove  the  minute  traces  of  lime 
from  the  filtrate. 


610  SEPARATION.  [§  135. 

4.  MAGNESIUM  FROM  POTASSIUM  AND  SODIUM.* 

a.  Methods  Ixised  upon  the  sparing  solubility  of  Magnesium 
Hydroxide  in  Water. 

a.  Make  the  solution  as  neutral  as  possible,  and  free  from  1& 
ammonium  salts  (it  is  a  matter  of  indifference  whether  the  mag- 
nesium and  alkali  metals  are  present  as  sulphates,  chlorides,  or 
nitrates),  add  baryta-water  as  long  as  a  precipitate  forms,  heat 
to  boiling,  filter,  and  wash  the  precipitate  with  boiling  water. 
The  precipitate  contains  the  magnesium  as  hydroxide.  Dis- 
solve it  in  hydrochloric  acid,  precipitate  the  barium  with  sul- 
phuric acid,  and  then  the  magnesium  as  ammonium-magnesium 
phosphate  (§  104,  2).  The  alkalies,  which  are  contained  in  the 
solution,  according  to  circumstances,  as  chlorides,  nitrates,  or 
caustic  alkalies,  are  separated  from  the  barium  as  directed  in 
10  or  12.  LIEBIG,  who  was  the  first  to  employ  this  method, 
proposes  crystallized  barium  sulphide  as  precipitant.  The 
method  is  riot  very  exact,  as  magnesium  is  somewhat  more 
soluble  in  solutions  of  alkali  salts  than  in  water.  On  this 
account  the  weighed  alkali  salt  must  always  be  tested  for 
magnesium,  and  the  latter  determined  if  required. 

ft.  Precipitate  the  solution  with  a  little  pure  milk  of  lime,  1$ 
boil,  filter,  and  wash.  Separate  the  calcium  and  magnesium  in 
the  precipitate  according  to  36;  the  calcium  and  the  alkalies -in 
the  filtrate  according  to  10  or  14.  This  method  may  be  em- 
ployed when  magnesium  has  to  be  removed  from  a  fluid  con- 
taining calcium  and  alkalies,  provided  the  alkalies  alone  are 
to  be  determined.  Minute  quantities  of  magnesium  also  in  this 
case  remain  with  the  alkali  salt  from  the  cause  mentioned  in  a. 

y.  Evaporate  the  solution  of  the  chlorides  (which  must  IT 
contain  no  other  acids)  to  dryness,  and  if  ammonium  chloride 
is  present,  ignite ;  warm  the  residue  with  a  little  water  (this 
will  dissolve  it  with  the  exception  of  some  magnesium  oxide, 
which  separates).  Add  mercuric  oxide  shaken  up  with  water, 
evaporate  to  dryness  on  the  water-bath  with  frequent  stirring, 
dry  thoroughly,  ignite  with  increasing  temperature  till  all  the 
resulting  mercuric  chloride  is  volatilized,  proceeding  exactly  as 
detailed  in  §  104,  3,  b.  (Avoid  inhaling  the  fumes.)  There  is 

*The  methods  a,  a,  and  J3  are  suitable  for  the  separation  of  magnesium 
from  lithium. 


§  153.]  BASKS    OF    GROUP   II.  611 

no  need  to  continue  the  ignition  until  the  whole  of  the  mer- 
curic oxide  is  expelled ;  on  the  contrary,  part  of  it  may  be  fil- 
tered off  together  with  the  magnesium  oxide  and  subsequently 
volatilized  upon  the  ignition  of  the  latter.  Treat  the  residue 
with  small  quantities  of  hot  water,  filter  off  rapidly,  and  wash 
the  magnesium  oxide  with  hot  water,  using  small  quantities  at  a 
time,  and  not  continuing  the  operation  unnecessarily.  The 
solution  contains  the  alkalies  in  form  of  chlorides.  This 
method,  proposed  by  BEKZELIUS,  gives  satisfactory  results, 
and,  so  far  as  my  experience  goes,  is  the  best  of  those  given 
under  a.  Take  care  to  add  the  mercuric  oxide  only  in  proper 
quantity,  and  always  test  the  alkali  chlorides  for  magnesium, 
a  trace  of  which  will  generally  be  found. 

d.  If  the  bases  are  present  as  chlorides,  add  as  much  pure  18 
oxalic  acid  *  as  may  be  necessary  to  unite  with  all  the  bases 
present,  viewed  as  potassium,  to  form  a  tetroxalate,  add  a 
little  water,  evaporate  to  dryness  in  a  platinum  dish,  and 
ignite.  By  this  operation  the  magnesium  chloride  is  completely, 
the  alkali  chlorides  partially,  converted  into  oxalates,  which 
on  ignition  yield  alkali  carbonates  and  magnesia.  Treat  the 
residue  repeatedly  with  small  quantities  of  boiling  water ;  it 
is  immaterial  whether  the  precipitate  is  transferred  to  the 
filter  or  remains  in  the  platinum  dish.  "When  all  the  alkali 
salt  is  washed,  dry  the  filter,  incinerate  it  in  the  dish,  ignite 
strongly,  and  weigh  the  magnesia.  Should  the  solution  ob- 
tained be  slightly  turbid,  evaporate  it  to  dryness,  take  up 
the  residue  with  water,  and  filter  off  the  trifling  residual 
magnesia;  finally  add  hydrohcloric  acid  to  the  filtrate  and  esti- 
mate the  alkalies  as  chlorides. 

If  the  bases  are  present  as  sulphates,  add  barium  chloride    19 
to  the  boiling  solution  until  a  precipitate  just  ceases  to  form, 
evaporate  the  filtrate  with  excess  of  oxalic  acid,  and  proceed 
as  in   18.      Separate  the  slight  quantity  of  barium  carbonate 
remaining  with  the  magnesia  as  directed  in  29. 

These    methods  were   devised   by  E.  MITSCHERLICH    and    20 
described    by    LAScii.f     According    to    my    experience    the 

*  TH.    SCHERER  (Zeitschr.  f.   analyt.   Chem.,   xi,  197)  recommends  pure 
ammonium  oxalate  instead  of  oxalic  acid. 
•\Journ.f.  prakt.  CJiem.,  LXIII,  343. 


612  SEPARATION.  [§  153. 

results  are  not  particularly  good.  As  a  rule  too  little  mag- 
nesia is  obtained ;  hence  the  weighed  alkali  salt  should  always 
be  tested  with  sodium  phosphate  and  ammonia  for  magnesia. 
Not  infrequently  a  quite  weighable  precipitate  is  obtained, 
which  must  not  be  neglected.* 

The  method  described  in  18  is  also  applicable  to  nitrates, 
and  was  recommended  for  these  by  DEVILLED  During 
evaporation  there  are  evolved  carbon  dioxide  and  nitrous  acid. 

h.  Methods  based  on  the  Precipitation  of  Magnesium  ~by 
Ammonium  Phosphate  (or  Ar  senate}. 

Add  ammonia  in  excess  to  the  solution  containing  mag-  21 
nesium,  potassium,  and  sodium,  and  add  some  ammonium 
chloride  should  this  not  be  already  present ;  then  precipitate 
the  magnesium  with  only  a  slight  excess  of  pure  ammonium 
phosphate.  Expel  the  free  ammonia  from  the  filtrate  by 
evaporation,  and  precipitate  the  phosphoric  acid  with  lead 
acetate  as  a  compound  of  lead  phosphate  and  lead  chloride. 
Remove  the  excess  of  lead  oxide  with  ammonia  and  ammonium 
carbonate,  or  with  hydrogen  sulphide,  from  the  still  warm 
fluid,  and  in  the  filtrate  determine  the  potassium  and  sodium 
according  to  §§.97  and  98  (O.  L.  EKDMANN  J ;  HEINTZ§). 
The  method  is  rather  inconvenient,  but  quite  accurate.  If  the 
solution  contains  much  ammonium  chloride,  the  greater  part 
should  be  first  removed  by  volatilization. 

The  excess  of  phosphoric  acid  may  also  be  removed  with    22 
ferric  oxide  or  silver  oxide  instead  of  with  lead  oxide 

a.  With  ferric  oxide.  Expel  any  ammonia  from  the 
liquid  with  heat,  neutralize  if  necessary  with  hydrochloric  acid, 
and  add  ferric-chloride  solution  until  the  liquid  is  yellowish ; 
then  add  ammonium  carbonate  until  the  liquid  is  neutral  or 
only  acid  from  the  carbonic  acid  present,  boil,  filter  off  the 
basic  ferric  phosphate  (which,  if  sufficient  ferric  chloride  has 
been  used,  will  have  a  reddish-brown  color),  wash,  evaporate 

*  SONNENSCHEIN'S  method  (boiling  the  chlorides  with  silver  carbonate)  I 
cannot  recommend,  as  the  filtrate  always  contains  magnesia,  and  in  fact  more 
than  mere  traces. 

\Journf.prakt.  Chem.,  LX,  17. 

j  lb.,  XXXTX,  278. 

§  Pogg.  AnnaL,  LXXIII,  119. 


§  153.]  BASES   OF   GROUP   II.  613 

the  filtrate  to  dryness,  expel  the  ammonium  salt,  and  deter- 
mine the  potassium  and  sodium  according  to  §§  97,  98.  A 
good  and  convenient  method. 

(3.  With  silver  oxide.  Evaporate  to  dryness  the  fluid  fil- 
tered off  from  the  ammonium-magnesium  phosphate,  ignite 
cautiously,  dissolve  in  water,  and  mix  with  silver  nitrate  and 
a  slight  excess  of  silver  carbonate.  After  filtering,  remove 
the  excess  of  silver  from  the  filtrate  with  hydrochloric  acid, 
and  evaporate  the  solution  to  dryness  with  an  excess  of  hydro- 
chloric acid  (CHANCEL  *). 

The  separation  is  somewhat  shorter,  if  less  precise  and 
convenient,  when  the  magnesium  is  precipitated  with  ammo- 
nium arsenate(§  127,  2)  instead  of  ammonium  phosphate,  and 
the  liquid,  with  some  ammonium  chloride  added,  evaporated 
to  dryness  and  the  residue  ignited  under  a  good  chimney. 
By  this  treatment  the  excess  of  arsenate  added  is  volatilized, 
while  the  alkalies  remain  behind  as  chlorides  (always  retaining, 
however,  a  little  magnesium  chloride).  C.  v.  HAUERf 
recommended  a  similar  method. 

c.  Method  based  on  the  Precipitation  of  the  Magnesium 
as  Ammonium- Magnesium  Carbonate. 

Mix  the  solution  of  sulphates,  nitrates,  or  chlorides  (it  23 
must  be  very  concentrated)  with  an  excess  of  a  concentrated 
solution  of  sesquicarbonate  of  ammonia  in  water  and  ammonia 
(230  grm.  of  the  salt,  360  c.  c.  solution  of  ammonia,  sp.  gr. 
0*96,  and  water  to  one  litre).  After  twenty-four  hours  filter 
off  the  precipitate  (MgCO,*[NH4]aCO3  +  4H2O),  wash  it  with 
the  solution  of  ammonia  and  ammonium  carbonate  used  for  the 
precipitation,  dry,  ignite  strongly  and  for  a  sufficient  length 
of  time,  and  weigh  the  magnesium  oxide.  Evaporate  the 
filtrate  to  dryness,  keeping  the  heat  at  first  under  100°,  expel 
the  ammonium  salts,  and  determine  the  alkalies  as  chlorides 
or  sulphates.  When  sodium  alone  is  present  the  results  are 
fairly  satisfactory.  In  the  presence  of  potassium  the  ignited 
magnesium  oxide  must  be  extracted  with  water  before  weigh- 
ing, as  it  contains  an  appreciable  quantity  of  potassium  carbo- 

*  Compt.  rend.,  L,  94. 

f  Jahrb.  der  k.  k.  geolog.  JKeicJisanstalt,  iv,  863. 


614  SEPARATION.  [§  153. 

nate ;  the  washings  are  to  be  added  to  the  principal  filtrate. 
This  last  measure  is  unnecessary  in  the  absence  of  potassium. 
The  magnesium  is  always  a  little  too  low.  Mean  error  0'009 

(F.    G.    SCHAFFGOTSCH,*    PI.    WEBEK  f). 

d.  Method  based  on  the  Precipitation  of  the  Alkalies  as 
jSilicofluorides  (STOLE A  ;£). 

When  a  solution  contains  a  mixture  of  potassium  and  mag-  24 
nesium  chlorides  or  potassium  and  magnesium  nitrates,  the 
potassium  in  one  aliquot  portion  may  be  precipitated  and  de- 
termined as  silicofi uoride  (§  97,  5),  while  in  another  the  magne- 
sium may  be  precipitated  as  ammonium-magnesium  phosphate 
(§  104,  2).  If  it  is  desired  to  make  both  determinations  in  the 
same  portion  of  fluid,  remove  the  excess  of  silicofluoric  acid 
from  the  fluid  filtered  off  frorn  the  potassium  silicofluoride,  by 
an  alcoholic  solution  of  potassium  acetate,  wash  the  precipitate 
with  a  mixture  of  equal  volumes  of  alcohol  and  water,  and 
determine  the  magnesium  in  the  filtrate.  If  sulphates  are  pres- 
ent, the  method,  in  my  opinion,  is  rendered  so  difficult  as  to 
make  it  untrustworthy,  because  of  the  difficult  solubility  of  the 
magnesium  sulphate  in  dilute  alcohol. 

This  method  is  less  adapted  for  the  separation  of  sodium 
from  magnesium  than  for  potassium  from  magnesium,  because 
sodium  silicofluoride  is  more  soluble  in  alcohol  than  is  the  po- 
tassium salt.  In  the  case  of  sulphates  it  is  unserviceable,  and 
in  the  case  of  chlorides  or  nitrates,  to  obtain  results  of  any 
value,  it  is  necessary  to  add  2  volumes  of  strong  alcohol  after 
adding  the  hydrosilicofluoric  acid,  and  to  allow  the  precipi- 
tate to  settle  completely  before  filtering. 

e.  Indirect  methods  which  give  simultaneously  the  quan- 
tity of  Potassium  and  Sodium,  if  both  are  present. 

a.  Weigh  the  sulphates,  dissolve,  divide  the  solution  into  25 
two  parts,  and  in  one  determine  the  magnesium  according  to 
§  104,  2;   in  the  other  determine  the  potassium  as  in  2,  cal- 
culate the  magnesium  sulphate  and  potassium  sulphate,  and 
from  the  difference  find  the  sodium  sulphate. 

*  Pogg.  Annal.,  civ,  482.        f  Vierteljahresschrift  f.  prakt.  Pharm.,  vm,  161 
IZeitschr.f.  analyt.  Chem.,  iv,  160. 


§  154.]  BASES    OF   GROUP  II.  615 

ft.  With  proper  caution  convert  the  bases  into  pure  neutral  26 
sulphates,  weigh  these,  dissolve  in  water,  determine  the  sul- 
phuric acid  present  with  barium  chloride  (§  132),  precipitate 
the  excess  of  barium  chloride  in  the  filtrate  with  sulphuric  acid, 
filter  again,  and  in  the  filtrate,  concentrated  by  evaporation, 
determine  the  magnesium  as  directed  in  §  104,  2  (K.  LIST*). 

Deduct  the  weight  of  the  magnesium  calculated  as  mag- 
nesium sulphate  from  the  weight  of  the  total  sulphates;  the 
difference  will  give  the  weight  of  the  alkali  sulphates.  Further, 
deduct  the  weight  of  the  sulphuric  acid  combined  with  the 
magnesium  from  the  total  sulphuric  aci$ ;  the  difference  will 
give  that  combined  with  the  alkalies.  See  §  152,  3  (6). 

It  is,  of  course,  evident  that  the  indirect  methods  can  give 
accurate  results  only  when  the  most  painstaking  care  is  exer- 
cised. The  accuracy  of  method  /?  is,  besides,  impaired  by 
the  tendency  of  barium  sulphate  to  carry  down  with  it  readily 
soluble  salts. 

II.  SEPARATION  OF   THE  BASIC  RADICALS  OF  THE  SECONP 

GROUP    FROM    EACH    OTHER. 

§154. 

INDEX.     (The  numbers  refer  to  those  in  the  margin.) 
Barium  from  strontium,  28,  31,  40. 

calcium,  28,  30,  31,  35,  40. 
"  magnesium,  27,  29. 

Strontium  from  barium,  28,  31,  40. 
««  calcium,  34,  38,  39. 

"  magnesium,  27,  29. 

Calcium  from  barium,  28,  30,  31,  35,  40. 
strontium,  34,  38,  39. 
magnesium,  27,  32,  33,  36,  37. \ 
Magnesium  from  barium,  27,  29. 
"  strontium,  27,  29. 

calcium,  27,  32,  33,  36,  37.  f 


*  Annal.  d.  Chem.  u.  Pharm.,  LXXXI,  117. 

f  Compare  also  the  method  of  OEFFINGEB,  Zeitschr.  f.  analyt.  Chem.,  vm, 
456. 


616  SEPARATION.  [§  154. 

A.  General  Method. 

THE    WHOLE    OF    THE    ALKALI-EARTH    METALS    FROM    EACH 
OTHER. 

Proceed  as  in  10.  The  magnesium  is  precipitated  from  the  27 
filtrate  as  ammonium-magnesium  phosphate  (see  foot-note,  p. 
620).  The  precipitated  carbonates  of  barium,  strontium,  and 
calcium  are  dissolved  in  hydrochloric  acid,  and  the  bases  sepa- 
rated as  directed  in  28.  The  traces  of  magnesium,  which  may 
be  present  in  the  ammonium  carbonate  precipitate,  are  obtained 
by  evaporating  the  nitrate  from  the  strontium  or  calcium  sul- 
phate to  dryness,  taking  up  the  residue  with  water,  and  precipi- 
tating the  solution  with  sodium  phosphate  and  ammonia. 

B.  Special  Methods. 

1.   Methods  based  upon  the  Insolubility  of  Barium 

Silicofluoride. 

BARIUM  FROM  STRONTIUM  AND  FROM  CALCIUM. 

Mix  the  neutral  or  slightly  acid  solution  with  hyclrofluosi-  28 
licic  acid  *  in  excess,  add  one  third  of  the  volume  of  alcohol  of 
0-81  sp.  gr.,  let  the  mixture  stand  twelve  hours,  collect  the  pre- 
cipitate of  'barium  silicofluoride  on  a  weighed  filter,  wash  with 
a  mixture  of  equal  parts  of  water  and  alcohol  until  the  wash- 
ings cease  to  show  even  the  least  trace  of  acid  reaction  (but  no 
longer),  and  dry  at  100°.  Precipitate  the  strontium  or  calcium 
from  the  filtrate  by  dilute  sulphuric  acid  (§  102,  1,  a,  and  §  103, 
1).  The  results  are  satisfactory.  For  the  properties  of  barium 
silicofluoride,  see  §  71.  If  both  strontium  and  calcium  are  pres- 
ent, the  sulphates  are  weighed,  and  then  separated  according 
to  34,  or  they  are  converted  into  carbonates  (§  132,  II.,  J),  and 
separated  according  to  38  or  39. 

*  If  not  kept  in  a  gutta-percha  bottle  it  should  be  freshly  prepared, 


§  154.]  BASES   OF   GROUP  II.  617 

2.  Methods  based  upon  the  Insolubility  of  Barium 
Sulphate  or  Strontium  Sulphate,  as  the  case  may  be,  in  • 
Water  and  in  Solution  of  Sodium  T hiosulphate. 

a.  BARIUM  AND  STRONTIUM  FROM  MAGNESIUM. 

Precipitate  the  barium  and  strontium  witli  sulphuric  acid  29 
(§  101,  1,  a  and  §  102,  1,  a\  and  the  magnesium  from  the  fil- 
trate   with    ammonia     and      sodium-ammonium      phosphate 
(§  101,  2). 

b.  BARIUM  FROM  CALCIUM. 

Mix  the  solution  with  hydrochloric  acid,  then  with  highly  30 
dilute  sulphuric  acid  (1  part  acid  to  300  water),  as  long  as  a  pre- 
cipitate forms  ;  allow  to  deposit,  and  determine  the  barium  sul- 
phate as  directed  §  101,  1,  a.  Concentrate  the  washings  by 
evaporation  and  add  them  to  the  filtrate,  neutralize  the  acid 
with  ammonia,  and  precipitate  the  calcium  as  oxalate  (§  103,  2, 
b,  a).  The  method  is  principally  to  be  recommended  when 
small  quantities  of  barium  have  to  be  separated,  from  much  cal- 
cium. If  we  have  to  separate  calcium  sulphate  from  barium 
sulphate,  the  salts  may  (in  the  absence  of  free  acids)  be  treated 
repeatedly  with  a  solution  of  sodium  thiosulphate  at  a  gentle 
heat.  The  barium  sulphate  remains  undissolved,  the  calcium 
sulphate  dissolves.  The  calcium  is  precipitated  from  the  fil- 
trate by  ammonium  oxalate  (DiEHL*). 

3.  Method  based  upon  the  different  deportment  with 
Alkali  Carbonates  of  Barium  Sulphate  on  the  one  hand, 
and  Strontium  and  Calcium  Sulphates  on  the  other. 

BARIUM  FROM  STRONTIUM  AND  CALCIUM. 

Digest  the  three  precipitated  sulphates  for  twelve  hours  at  31 
the  common  temperature  (15° — 20°),  with  frequent  stirring, 
with  a  solution  of  ammonium  carbonate,  decant  the  fluid  on  to 
a  filter,  treat  the  residue  repeatedly  in  the  same  way,  wash 
finally  with  water,  and  in  the  still  moist  precipitate,  separate 
the  undecomposed  barium  sulphate  by  means  of  cold  dilute 
hydrochloric  acid  from  the  strontium  and  calcium  carbonates 
formed.  To  hasten  the  separation  you  may  boil  the  sulphates 
for  some  time  with  a  solution  of  potassium  (not  sodium)  car- 
bonate, to  which  \  the  amount  of  the  carbonate,  or  more,  of 

*Jouru.f.  prakt.  Chem.,  LXXIX,  430. 


618  SEPARATION.  [§  154. 

potassium  sulphate  has  been  added.  By  this  process,  also,  the 
strontium  and  calcium  sulphates  are  decomposed,  the  barium 
sulphate  remaining  unacted  on.  If  the  basic  metals  are  in  solu- 
tion, the  above  solution  of  potassium  carbonate  and  sulphate  is 
added  in  excess  at  once,  and  the  whole  boiled.  The  precipitate, 
consisting  of  barium  sulphate  and  strontium  and  calcium  car- 
bonates, is  to  be  treated  as  above  with  cold  hydrochloric  acid 
(H.  ROSE*). 

4.  Methods  based  on  the  Insolubility  of  Calcium  Sul- 
phate in  Alcohol. 

CALCIUM  FROM  MAGNESIUM. 

a.  Remove  water  and  free  hydrochloric  from  a  solution  of  32 
the  chlorides  by  evaporation,  dissolve  the  residue  in  strong  (but 
not  absolute)  alcohol,  add  a  slight  excess  of  pure  strong  sulphu- 
ric acid,  digest  in  the  cold,  allow  to  stand  for  some  hours,  trans- 
fer the  precipitate  consisting  of  calcium  sulphate  and  some 
magnesium  sulphate  to  a  filter,  wash  away  the  acid  thoroughly 
with  nearly  absolute  alcohol,  and  then,  but  only  after  the  acid 
has  been  completely  removed,  continue  the  washing  with  alcohol, 
sp.  gr.  0-96 — 0-95,  till  a  few  drops  of  the  washings  give  no 
residue  on  evaporation.  Weigh  the  calcium  sulphate  accord- 
ing to  §  103,  1.  Evaporate  the  alcohol  from  the  filtrate,  and 
determine  the  magnesium  according  to  §  104,  2.  The  method 
is  in  itself  not  new,  but  A.  CmzYNSKijf  adopting  the  precautions 
here  given,  has  obtained  excellent  results,  even  in  the  presence 
of  phosphoric  acid. 

b.  SMALL  QUANTITIES  OF  CALCIUM  FROM  MUCH  MAGNESIUM.  33 
Convert  into  neutral  sulphates,  dissolve  the  mass  in  water,  and 
add  alcohol,  with  constant  stirring,  till  a  slight  permanent  tur- 
bidity is  produced,  Wait  a  few  hours  and  then  filter,  wash  the 
precipitated  calcium  sulphate  with  alcohol  which  has  been 
diluted  with  an  equal  volume  of  water,  and  determine  it  after 
§  103,  1,  a  (in  which  case  the  weighed  sulphate  must  be  tested 
for  magnesium),  or  dissolve  the  precipitate  in  water  containing 
hydrochloric  acid  and  separate  the  calcium  from  the  small  quan- 
tity of  magnesium  possibly  coprecipitated  according  to  36 

(SCHEERER^). 

*Pogg.  Ann.,  xcv,  286,  299,  427.         f  Zeilschr.f.  analyt.  Chem.,  iv,  348. 
%Annal.  d.  Chem  u.  Pharm.,  ex,  237. 


§  154.]  BASES   OF   GROUP   II.  619 

5.  Methods  'based  on  the  Insolubility  of  Strontium,  and 
Barium  Sulphates  in  solution  of  Ammonium  Sulphate. 

STRONTIUM  FROM  CALCIUM. 

If  the  mixture  is  soluble,  dissolve  in  the  smallest  quantity  34 
of  water,  add  about  50  times  the  quantity  of  the  substance  of 
ammonium  sulphate  dissolved  in  four  times  its  weight  of  water, 
and  either  boil  for  some  time  with  renewal  of  the  water  that 
evaporates  and, addition  of. a  very  little  ammonia  (as  the  solu- 
tion of  ammonium  sulphate  becomes  acid  on  boiling),  or  allow  to 
stand  at  the  ordinary  temperature  for  twelve  hours.  Filter  and 
wash  the  precipitate,  which  consists  of  strontium  sulphate  and 
a  little  ammonium  strontium  sulphate,  with  a  concentrated  solu- 
tion of  ammonium  sulphate,  till  the  washings  remain  clear  on 
addition  of  ammonium  oxalate.  The  precipitate  is  cautiously 
ignited,  moistened  with  a  little  dilute  sulphuric  acid  (to  convert 
the  small  quantity  of  strontium  sulphide  into  sulphate),  reig- 
nited  and  weighed.  The  highly  dilute  filtrate  is  precipitated 
with  ammonium  oxalate,  and  the  calcium  determined  according 
to  §  103,  2,  &,  a.  If  you  have  the  solid  sulphates  to  analyze, 
they  are  very  finely  powdered  and  boiled  with  concentrated  solu- 
tion of  ammonium  sulphate  with  renewal  of  the  evaporated 
water  and  addition  of  a  little  ammonia.  Results  very  close,  e.g., 
1 -048  Sr(NO3)2  instead  of  1-053,  and  0 -497  CaCO3,  instead 
of  0-504:  (H.  ROSE*). 

.    BARIUM  may  be  separated  FROM  CALCIUM  in  the  same  way.   35 

6.  Methods  based  upon  the  Insolubility  of  Calcium 
Oxalate  in  Ammonium  Chloride  and  in  Acetic  Acid. 
CALCIUM  FROM  MAGNESIUM. 

a.  Mix  the  properly  diluted  solution  with  sufficient  ammo-  36 
ninm  chloride  to  prevent  the  formation  of  a  precipitate  by 
ammonia,  which  is  added  in  slight  excess ;  add  ammonium  oxa- 
late as  long  as  a  precipitate  forms,  then  a  further  portion  of  the 
same  reagent,  about  sufficient  to  convert  the  magnesium  also 
into  oxalate  (which  remains  in  solution).  This  excess  is  abso- 
lutely indispensable  to  insure  complete  precipitation  of  the  cal- 
cium, as  calcium  oxalate  is  slightly  soluble  in  magnesium  chlo- 
ride not  mixed  with  ammonium  oxalate  (Expt.  No.  83).  Let 


*  Pogg.  Annal.,  ex,  296. 


620  SEPARATION.  [§  154 

the  mixture  stand  twelve  hours,  decant  the  supernatant  clear 
fluid,  as  far  as  practicable,  from  the  precipitated  calcium  oxa- 
late,  mixed  with  a  little  magnesium  oxalate,  on  to  a  filter,  wash 
the  precipitate  once  in  the  same  way  by  decantation,  then  dis- 
solve in  hydrochloric  acid,  add  water,  then  ammonia  in  slight 
excess,  and  a  little  ammonium  oxalate.  Let  the  fluid  stand 
until  the  precipitate  has  completely  subsided,  then  pour  on  to 
the  previous  filter,  transfer  the  precipitate  finally  to  the  latter, 
and  proceed  exactly  as  directed  §  103,  2,  5,  a.  The  first  filtrate 
contains  by  far  the  larger  portion  of  the  magnesium,  the  second 
the  remainder.  Evaporate  the  second  filtrate,  acidified  with 
hydrochloric  acid,  to  a  small  volume,  then  mix  the  two  fluids, 
and  precipitate  the  magnesium  with  sodium  ammonium  phos- 
phate (HNaNH4)PO4  ,*as  directed  §  104,  2.  If  the  quantity  of 
ammonium  salts  present  is  considerable,  the  estimation  of  the 
magnesium  is  rendered  more  accurate  by  evaporating  the  fluids 
in  a  large  platinum  or  porcelain  dish  to  dryness,  and  igniting 
the  residuary  saline  mass,  in  small  portions  at  a  time,  in  a  smaller 
platinum  dish,  until  the  ammonium  salts  are  expelled.  The 
residue  is  then  treated  with  hydrochloric  acid  and  water, 
warmed,  allowed  to  cool,  and  rendered  just  alkaline  writh  ammo- 
nia. If  enough  ammonium  chloride  is  present,  no  magnesium 
hydroxide  will  fall  down,  but  occasionally  small  flocks  of  silica 
or  alumina  are  to  be  seen.  Filter  them  off  and  finally  precipi- 
tate with  ammonia  and  (IEN"aNH4)PO4.  If  the  precipitate  pro- 
duced by  ammonia  is  at  all  considerable,  dissolve  it  in  hydro- 
chloric acid,  evaporate  the  solution  on  a  water-bath  to  dryness, 
treat  the  residue  with  hydrochloric  acid  and  water,  render  alka- 
line with  ammonia,  filter,  and  add  the  filtrate  to  the  principal 
solution. 

Numerous  experiments  have  convinced  me  that  this  method, 
which  is  so  frequently  employed,  gives  accurate  results  only  if 
the  foregoing  instructions  are  strictly  complied  with.  It  is  only 
in  cases  where  the  quantity  of  magnesium  present  is  relatively 
small  that  a  single  precipitation  with  ammonium  oxalate  may 
be  found  sufficient  (conip.  Expt.  No.  84  f). 


*  This  is  preferable  to   sodium  phosphate  as  a  precipitant.     See 
Zeitschr.f.  analyt.  Chem.,  xn,  36 

f  Further  experiments  will  be  found  in  Zeitschr.  f.  analyt.  Chem.,  vn,  310. 


§  154.]  BASES   OF   GROUP  II.  621 

b.  In  the  case  of  calcium  and  magnesium  phosphates,  dis-  37 
solve  in  the  least  possible  quantity  of  hydrochloric  acid,  add 
ammonia  until  a  copious  precipitate  forms ;  redissolve  this  by 
addition  of  acetic  acid,  and  precipitate  the  calcium  with  an 
excess  of  ammonium  oxalate.  To  determine  the  magnesium, 
precipitate  the  filtrate  with  ammonia  and  (H!NaNH4)PO4.  As 
free  acetic  acid  by  no  means  prevents  the  precipitation  of  small 
quantities  of  magnesium  oxalate,  the  precipitate  contains  some 
magnesium,  and  as  calcium  oxalate  is  not  quite  insoluble  in 
acetic  acid,  the  filtrate  contains  some  calcium  ;  these  two  sources 
of  error  compensate  each  other  in  some  measure.  In  accurate 
analysis,  however,  these  trifling  admixtures  of  magnesium  and 
calcium  are  afterwards  separated  from  the  weighed  precipi- 
tates of  calcium  carbonate  or  oxide  and  magnesium  pyrophos- 
phate  respectively. 

Y.  Method  based  upon  the  Insolubility  of  Strontium 
Nitrate  in  Alcohol  and  Ether. 

STRONTIUM  FROM  CALCIUM  (after  STROMEYER). 

Digest  the  perfectly  dry  nitrates  in  a  closed  flask  with  abso-  38 
lute  alcohol,  to  which  an  equal  volume  of  ether  should  be  added 
(H.  HOSE).  Filter  off  the  undissolved  strontium  nitrate  in  a 
covered  funnel,  wash  with  the  mixture  of  alcohol  and  ether,  dis- 
solve in  water,  and  determine  as  strontium  sulphate  (§  102,  1). 
Precipitate  the  calcium  from  the  filtrate  by  sulphuric  acid. 
The  results  are  satisfactory. 

8.  Indirect  Method. 

STRONTIUM  FROM  CALCIUM. 

Determine  both  bases  first  as  carbonates  or  oxides,  precipi-  39 
tating  them  either  with  ammonium  carbonate  or  oxalate (§§  102, 
103) ;  then  estimate  the  amount  of  carbonic  acid  in  them,  and 
calculate  the  amount  of  strontium  and  calcium  as  directed  in 
"  Calculation  of  Analyses  "  (§  200).  The  determination  of  the 
carbonic  acid  may  be  effected  by  fusion  with  vitrified  borax 
(§  139,  II.,  <?),  but  the  application  of  a  moderate  white  heat, 
such  as  is  given  by  a  good  gas  blowpipe  without  the  use  of  a 

Compare  also  WITTSTEIN,  Zeitschr.  f.  analyt  Cliem  ,  u,  318.  and  COSSA,  t'5., 
vin,  141.  According  to  HAGER,  ib.,  ix,  254.  the  precipitate  of  calcium  oxalate 
will  be  free  from  magnesium  if  filtered  off  immediately,  however,  I  fear  that  a 
little  calcium  might  in  this  case  be  left  in  solution. 


(522  SEPAKATIOT*.  [§  154. 

crucible  jacket,  is  alone  sufficient  to  drive  out  all  the  carbonic 
acid  from  both  the  carbonates  (F.  G.  SCHAFFGOTSCH  *).  I  can 
strongly  recommend  this  method.  It  is  well  to  precipitate  the 
carbonates  hot,  to  press  the  precipitate  cautiously  down  in  the 
platinum  crucible  and  turn  over  the  agglomerated  cake  every 
now  and  then  till,  after  repeated  ignitions,  the  weight  has  become 
constant.  The  results  are  good  if  neither  of  the  bases  is  present 
in  too  minute  quantity. 

The  indirect  separation  may,  of  course,  be  effected  by  means  40 
of  other  salts,  and  can  be  used  also  for  the  determination  of  CAL- 
CIUM   IN    PRESENCE    OF    BARIUM  Or  of  BARIUM    IN    PRESENCE    OF 

STRONTIUM.  In  the  expulsion  of  carbonic  acid  from  barium  car- 
bonate vitrified  borax  must  be  used  (§  139,  II.,  c). 

Third  Group. 

ALUMINIUM CHROMIUM. 

I.  SEPARATION  OF  ALUMINIUM   AND   CHROMIUM  FROM  THE 

ALKALIES. 

§  155. 

1.  FROM  AMMONIUM. 

a.  Aluminium  and  chromium  salts  may  oe  separated  from  41 
ammonium  salts  by  ignition.  However,  in  the  case  of  alu- 
minium, this  method  is  applicable  only  in  the  absence  of  chlo- 
rine (volatilization  of  aluminium  chloride).  The  safest  way, 
therefore,  is  to  mix  the  compound  with  sodium  carbonate 
before  igniting. 

1}.  Determine  the  ammonium  by  one  of  the  methods  given  42 
in  §  99,  3,  using  solution  of  potassa  or  soda  to  effect  the  expul- 
sion of  ammonia.     The  aluminium   and   chromium   are  then 
determined  in  the  residue  in  the  same  way  as  in  43. 

2.  FROM  POTASSIUM  AND  SODIUM. 

a  Precipitate  and  determine  the  chromium  and  aluminium  43 
with  ammonia  as  directed  in  §  105,  «,  and  §  106,  1,  a.    The  ni- 
trate contains  the  alkalies,  which  are  then  freed  from  the  ammo- 
nium salt  formed,  by  evaporation  to  dry  ness  and  ignition.     In 
the  presence  of  large  quantities  of  alkali  salts  it  is  well  to  dis- 

*  Pogg.  AnnaL,  cxiu,  615. 


§  156.]  BASES    OF   GROUP   III.  623 

solve  the  moderately  ignited  precipitate  in  hydrochloric  acid 
and  reprecipitate  with  ammonia. 

~b.   Aluminium  may  be  separated  also  from  potassium  and  44 
sodium  by  heating  the  nitrate  (see  46). 

II.   SEPARATION  OF  ALUMINIUM  AND  CHROMIUM  FROM  THE 
ALKALI-EARTH  METALS. 

§  156. 
INDEX.     (The  numbers  refer  to  those  in  the  margin. 

a.  Aluminium  from  barium,  45-50,  and  51. 

"  "    strontium,  45-50,  and  51. 

"  "     calcium,  45-50,  and  52,  53,  54. 

"     magnesium,  45-50,  and  53,  54. 

b.  Chromium  from  the  alkali-earth  metals,  55-58. 

a.  SEPARATION  OF  ALUMINIUM  FROM  THE  ALKALI-EARTH 

METALS. 

.     A.   General  Methods. 

TlIE    WHOLE     OF     THE    ALKALI-EARTH     METALS    FROM    ALU- 
MINIUM. 

1.  Method  based  upon   the   Precipitation   of  Alu- 
minium Hydroxide  by  Ammonia,  and  upon  its  solution 

in  Soda. 

Put  the  solution  in  a  platinum  dish  or,  with  less  advantage,  45 
a  porcelain  dish.  Let  it  be  dilute  and  warm.  Add  a  tolerable 
quantity  of  ammonium  chloride,  if  such  be  not  already  present, 
then  very  gradually,  almost  drop  by  drop  (WRINKLE*),  amnto- 
nia  as  free  as  possible  from  carbonic  acid,  in  moderate  excess, 
and  boil  till  no  more  free  ammonia  is  observable.  Under  these 
circumstances,  a  little  magnesium  hydroxide,  and  also  a  small 
quantity  of  calcium,  barium,  or  strontium  carbonates  are  at  first 
precipitated  along  with  the  aluminium  hydroxide ;  on  che  boil- 
ing with  ammonium  chloride,  the  coprecipitated  alkali-earth 
metal  compounds  redissolve,  so  that  the  aluminium  hydroxide 
finally  retains  only  an  unweighable  or  scarcely  weighabk-  trace 
of  them.  Allow  to  deposit,  and  proceed  with  the  aluminium 
determination  according  to  §  105,  a.  In  very  exact  analysis  it 
is  well,  after  moderately  washing  the  aluminium  precipitate,  to 

*  ZeitscJir.  f.  analyt.  Chem.,  x,  96. 


624  SEPAKATION.  [§  156. 

redissolve  it  in  hydrochloric  acid,  and  reprecipitate  with  ammo- 
nia as  above.  In  separations  of  aluminium  from  calcium  or 
magnesium  this  double  precipitation  is  especially  necessary  in 
the  presence  of  sulphates.  After  the  aluminium  oxide  has 
been  weighed,  fuse  it  for  a  long  time  with  potassium  disul- 
phate,  dissolve  the  fused  mass  in  water,  and  determine  any  sili- 
cic acid*  that  may  remain.  The  solution,  when  mixed  with 
potassa  in  excess,  will  often  not  appear  perfectly  clear,  but  will 
contain  a  few  flocks  of  magnesium  hydroxide  (perhaps  also 
traces  of  barium,  strontium,  or  calcium  carbonates).  If  there  is 
any  amount  of  the  latter,  filter  it  off,  dissolve  in  nitric  acid,  pre- 
cipitate with  ammonia,  boil  till  the  fluid  ceases  to  smell  of 
ammonia,  filter,  evaporate  the  small  quantity  of  fluid  in  a  pla- 
tinum capsule,  ignite,  weigh  the  residual  magnesium  oxide 
(which  may  contain  traces  of  other  alkali-earth  metals),  deduct 
it  from  the  aluminium  oxide,  dissolve  it  in  hydrochloric  acid, 
and  add  to  the  original  filtrate.  In  order  to  the  further  separa- 
tion of  the  alkali-earth  metals,  acidify  the  fluid  containing  them 
with  hydrochloric  acid,  evaporate  (preferably  in  a  platinum  dish) 
to  a  small  bulk,  and  while  still  warm  add  ammonia  just  in 
excess.  A  small  precipitate  of  aluminium  hydroxide  is  some- 
times formed  at  this  stage ;  filter  off,  wash,  and  weigh  with  the 
principal  precipitate.  In  the  filtrate  determine  the  alkali-earth 
metals  according  to  §  154. 

Instead  of  precipitating  the  aluminium  hydroxide  as  di- 
rected, the  following  process  may  be  followed  :  Add  a  moderate 
excess  of  ammonia  to  the  boiling-hot  solution,  boil  about  two 
minutes,  add  acetic  acid  until  distinctly  acid,  heat  again  for  a 
•few  minutes,  add  ammonia  once  more  until  weakly  alkaline, 
and  proceed  as  above. f 

[The  difficulty  of  washing  aluminium  hydroxide  usually 
increases  with  lapse  of  time  between  precipitation  and  filtra- 
tion. This  difficulty  may  be  to  some  extent  obviated  by  the 
following  slight  modification  of  the  above-described  manipula- 
tion. Add  ammonia  to  the  solution,  which  may  occupy  a  volume 
of  400  c.c.  for  0-2  gr.  A1,O8  ,  until  free  acid  is  partially  neu- 
tralized, but  not  until  a  permanent  precipitate  is  formed ;  add 

*  A  small  quantity  will  always  be  found  if  you  have  boiled  in  a  glass  or 
porcelain  vessel. 

\  Handb.  der  anal.  CJiem,  von  H.  ROSE,  6.  Aufl.  von  FINKENER,  u,  647. 


§  156.]  BASES    OF    GROUP  III.  625 

also  ammonium  chloride  if  but  little  free  acid  was  present.  Heat 
nearly  to  boiling,  and  add  ammonia  slowly  until  a  permanent 
precipitate  begins  to  form,  then  drop  by  drop  until  a  slip  of  red 
litmus-paper  dipped  into  the  fluid  changes  to  blue  and  the  odor 
of  ammonia  becomes  perceptible  on  boiling.  Carefully  avoid 
the  use  of  more  ammonia  than  is  sufficient  to  produce  these 
indications  of  a  slight  excess.  Boil  rapidly  7  to  10  minutes, 
allow  the  precipitate  to  settle  5  to  10  minutes,  filter  and  wash 
the  precipitate  moderately  upon  the  filter.  Remove  the  filter 
with  the  moist  precipitate  from  the  funnel,  and  unfold  it  upon 
the  side  of  a  beaker  having  a  height  exceeding  the  diameter  of 
the  filter,  so  that  the  latter  may  not  extend  to  the  bottom 
of  the  beaker.  Rinse  the  precipitate  from  the  filter  down 
to  the  bottom  of  the  beaker  with  a  strong  jet  of  water 
and  dissolve  (completely  or  nearly)  by  adding  concentrated 
hydrochloric  acid.  Moisten  also  the  filter  with  acid  somewhat 
diluted,  and  rinse  the  small  amount  of  aluminium  chloride  solu- 
tion thus  formed  out  of  the  paper  with  a  jet  of  water.  Push 
up  the  filter  now,  if  necessary  with  a  rod,  so  that  it  may  be 
above  the  solution,  and  allow  it  to  remain  adhering  to  the  side 
of  the  beaker.  The  solution  need  not,  for  this  second  precipi- 
tation, occupy  a  volume  above  200 — 250  c.c.  Precipitate  the 
aluminium  precisely  as  before,  moistening  also  the  filter  with 
ammonia  solution.  Immediately  after  boiling  pour  the  solution 
with  the  precipitate  upon  a  filter.  Push  the  old  filter  down  to 
the  bottom  of  the  beaker,  wash  it  by  adding  and  decanting 
small  successive  portions  of  hot  water,  stirring  and  pressing  the 
paper  with  a  rod  and  pouring  the  water  upon  the  precipitate, 
until  a  few  drops  of  the  decanted  water  give  no  turbidity  with 
silver  nitrate.  Next  complete  the  washing  of  the  precipitate 
on  the  filter  with  hot  water.  After  the  washing  is  complete, 
beat  up  the  old  filter  in  the  beaker  with  a  glass  rod  and  rinse  it 
out  upon  the  top  of  the  washed  precipitate — the  old  filter  must 
on  no  account  be  thrown  away,  since  it  may  retain  a  little  alu- 
minium hydroxide  which  treatment  with  hydrochloric  arid 
failed  to  dissolve.  Add  to  the  united  filtrates  ammonia  to 
decided  alkaline  reaction  ;  heat  until  the  solution  becomes  neu- 
tral. If  more  aluminium  hydroxide  separates,  collect  it  on  a 
small  filter.] 


626  SEPARATION.  [§  156, 

2.  Method  based  upon  the  unequal  Deeomposability 
of  the  Nitrates  at  a  Moderate  Heat  (DEVILLE*). 

To  make  this  simple  and  convenient  method  applicable,  the  46 
basic  metals  must  be  present  as  pure  nitrates.  Evaporate  to  dry- 
ness  in  a  platinum  dish,  and  heat  gradually,  with  the  cover  on, 
in  the  sand-  or  air-bath — or,  better  still,  on  a  thick  iron  disk, 
with  two  cavities,  one  for  the  platinum  dish,  the  other,  filled 
with  brass  turnings,  for  inserting  a  thermometer  (compare  §  31) 
— to  from  200°  to  250°,  until  a  glass  rod  moistened  with  am- 
monia ceases  to  indicate  further  evolution  of  nitric-acid  fumes. 
You  may  also,  without  risk,  continue  to  heat  until  nitrous:acid 
vapors  form .  The  residue  consists  of  aluminium  oxide,  barium, 
strontium  and  calcium  nitrates,  and  normal  and  basic  magne- 
sium nitrates. 

Moisten  the  mass  with  a  concentrated  solution  of  ammonium 
nitrate,  and  heat  gently,  but  do  not  evaporate  to  dryness. 
Repeat  this  operation  until  no  further  evolution  of  ammo- 
nia is  perceptible.  (The  basic  magnesium  nitrate,  insoluble  in 
water,  dissolves  in  nitrate  of  ammonia,  with  evolution  of  ammo- 
nia, as  normal  magnesium  nitrate.)  Add  water,  and  digest  at 
a  gentle  heat. 

[If  the  ammonium  nitrate  has  evolved  only  imperceptible 
traces  of  ammonia,  pour  hot  water  into  the  dish,  stir,  and  add  a 
drop  of  dilute  ammonia ;  this  must  cause  no  turbidity  in  the 
fluid  ;  should  the  fluid  become  turbid,  this  proves  that  the  heat- 
ing of  the  nitrates  has  not  been  continued  long  enough  ;  in 
which  case  you  must  again  evaporate  the  contents  of  the  dish? 
and  heat  once  more.] 

The  aluminium  oxide  remains  undissolved  in  the  form  of  a 
dense  granular  substance.  Decant  after  digestion,  and  wash  with 
boiling  w^ater  ;  ignite  strongly  in  the  same  vessel  in  which  the 
separation  has  been  effected,  and  weigh.  Test  the  weighed  alu- 
minium oxide  according  to  45.  Separate  the  alkali-earth  metals 
as  directed  in  §  154. 

In  the  same  way  aluminium  may  be  separated  also  from 
potassium  and  sodium  (44). 

3.   Method  in  which  the  processes  of  1  and  2  are 
combined. 

Precipitate  the  aluminium  as  in  45,  wash  in  the  same  way  47 
*  Journ.  /.  prakt.  Chem.,  1853,  LX,  9. 


§J56.]  BASES   OF   GROUP  III.  627 

as  there  directed,  then  treat,  while  still  moist,  with  nitric  acid, 
and  proceed  according  to  46,  to  remove  the  trifling  amount  of 
magnesium,  etc.,  coprecipitated ;  add  the  solution  obtained  to 
the  principal  solution  of  the  alkaline  earths  and  treat  the  fluid 
as  directed  in  45.  This  method  may  be  employed  also  in  the 
case  of  chlorides ;  it  will  be  sometimes  found  useful. 

4.  Method  based  upon  the  Precipitation  of  Alumin- 
ium by  Sodium  Acetate  or  Formate  upon  boiling. 

The  same  process  as  for  the  separation  of  ferric  iron  from  48 
the  alkali-earth  metals.  The  method  is  employed  more  par- 
ticularly when  both  aluminium  and  ferric  iron  have  to  be 
separated  from  alkali-earth  metals  at  the  same  time.  The 
precipitation  of  the  aluminium  is  usually  not  quite  complete, 
so  that  it  will  be  necessary  to  separate  the  aluminium  which 
remains  in  solution  from  the  filtrate  (45). 

5 .  Method  based  on  the  Precipitation  ofAlmmnium 
by  Ammonium  Succinate. 

Proceed  as  for  the  precipitation  of  ferric  iron  by  the  same  49 
reagent  (§  159) ;   especially  to  be  employed  when  aluminium 
and  ferric  iron  are  both  to  be  separated  from  alkali- earth  metals 
at  the  same  time.     The  filtrate  must  be  tested  according  to  48. 

6.  Method   based  upon  the  Formation  of  a  Solu- 
ble  Alkali  Aluminate  in  the  dry  way.     (See  §  161.)     50 

B.  Special  Methods. 
SOME  OF  THE  ALKALI-EARTH  METALS  FROM  ALUMINICTM. 

1 .  Methods  based  upon  the  Precipitation  of  some  of 
the  /Salts  of  the  Alkali- earth  Metals. 

a.  BARIUM  AND  STRONTIUM  FROM  ALUMINIUM. 
Precipitate  the  barium  and  strontium  with  sulphuric  acid  51 

(§§  101  and  102),  and  the  aluminium  from  the  filtrate  as 
directed  in  §  105,  a.  This  method  is  especially  suited  for  the 
separation  of  barium  from  aluminium.  In  accurate  analyses 
the  barium  sulphate  must  be  purified  according  to  12. 

b.  CALCIUM  FROM  ALUMINIUM. 

Add  ammonia  to  the  solution  until  a  permanent  precipitate  52 
forms,  then  acetic  acid  until  this  precipitate  is  redissolved,  then 


628  SEPARATION.  [§  156. 

ammonium  acetate,  and  finally  ammonium  oxalate  in  slight 
excess  (§  103,  2,  &,  /3) ;  allow  the  precipitated  calcium  oxalate 
to  deposit  in  the  cold,  then  filter,  and  precipitate  the  alumin- 
ium from  the  filtrate  as  directed  in  §  105,  a.  Compare  also 
J  161,  4,  I. 

V.   Magnesium    and  small  quantities  of  Calcium  from 
Aluminium. 

Add  a  little  tartaric  acid,  supersaturate  with  ammonia,  53 
and  precipitate  from  the  clear  liquid  (if  sufficient  aluminium 
is  present  110  calcium  tartrate  is  precipitated)  first  the  calcium, 
by  ammonium  oxalate,  and  then  the  magnesium,  with  sodium- 
ammonium  phosphate.  If  the  aluminium  is  to  be  estimated 
in  the  filtrate,  add  to  the  latter  some  sodium  carbonate  and 
potassium  nitrate,  evaporate  to  dryness,  ignite  the  residue, 
soften  with  water,  dissolve  in  hydrochloric  acid  (not  in  a  plat- 
inum dish),  and  precipitate  the  aluminium  with  ammonia. 
The  ammonium-magnesium  phosphate,  which  may  contain 
basic  magnesium  tartrate,  dissolve  in  hydrochloric  acid,  and 
again  precipitate  with  ammonia  and  a  little  sodium-ammo- 
nium phosphate,  and  then  ignite  and  weigh. 

2.  Method  "based  upon  the  Precipitation  of  Alumin- 
ium by  Barium  Carbonate. 

ALUMINIUM  FROM  MAGNESIUM  AND  SMALL   QUANTITIES    OF 
CALCIUM. 

Mix  the  slightly  acid  dilute  fluid  in  a  flask  with  a  moder-  54 
ate  excess  of  barium  carbonate  shaken  up  with  water,  cork  the 
flask  and  let  the  mixture  stand  in  the  cold  until  the  aluminium 
hydroxide  has  subsided,  wash  by  decantation  three  times, 
filter,  and  then  determine  the  aluminium  in  the  precipitate  as 
directed  in  51 ;  in  the  filtrate,  first  precipitate  the  barium  by 
sulphuric  acid  (30)  and  then  separate  the  calcium  and  mag- 
nesium according  to  §  154. 

5.    SEPARATION   OF   CHROMIUM   FROM    THE    ALKALI-EARTPI 
[METALS. 

1.   The  best  way  to  separate  THE  WHOLE  OF  THE  ALKALI- 
EARTH  METALS  from  chromium  at  the  same  time  is  to  convert 


§  156.]  BASES   OF  GROUP  III.  629 

the  latter  into  chromic  acid.     This  may  be  done  in  the  dry  or 
the  wet  way. 

a.  Dry  way.     Mix  the  powdered  substance  with  about  8  55 
times  its  weight  of  a  mixture  of  2  parts  of  sodium  carbonate 
and  1  part  of  nitre  and  fuse  in  a  platinum  crucible.     On  treat- 
ing the  fused  mass  with  hot  water,  the  chromium  dissolves  as 
alkali  chromate  (to  be  determined  according  to  §  130),  while 
the  alkali-earth  metals  remain  in  the  residue  as  carbonates  or 
oxides   (magnesium  oxide).     If  the  residue  is  not  perfectly 
white,  extract  the  remainder  of  the  chromic  acid  from  it  by 
boiling  with  solution  of  sodium  carbonate. 

b.  Wet   way.      Suitable    for   separating   chromium   from  56 
barium,  strontium,  and  calcium. 

a.  Nearly  neutralize  the  acid  fluid  with  sodium  carbonate, 
add  excess  of  sodium  acetate,  warm,  and  pass  chlorine,  adding 
sodium  carbonate  occasionally  to  keep  the  fluid  nearly  neutral. 
As  soon  as  all  the  chromium  is  oxidized,  precipitate  with  sodium 
carbonate  by  ths  aid  of  heat,  and  proceed  for  the  rest  according 
to  55  (GIBBS *).  Bromine  instead  of  chlorine  may  be  used; 
however,  the  oxidation  is  but  tardily  effected  by  the  mere  addi- 
tion of  bromine  water. 

ft.  Neutralize  the  solution  with  sodium  carbonate,  add 
sodium  hypochlorite,  and  heat,  if  necessary,  with  more  hypo- 
chlorite,  until  all  the  chromium  is  converted  into  chromate. 
Then  add  again  sodium  carbonate,  heat,  decant  the  yellow 
solution  through  a  filter,  boil  the  residue  anew  with  a  sodium- 
carbonate  solution  and  proceed  as  in  §5. 

2.  CHROMIUM  FROM  BARIUM,  STRONTIUM,  AND  CALCIUM.  To  57 
separate  barium  and  strontium, precipitate  the  moderately  acid, 
hot,  dilute  solution  with  sulphuric  acid — in  the  presence  of 
strontium,  allow  to  cool  and  add  alcohol — and  when  the  pre- 
cipitate has  settled,  filter.  Chromium  cannot  be  separated,  by 
ammonia  from  the  alkali-earth  metals,  since,  even  though  car- 
bonic acid  be  completely  excluded,  they  are  partially  precipi- 
tated along  .with  the  chromic  hydroxide.  From  solutions 
containing  a  salt  of  chromium,  calcium  cannot  be  precipitated 
completely  by  ammonium  oxalate;  but  it  may  be  by  sulphuric 
acid  and  alcohol  (§  103,  1). 

*Zeitschr.  f.  analyt.  Chem.,  HI,  328. 


630  SEPARATION.  [§  157. 

3.   CHROMIUM  may  also  be  separated  from  MAGNESIUM  and  58 
small   quantities  of  CALCIUM  by  means   of  barium  carbonate. 
See  54. 

III.  SEPARATION  OF  CHROMIUM  FROM  ALUMINIUM.* 
§157. 

a.  Fuse  the  oxides  with  2  parts  of  potassium  nitrate  and  4:  59 
parts  of  sodium  carbonate  in  a  platinum  crucible,  treat  the  fused 
mass  with  boiling  water,  rinse  the  contents  of  the  crucible  into 
a  porcelain  dish  or  beaker,  add  a  somewhat  large  quantity  of 
potassium  chlorate,  supersaturate  slightly  with  hydrochloric 
acid,  evaporate  to  the  consistence  of  syrup,  and  add,  during  the 
latter  process,  some  more  potassium  chlorate  in  portions,  to 
remove  the  free  hydrochloric  acid.  Dilute  now  with  water, 
and  separate  the  aluminium  and  chromium  as  directed  in  §  130, 
II.,  <?,  a.  If  you  omit -the  evaporation  with  hydrochloric  acid 
and  potassium  chlorate,  part  of  the  chromic  acid  will  be  reduced 
by  the  nitrous  acid  in  the  fluid,  and  chromic  hydroxide  will 
accordingly,  upon  addition  of  ammonia,  be  precipitated  with 
the  aluminium  hydroxide  (Dp;xTERf). 

&.  Dissolve  the  oxides  in  hydrochloric  acid,  add  soda  or  60 
potassa  solution  in  sufficient  excess  and  saturate  the  clear  green 
solution  with  chlorine  gas.  The  chromium  will  be  con  verted 
into  chromic  acid,  and  the  aluminium  partially  separated. 
When  the  fluid  has  become  of  a  pure  yellow  color,  heat  to 
remove  the  excess  of  chlorine,  add  ammonium  carbonate,  and 
digest  to  destroy  the  hypochlorous  acid  and  precipitate  the  still 
dissolved  aluminium,  and  proceed  according  to  §  IftO,  IL,  c,  a 

(WOHLER  $). 

c.  Nearly  neutralize  the  acid  solution  with  sodiuy>.i  carbonate,  61 
add  sodium  acetate  in  excess,  pass  chlorine  or  add  bromine  and 
warm.  •  The  chromium  will  readily  be  converted  into  chromic 
acid,  especially  if  sodium  carbonate  is  added  every  now  and  then 
to  keep  the  fluid  nearly  neutral.  As  soon  as  t\  ds  is  effected 
proceed  according  to  §  130,  IL,  <?,  a  (GIBBS§). 

*  The  separation  of  aluminium  from  titanic  acid  will  be  gi  /on  under  the 
Analysis  of  Silicates. 

\Pogg.  AnnaL,  LXXXIX,  142. 

\  AnnaL  d.  Chem.  u.  Pharm.,  cvi,  121.     §  Zeitsckr.  f.  analyt.  Chem.,  in,  327. 


§  158.]  BASES   OF   GROUP   IV.  631 

Fourth  Group. 

ZINC MANGANESE NICKEL COBALT FERROUS   IRON FEREIO 

IRON — (URANIUM). 

I.  SEPARATION  OF  THE  METALS  OF  THE  FOURTH  GROUP  FROM 
THE  ALKALIES. 

§158. 
A,  General  Methods. 

1.  ALL  METALS  OF  THE  FOURTH  GROUP  FROM  AMMONIUM. 

Proceed  as  for  the  separation  of  chromium  and  aluminium  62 
from  ammonium,  §  155  (41).  It  must  be  borne  in  mind  that  the 
oxides  of  the  fourth  group  comport  themselves,  upon  ignition 
with  ammonium  chloride,  as  follows :  Ferric  oxide  is  partly 
converted  into  ferric  chloride  which  volatilizes ;  the  oxides  of 
manganese  are  converted  into  manganous  chloride  and  manga- 
nous  oxide  with  volatilization  of  some  of  the  former;*  the 
oxides  of  nickel  and  cobalt  are  reduced  to  the  metallic  state,  no 
chloride  being  lost  by  volatilization  ;  f  oxide  of  zinc  is  converted 
into  chloride  which  volatilizes.  It  is,  therefore,  generally  the 
safest  way  to  add  sodium  carbonate.  The  ammonium  is  deter- 
mined in  a  separate  portion. 

2.  ALL  METALS  OF   THE  FOURTH  GROUP  FROM  POTASSIUM 
AND  SODIUM. 

Mix  the  solution  in  a  flask  with  ammonium  chloride  if  63 
necessary,  add  ammonia  till  neutral  or  slightly  alkaline,  then 
yellow  ammonium  sulphide  saturated  with  hydrogen  sulphide, 
fill  the  flask  nearly  to  the  top  with  water,  cork  it,  allow  the 
precipitated  sulphides  to  subside,  and  then  filter  them  off  from 
the  fluid  containing  the  alkalies.  In  performing  this  process 
the  precautionary  rules  given  under  the  heads  of  the  several 
metals  in  question  (§§  108 — 113)  must  be  borne  in  mind.J  (If? 
notwithstanding,  the  filtrate  is  brownish,  acidify  it  with  acetic 

*  Zeitadir.  f.  analyt.  Chem.,  xr,  424.  \  Ib.,  xn,  73. 

|  Manganese  may  be  separated  from  the  alkalies  according  to  §  109,  2,  &. 
Nickel  and  cobalt  may  be  separated  from  the  alkalies  according  to  66,  substi- 
tuting ammonium  acetate  for  sodium  acetate. 


632  SEPARATION.  [§  15& 

acid,  pass  hydrogen  sulphide,  boil,  and  filter  off  the  small  quan- 
tity of  the  nickel  sulphide  which  then  separates.)  Acidify  the 
filtrate  with  hydrochloric  acid,  evaporate,  filter  off  the  sulphur 
if  necessary,  continue  the  evaporation  to  dryness,  ignite  the 
residue  to  remove  ammonium  salts,  and  determine  the  alkalies 
by  the  methods  given  in  §  152. 

B.   Special  Methods. 

1 .  ZINC  FROM  POTASSIUM  AND  SODIUM,  by  precipitating  the  64 
zinc  from  the  solution  of  the  acetates  with  hydrogen  sulphide. 
(See  87.) 

2.  NICKEL(OUS)  AND  CoBALT(oUs)   FROM    THE  ALKALIES,   by 

igniting  the  chlorides  in  a  current  of  hydrogen  and  treating 
the  residue  with  water.  Precipitation  of  the  alkalies  as  silico- 
fluorides  (§  97,  5 — STOLBA*)  is  less  suitable  for  sodium  than 
for  potassium. 

3.  FERRIC  IKON  FROM  POTASSIUM  AND  SODIUM,  by  precip- 
itating with  ammonia,  or  by  heating  the  nitrates.    (See  45  and 
46.) 

4.  MANGANESE  FROM  THE  ALKALIES.     Mix  the  neutral  or  65 
slightly  acid  solution  with  ammonium  chloride  and  precipitate 
the  manganese  with  a  slight  excess  of  ammonium  carbonate. 
Allow  the  precipitate  to  settle  in  a  warm  place,  filter  through 

a  thick  filter,  wash  With  hot  water,  and  weigh  as  protosesqui- 
oxide  (H.  TAMM  f ).  In  the  filtrate  separate  the  alkalies  from 
ammonium  salts  by  gentle  ignition.  The  separation  of  man- 
ganese as  hydrated  peroxide  cannot  be  recommended,  as  the 
precipitate  retains  alkali.:}: 


*  Zeitschr.  f.  analyt.  Chem.,  ix,  100. 
f/6.,  xi.  425. 
J  lb.t  xi,  298. 


§  159.]  EASES    OF   GROUP    IV.  633 

II.  SEPARATION  OF  THE  METALS  OF  THE  FOURTH  GROUP  FROM 

THOSE    OF    THE    SECOND. 

§159. 
INDEX.     (The  numbers  refer  to  those  in  the  margin.) 

Zinc  from  barium  and  strontium,  66,  67,  68,  73. 

calcium,  66,  68,  73. 
"          magnesium,  66,  68. 
Manganese  from  barium  and  strontium,  66,  67,  70,  71,  72. 

."  calcium  and  magnesium,  66,  70,  71,  72. 

Nickel  and  cobalt  from  barium  and  strontium,  66,  67,  73,  75. 
"  calcium,  66,  73,  75. 

magnesium,  66,  74. 

Ferric  iron  from  barium  and  strontium,  66,  67,  69. 
"  calcium  and  magnesium,  66,  69. 

A.    General  Method. 

ALL  METALS  OF  THE  FOURTH  GROUP  FROM  THE  ALKALI- 
EARTH  METALS. 

Add  ammonium  chloride,  and,  if  acid,  also  ammonia,  and  66 
precipitate  with  ammonium  sulphide,  as  in  63.  Take  care  to 
use  slightly  yellow  ammonium  sulphide,  perfectly  saturated 
with  hydrogen  sulphide,  and  free  from  ammonium  carbonate 
and  sulphate,  and  to  employ  it  in  sufficient  excess.  Insert  the 
cork,  and  let  the  flask  stand  for  some  time  to  allow  the  precipi- 
tate to  subside,  then  wash  quickly,  and  so  far  as  practicable 
out  of  the  contact  of  air,  with  water  to  which  some  ammonium 
sulphide  has  been  added.  Acidify  the  filtrate  with  hydro- 
chloric acid,  heat,  filter  from  the  sulphur,  and  separate  the 
alkali-earth  metals,  as  directed  in  §  154.  If  the  filtrate  is  brown- 
ish from  a  little  dissolved  nickel  sulphide,  make  it  slightly  acid 
with  acetic  acid  instead  of  with  hydrochloric  acid,  add  some 
alkali  acetate,  pass  hydrogen  sulphide,  boil,  and  filter. 

If  the  quantity  of  the  alkali-earth  metals  is  rather  consider- 
able, it  is  advisable  to  trea£  the  slightly  washed  precipitate  once 
more  with  hydrochloric  acid  (in  presence  of  nickel  or  cobalt  it 
is  not  necessary  to  effect  complete  solution),  heat  the  solution 
gently  for  some  time,  and  then  reprecipitate  in  the  same  way. 

If  we  have  merely  to  effect  removal  of  nickel  and  cobalt, 
we  may  also  add  ammonium  sulphide,  acidulate  with  acetic 


634  SEPARATION.  [§  159. 

acid,  add  alkali  acetate,  heat,  pass  hydrogen-sulphide  gas 
through  the  solution  while  boiling',  and  filter.  It  is  always 
necessary  to  test  the  filtrate  with  ammonium  sulphide,  how- 
ever, to  make  sure  that  all  the  nickel  and  cobalt  are  precipitated. 
(Compare  90.) 

In  separating  manganese  from  the  alkaline  earths  the 
method  of  precipitation  described  in  §  109,  2,  5  may  be 
employed,  but  even  in  this  method  a  double  precipitation  is 
advisable. 

B.  Special  Methods. 

1.  BARIUM  AND  STRONTIUM  FROM  THE  WHOLE  OF  THE  METALS  67 
OF  THE  FOURTH  GROUP. 

Precipitate  the  barium  and  strontium  from  the  slightly  acid 
solution  with  sulphuric  acid  (§§  101 ,  102).  The  barium  sulphate 
should  first  be  washed  with  water  acidified  with  hydrochloric 
acid,  but  even  then  you  cannot  be  sure  of  getting  it  free  from 
iron.  The  sulphates,  after  weighing,  must  therefore  always 
be  tested  for  iron,  etc. 

2.  ZlNO  FROM  THE  ALKALI-EARTH  METALS.  68 

a.  Convert  the  basic  metals  into  acetates,  and  precipitate 
the  zinc  from  the  solution  according  to  §  108,  1,  1). 

b.  Evaporate  the  solution  of  the  chloride  with  an  excess 
of  ammonium  chloride,  ignite  the  residue,  and,  if  necessary, 
repeat  the  operation.     The  zinc  is  completely  volatilized  as 
zinc  chloride,  and  the  alkaline  earths  remain  behind. 

3.  FERRIC  IRON  FROM  THE  ALKALI-EARTH  METALS.  69 

a.  Mix  the  somewhat  acid  solution  with  enough  ammonium 
chloride,  boil,  add  slight  excess  of  ammonia,  boil  till  the  excess 
of  the  latter  is  nearly  expelled,  and  filter.  The  solution  is  free 
from  iron;  the  precipitate  is  free  from  calcium,  barium,  and 
strontium,  but  contains  a  very  slight  trace  of  magnesium  (H. 
HOSE*).  In  delicate  analyses,  after  moderately  washing  the 
ferric  hydroxide,  redissolve  it  in  hydrochloric  acid,  and  repeat 
the  precipitation. 

5.  Precipitate  the  iron  as  basic  ferric  acetate  or  formate, 

*  Pogg.  Annal.,.cx,  300. 


§  159.]  BASES   OF   GROUP   IV. 

compare  84  and  85.      The  method  is  good,  and  can  frequently 
be  employed. 

c.  Precipitate  the  iron  with  ammonium  succinate  (86). 

d.  Decompose  the  nitrates  by  heat  (46).     A  good  method.* 

e.  Precipitate  the  diluted,  weakly  acid  solution  with  ba- 
rium carbonate,  and  filter  after  a  short  digestion  in  the  cold 
(54).      Only  applicable  for  the  separation  of  iron  from  calcium 
and  magnesium. 

4.   MANGANESE  FROM  THE  ALKALINE  EARTHS. 

a.   Methods  based  upon  the  Precipitation  of  Man- 
ganese as  Sesquioxide  or  Dioxide. 

a.  The  methods  of  GIBBS,  f  SCHIEL,  J  II.  ROSE,§  and  others  70 
{described  in  the  previous  edition),  in  which  the  manganese  is 
precipitated  as  a  hyd rated  manganic  oxide  by  lead  peroxide, 
by  adding  sodium  acetate  and  passing  in  a  current  of  chlorine 
gas,  or  by  bromine,  cannot  be  recommended,  because  notable 
traces  of  the  alkaline  earths  are  precipitated  with  the  hydrated 
manganic  oxide.  Compare  also  R.  FINKENER,  ||  who  lias  made 
similar  observations.  According  to  GIBBS  T  the  errors  may 
be  decreased  by  double  precipitation.  As  the  precipitate, 
however,  as  a  rule  contains  alkali  also,**  and  is  therefore  un- 
fit for  weighing  after  simple  ignition,  these  methods  can  be 
but  rarely  made  use  of. 

ft.  After  DEVILLED  The  bases  must  be  present  as  71 
nitrates.  Heat  in  a  covered  platinum  dish  to  from  200°  to 
250°  until  all  fumes  cease  to  form,  and  the  mass  has  become 
black,  then  proceed  as  in  46.  If  a  small  quantity  of  organic 
matter  be  present,  or  if  the  heat  applied  is  too  strong,  some 
manganese  dioxide  may  be  reduced  and  be  dissolved  in  the 
ammonium  nitrate,  hence  the  solution  must  always  be  tested 
for  manganese.  According  to  my  experience  the  precipitate 
is  not  entirely  free  from  alkaline  earths. 

*  Compare  LATSCHINOW,  Zeitschr.f.  analyt.  Chem.,  vn,  213. 

f  Annai.  de  Chem.  u.  Pharm.,  LXXXVI,  54. 

\Siltim.  Journ.,  xv,  275. 

%Pogg.  A  tinal.,  ex,  305. 

|  Handb.  d.  analyt.  Chem.,  v.  H.  ROSE,  6.  Aufl.  v.  R.  FINKENER,  II,  925. 

Tf  Zeittichr.  f.  analyt.  Chem.,  in,  831. 

**lbid.,  xi,  298.  \\  Journ.  f.  prakt.  Chcm.t  LX,  11. 


636  SEPARATION.  [§  159. 

J.   Methods  leased  on  the  Volumetric  Determination 
of  Manganese,  according  to  BUNSEN  and  KRIEGER.* 

a.  MANGANESE  FROM  MAGNESIUM. — Precipitate  with  soda  72 
(§109,  1,  J).  Ignite  and  weigh  the  well- washed  precipitate. 
If  the  quantity  of  magnesium  present  is  sufficient,  the  residue 
has  the  composition  Mn2O3*MgO  -f-  #MgO.  Treat  a  weighed 
sample  of  it  according  ito  §  142 ;  this  will  give  the  quantity 
of  manganese  (1  eq.  of  chlorine,  corresponding  to  1  eq.  of 
iodine  liberated,  is  the  equivalent  of  1  eq.  Mn2O3) ;  the  dif- 
ference will  give  the  magnesia. 

ft.  From  BARIUM  and  STRONTIUM. — Precipitate  with  so- 
dium carbonate  (§  109,  1,  a).  The  ignited  precipitate  has 
the  formula  Mn,O3  •  BaO  +  #BaCO3. 

Treat  a  sample  as  in  a,  and  so  find  the  quantity  of  man- 
ganese. The  quantity  of  barium  carbonate  is  found  on  de- 
ducting the  weight  of  the  manganese  sesquioxide  from  that  of 
the  weighed  precipitate,  and  adding  to  the  difference  as  much 
carbonic  acid  as  has  been  expelled  by  the  sesquioxide,  i.e.,  for 
each  eq.  of  MnaOs ,  1  eq.  of  CO3. 

y.  FROM  CALCIUM.  Proceed  as  directed  under  barium 
and  strontium,  but,  after  ignition,  moisten  repeatedly  with 
ammonium  carbonate,  dry,  and  ignite  gently  until  the  weight 
remains  constant.  Here,  however,  it  is  advisable  to  ignite  the 
precipitate  in  a  blast-lamp  flame  until  calcium  oxide  has 
formed. 

N.  B. — This  method  of  volumetric  estimation  of  manga- 
nese presupposes  that  more  than  1  eq.  of  MgOCaO,  etc.,  is 
present  for  every  eq.  of  Mn3O8,  otherwise  the  residue  will 
contain,  besides  Mn2O3,  some  Mn3O4  also.  In  order  to  adapt 
the  method  in  such  a  case  also,  KRIEGER  recommends  to  dis- 
solve a  sample  of  the  weighed  precipitate,  add  half  its  weight 
of  zinc  oxide,  precipitate  with  sodium  carbonate,  determine 
the  quantity  of  precipitate  after  long-continued  ignition  with 
access  of  air,  and  then  to  use  all  or  a  part  of  the  residue  so 
obtained  for  the  volumetric  determination.  The  precipitate 
contains  all  the  manganese  as  Mn2O3.  As  will  be  seen,  how- 
ever, this  modification  greatly  complicates  the  process 

*  Annal.  d.  Chem.  u.  Pharm.,  LXXXVII,  268. 


§  159.]  BASES    OF   GROUP   IV.  637 

When  employing  the  methods  under  72  it  must  always  be 
remembered  that  the  precipitation  of  manganese  by  caustic  soda 
or  sodium  carbonate  can  be  complete  only  then  when  the  pre- 
cautions stated  under  §  109,  1,  a  and  J,  are  carefully  ob- 
served ;  also  that  the  precipitates  are  obtained  free  from  alkali 
only  when  again  exhausted  with  boiling  water  after  ignition. 

d.  Solutions  containing  manganese,  calcium,  and  magne- 
sium must  not  contain  ammonium  salts.  The  manganese, 
calcium,  and  magnesium  may  be  present  as  chlorides,  nitrates, 
or  acetates  (or  sulphates  if  but  little  calcium  is  in  the  solution 
and  care  be  taken  to  avoid  deposition  of  calcium  sulphate)". 
Neutralize  any  free  acid  which  may  be  present  by  adding 
sodium  carbonate  till  a  slight  precipitate  is  formed.  Redis- 
solve  this  precipitate  by  the  addition  of  just  sufficient  HC1. 
Add  next  sodium  acetate  to  the  solution,  then  aqueous  solution 
of  bromine.  The  solution  should  at  this  point  be  rather  dilute. 
Expose  to  a  temperature  of  50°  to  70°  a  few  hours,  till  free 
bromine  is  all  or  nearly  all  expelled  from  the  solution,  and 
filter.  Test  the  filtrate  by  adding  more  sodium  acetate  and 
more  bromine  water,  and  warming.  The  manganese  is  pre- 
cipitated as  hydrated  dioxide,  which  is  liable  to  contain  soda. 
If  the  quantity  is  very  small,  it  may,  unless  great  accuracy  is 
required,  be  converted  by  ignition,  after  careful  washing  with 
hot  water,  directly  into  Mn3O4,  and  weighed.  If,  however, 
the  quantity  is  considerable,  it  should  be  dissolved  in  HC1  and 
converted  into  some  other  suitable  form  for  weighing. 

According  to  FINKENEK,*  manganese  dioxide  precipitated 
as  above  described  (except  using  chlorine  instead  of  bromine) 
from  a  solution  containing  the  alkali-earth  metals,  will  not  be 
entirely  free  from  the  latter,  especially  from  barium  if  that  is 
present.  He  recommends  to  dissolve  the  manganese  precipi- 
tate, and  reprecipitate  boiling  hot  with  ammonium  sulphide,  by 
which  means  pure  manganese  sulphide  is  obtained.  GIBBS  t 
observes  that  when  manganese  is  separated  from  zinc,  calcium, 
and  magnesium  by  the  above  process  (precipitation  as  dioxide), 
a  repetition  of  the  process  is  necessary  to  secure  good  results ; 

*  Handbuch  d.  analyt.  Chem.  v.  H.  ROSE,  6.  Aufl.  v.  FINKENEK,  n,  926. 
f  Zeitschr.  /.  analyt.  Cliem. ,  in,  321. 


638  SEPARATION.  [§  159. 

but  in  case  manganese  is  to  be  separated  only  from  calcium  and 
magnesium,  the  second  treatment  may  be  omitted.* 

5.  COBALT,  NICKEL,  AND  ZINC  FROM  BARIUM,  STRONTIUM, 
AND  CALCIUM. 

Add  an  excess  of  sodium  carbonate,  then  add  potassium  73 
cyanide,  warm  very  gently  until  all  the  precipitated  cobalt, 
nickel,  and  zinc  carbonates  are  redissolved,  and  filter  off  the 
carbonates  of  the  alkaline  earths  from  the  solution  of  the  metal- 
lic cyanides  in  the  potassium- cyanide  solution.  Dissolve  the 
former  in  diluted  hydrochloric  acid,  and  separate  according  to 
§  154;  separate  the  latter  according  to  §  160  (HAIDLEN  and 
FRESENIUS  f ). 

6.  COBALT  AND  NICKEL  FROM  MAGNESIUM. 

Precipitate  the  solution  with  a  mixture  of  potassium -hypo-  74 
chlorite  solution  and  potassa  lye.  Thoroughly  wash  the  pre- 
cipitate, consisting  of  the  hydroxides  of  nickel,  cobalt,  and 
magnesium,  and  while  still  moist  digest  it  at  a  temperature  of 
from  30°  to  40°  with  an  excess  of  mercuric-chloride  solution. 
A  double  salt  having  the  composition  MgCla  -f-  3HgCl,  is 
formed,  and  the  magnesium  goes  into  solution,  while  a  corre- 
sponding equivalent  of  basic  mercury  chloride  is  precipitated 
(ULLGKREN  J).  Evaporate  the  solution  and  the  washings  with 
the  addition  of  pure  mercuric  oxide,  and  determine  the  magne- 
sium according  to  §  104,  3,  5. — Remove  the  nickel  and  cobalt 
oxides  by  ignition,  and  separate  the  metals  as  detailed  below. 

7.  COBALT  AND  NICKEL  FROM  BARIUM,   STRONTIUM,  AND 
CALCIUM. 

Ignite  the  chlorides  of  the  metals  in  a  current  of  hydrogen  75 
and  separate  the  reduced  cobalt  and  nickel  from  the  barium 
chloride,  etc.,  by  treatment  with  water. 

*  E.  A.  COLBY  (priv.  contrib.)  finds,  by  experiments  made  in  the  Sheffield 
Laboratory  on  the  separation  of  Ca  from  Mn,  that  by  proceeding  as  above  directed 
only  a  slight  unweighable  trace  of  Ca  goes  down  with  the  Mn  ;  while  if  the 
amount  of  free  acetic  acid  is  moderately  increased,  the  manganese  is  precipitated 
entirely  free  from  calcium.  Too  much  acetic  acid,  however,  prevents  or  delays 
precipitation  of  Mn. 

\Annal.  d.  Chem.  u.  Pharm.,  XLIII,  140. 

J  BERZELIUS'  Jahresber.,  xxi,  146. 


§  160.]  BASES   OF   GROUP  IV.  639 

III.   SEPARATION  OF  THE  METALS  OF  THE  FOURTH  GROUP 

FROM    THOSE    OF   THE    THIRD,   AND    FROM    EACH    OTHER. 
§   160. 

INDEX.     (The  numbers  refer  to  those  in  the  margin.) 

Aluminium  from  zinc,  76,  77,  85,  86,  87,  97. 

"  manganese,  76,  77,  78,  85,  86,  108. 

"  nickel  and  cobalt,  76,  77,  80,  85,  86,  97. 

"  ferrous  iron,  76,  77,  78,  85. 

ferric  iron,  77,  78,  84,  91,  92,  104. 
Chromium  from  zinc,  manganese,  nickel,  cobalt,  and 

iron,  76,  77,  93,  94. 
ferric  iron,  77,  91,  93,  94. 
Zinc  from  aluminium,  76,  77,  85,  86,  87,  97. 
"        chromium,  76,  77,  93,  94. 
manganese,  81,  87,  88,.  109. 
nickel,  88,  100,  101,  102. 
"        cobalt,  88,  96,  99,  101,  102. 

ferric  iron,  76,  82,  85,  86,  103,  106. 
Manganese  from  aluminium,  76,  77,  78,  85,  86,  108. 

chromium,  76,  77,  93,  94. 
"  zinc,  81,  87,  88,  109. 

nickel,  81,  89,  90,  101. 
cobalt,  89,  90,  96,  101. 
ferric  iron,  76.  82,  85,  86,  108. 
Nickel  from  aluminium,  76,  77,  80,  85,  86,  97. 
chromium,  76,  77,  93,  94. 
zinc,  88,  100,  101,  102. 
"          manganese,  81,  89,  90,  101. 
cobalt,  95,  96,  98,  110. 
ferric  iron,  76,  80,  82,  85,  86,  89,  106. 
Cobalt  from  aluminium,  76,  77,  80,  85,  86,  87. 
"          chromium,  76,  77,  93,  94. 
zinc,  88,  96,  99,  101,  102. 
"          manganese,  89,  90,  96,  101. 
"          nickel,  95,  96,  98,  110. 

ferric  iron,  76,  80,  82,  85,  86,  89,  106. 
Ferrous  iron  from  aluminium,  76,  77,  78,  85. 
«  chromium,  76,  77,  93,  94. 

ferric  iron,  76,  83,  85,  105,  107,  111. 
Ferric  iron  from  aluminium,  77,  78,  84,  91,  92,  104. 
"  chromium,  77,  91,-  93,  94. 

zinc,  76,  82,  85,  86,  103,  106. 
«  manganese,  76,  82,  85,  86,  108. 

"  nickel,  76,  80,  82,  85,  86,  89,  106. 

«'  cobalt,  76,  80,  82,  85,  86,  89,  106. 

"    .  ferrous  iron,  76,  83,  85,  105,  107,  111. 


640  SEPARATION.  [§  160. 

A.   General  Methods. 

1.  Method  Abased  upon   the  Precipitation  of  some 
Basic  Radicals  ly  Barium  Carbonate. 

FERRIC  IKON,  ALUMINIUM,  AND  CHROMIUM,  FROM  ALL  OTHER 
BASIC  RADICALS  OF  THE  FOURTH  GROUP. 

Mix  the  sufficiently  dilute  solution  of  the  chlorides  or  76 
nitrates,  but  not  sulphates,  which  must  contain  a  little  free 
acid,*  in  a  flask,  with  a  moderate  excess  of  barium  carbonate 
diffused  in  water ;  cork,  and  allow  to  stand  some  time  in  the 
cold,  with  occasional  shaking.  The  ferric  iron,  aluminium,  and 
chromium  are  completely  separated,  f  whilst  the  other  basic 
radicals  remain  in  solution,  with  the  exception  perhaps  of  traces 
of  cobalt  and  nickel,  which  will  generally  fall  down  with  the 
precipitate.  This  may  be  prevented,  at  least  as  regards  nickel, 
by  addition  of  ammonium  chloride  to  the  fluid  to  be  precipi- 
tated (SCHWARZENBERG^;).  Decant,  stir  up  with  cold  water, 
allow  to  deposit,  decant  again,  filter,  and  wash  with  cold  water. 
The  precipitate  contains,  beside  the  precipitated  metals, 
barium  carbonate;  and  the  filtrate,  besides  the  non-precipi- 
tated metals,  a  barium  salt. 

If  ferrous  iron  is  present,  and  it  is  wished  to  separate  it  by 
this  method  from  ferric  iron,  etc.,  the  air  must  be  excluded 
during  the  whole  of  the  operation.  In  that  case,  the  solution 
of  the  substance,  the  precipitation,  and  the  washing  by  decan- 
tation  are  effected  in  a  flask  (A,  Fig.  113)  through  which  car- 
bonic acid  is  transmitted  (5).  The  washing  water,  boiled  free 
from  air  and  cooled  out  of  contact  of  air  (preferably  in  a  cur- 
rent of  carbonic  acid),  is  run  in  through  c  and  the  fluid  drawn 
off  by  means  of  a  movable  tube,  d\  all  the  tubes  are  fitted  air- 
tight into  the  cork  and  are  smeared  with  tallow.  The  wash- 
water,  decanted  off  from  the  precipitate  as  much  as  possible, 
is  passed  through  the  asbestos  filter,  e.  It  is  evident  that  the 
flow  of  the  liquid  through  d  is  to  be  effected  by  the  pressure 


*  If  there  is  much  free  acid,  the  greater  part  of  it  must  first  be  saturated 
with  sodium  carbonate. 

f  The  separation  of  the  chromium  requires  the  most  time. 
\  Annal.  d.  Chem.  u.  Pharm.,  xcvn,  21(3. 


§  160.] 


BASES    OF   GROUP   IV. 


641 


of  carbonic  acid,   hence  this  must  be  taken  into  considera- 
tion when  constructing  the  carbonic-acid  evolution  apparatus. 


Fig.  113. 

If   e   is   placed   sufficiently  low,  d,  once   filled,  will  act  as  a 
siphon. 


64S  SEPARATION.  [§  16(X 

2.  Method  based  upon  the  Precipitation  of  the  Metals 
of  the  Fourth  Group  ~by  Sodium  Sulphide  or  Ammo- 
nium Sulphide,  from  Alkaline  Solution  effected  with 
the  aid  of  Tartar ic  Acid. 

ALUMINIUM   AND    CHROMIUM    FROM   THE   METALS   OF   THE 
FOURTH  GROUP. 

Mix  the  solution  with  pure  normal  potassium  tartrate,*  then  77 
with  pure  solution  of  soda  or  potassa  until  the  fluid  has  cleared 
again ;  f  add  sodium  sulphide  as  long  as  a  precipitate  forms, 
allow  it  to  deposit  until  the  supernatant  fluid  no  longer  exhibits 
a  greenish  or  brownish  tint ;  decant,  stir  the  precipitate  up 
with  water  containing  sodium  sulphide,  decant  again,  transfer  . 
the  precipitate,  which  contains  all  the  metals  of  the  fourth 
group,  to  a  filter,  wash  with  water  containing  sodium  sulphide, 
and  separate  the  metals  as  directed  in  B.  Add  to  the  filtrate 
potassium  nitrate,  and  evaporate  to  dryness ;  fuse  the  residue 
in  a  platinum  dish,  and  separate  the  aluminium  from  the 
chromic  acid  formed  as  directed  §  157.  If  you  have  merely  to 
separate  aluminium  from  the  metals  of  the  fourth  group,  it  is- 
better,  after  addition  of  potassium  tartrate,  to  supersaturate 
with  ammonia,  add  ammonium  chloride,  and  precipitate  in  a 
flask  with  ammonium -sulphide.  When  the  precipitate  has  set- 
tled it  is  filtered  off  and  washed  with  water  containing  ammo- 
nium sulphide.  The  filtrate  is  evaporated  in  a  platinum  dish 
with  sodium  carbonate  and  potassium  nitrate  to  dryness,  fused,, 
and  the  aluminium  determined  in  the  residue. 

B.  Special  Methods. 

1.  Methods  l>ased  upon,  the  Solubility  of  Aluminium 
Hydroxide  in  Caustic  Alkalies. 

a.  ALUMINIUM  FROM  FERROUS  AND  FERRIC  IRON,  AND  SMALL 
QUANTITIES  OF  MANGANESE  (but  not  from  nickel  and  cobalt). 

Mix  the  hydrochloric  solution  with  sodium  carbonate  or  7& 
pure  potassa  till  the  greater  part  of  the  free  acid  is  neutralized 
and  pour  the  solution  gradually  into  excess  of  pure  potassa 

*  Tartaric  acid  often  contains  aluminium,  therefore  this  is  best  made  from 
the  acid  lartrate. 

f  Chromium  and  zinc  cannot  be  obtained  together  in  alkaline  solution 
(CHANCEL,  Compt.  rend.,  XLIII,  927;  Journ.  f.  prakt.  Ohem.,  LXX,  378). 


§  160.]  BASES    OF   GROUP   IV.  643 

heated  nearly  to  boiling  in  a  platinum  or  silver  dish,  stirring  all 
the  while.  Porcelain  does  not  answer  so  well,  and  glass  should 
on  no  account  be  used.  The  iron,  if  present  as  ferric  chloride, 
separates  as  ferric  hydroxide,  while  the  aluminium  remains  in 
solution  as  alkali  aluminate.  Hydrated  protosesquioxide  of 
iron  is  more  easy  to  wash  than  ferric  hydroxide,  hence  when 
much  iron  is  present  it  is  better  to  reduce  a  part  by  cautiously 
adding  sodium  sulphite  and  heating,  so  that  when  the  solution 
is  added  to  the  boiling  potash  a  black  granular  precipitate  may 
be  formed.  The  iron  precipitate  is  sure  to  contain  alkali,  and 
must  be  dissolved  in  hydrochloric  acid,  the  solution  boiled  with 
nitric  acid  if  necessary,  and  reprecipitated  with  ammonia. 

To  the  alkaline  filtrate  add  a  few  drops  of  hydrochloric 
acid.  If  the  potassa  was  present  in  sufficient  excess  the  precipi- 
tate will  redissolve  readily  on  stirring.  Continue  adding  hydro- 
chloric acid  till  in  excess,  boil  with  a  little  potassium  chlorate 
(to  destroy  traces  of  organic  matter),  concentrate  by  evapora- 
tion, and  throw  down  the  aluminium  according  to  §  105,  a. 
The  above  is  the  best  method  of  procedure,  but  it  is  always  to 
be  feared  that  small  quantities  of  aluminium  will  be  retained 
by  the  iron  precipitate. 

It.  ALUMINIUM  FROM  FERROUS  AND  FERRIC  IRON,   COBALT, 

AND  NlCKEL. 

Fuse  the  oxides  with  potassium  hydroxide  in  a  silver  era-  79 
cible,  boil  the  mass  with  water,  and  filter  the  alkaline  fluid, 
which  contains  the  aluminium,  from  the  oxides,  which  are  free 
from  aluminium,  but  contain  potassa  (II.  ROSE). 

2.  Methods  based  on  the  different  behavior  of  Am- 
monia or  Ammonium  Carbonate  in  the  presence  of  Chlo- 
r></e  with  solutions  of  certain  basic  radicals. 

a.  ALUMINIUM  AND  FERRIC  IRON  FROM  COBALT  AND  NICKEL. 

Ferric  iron  may  be  completely  separated  from  these  metals  80 
by  mixing  the  hot  solution  with  ammonium  chloride,  and  then 
with  excess  of  ammonia,  digesting  for  several  hours,  washing 
the  precipitate,  redissolving  in  hydrochloric  acid,  reprecipitating 
with  ammonia,  and  repeating  the  operation  a  third  time.  Nickel 
and  cobalt  are  to  be  precipitated  from  the  filtrate  by  ammo- 
nium sulphide  and  subsequently  neutralizing  with  acetic  acid. 


644  SEPAKATION.  [§  160. 

If  it  is  intended  to  first  remove  ammonium  chloride  this 
may  be  accomplished  by  evaporating  the  fluid  to  dryness  and 
heating  the  residue  in  a  porcelain  dish  or  crucible.*  Plati- 
num vessels  should  not  be  used,  as  they  become  spotted  with, 
nickel  platinum  and  are  difficult  to  clean. 

In  separating  iron  and  aluminium  from  nickel  and  cobalt, 
it  is  well  to  substitute  ammonium  carbonate  for  ammonia,  so 
as  to  insure  the  complete  precipitation  of  the  aluminium. 

£>.  MANGANESE  FROM  NICKEL  AND  ZINC. 

The  solution  should  be  slightly  acid  and  contain  ammonium  81 
chloride.  Precipitate  the  manganese  as  white  carbonate  with 
ammonium  carbonate,  allow  to  settle  in  a  warm  place,  filter 
through  a  thick  paper  (double,  if  necessary),  wash  with  hot 
water,  dry  the  precipitate,  and  convert  it  into  protosesquioxide 
by  ignition  with  access  of  air.  This  excellent  method  was 
proposed  by  TAMM,f  and  has  given  me  good  results.  J  It  is 
not  adapted  to  the  separation  of  cobalt  from  manganese,  as 
the  former  is  partly  precipitated  with  the  latter. 

3.  Method   based   upon    the   different  deportment  of 
neutralized  Solutions  at  boiling  heat. 

a.  FERRIC  IRON  FROM  MANGANESE,  NICKEL,  AND  COBALT, 

AND      OTHER       STRONG       BASIC      METALS,       AFTER       IlERSCHEL,§ 
SCHWARZENBERG,  |    AND  MY  OWN  EXPERIMENTS. 

Mix  the  dilute  solution  largely  with  ammonium  chloride  82 
(at  least  40  of  ]STII4C1  to  1  of  MnO,  NiO,  etc.),  add  ammo- 
nium carbonate  in  small  quantities,  at  last  drop  by  drop  and 
in  very  dilute  solution,  so  long  as  the  precipitated  iron  redis- 
Bolves,  which  takes  place  promptly  at  first,  but  more  slowly 
towards  the  end.  As  soon  as  the  fiuid  has  lost  its  transpar- 
ency, without  showing,  however,  the  least  trace  of  a  distinct 
precipitate  in  it,  and  fails  to  recover  its  clearness  after  stand- 

*  This  method,  which  was  recommended  in  the  previous  German  edition 
for  the  separation  of  small  quantities  of  iron  from  nickel  and  cobalt,  has  been 
found  by  BAUMHAUEB,  (Zeitschr.  /.  analyt.  Chem.,  x,  218)  to  be  well  adapted 
for  the  separation  of  large  quantities  as  well. 

f  Chem.  News,  xxvr,  37.  ^.Zeitschr.  f.  analyt.  Chem.,  xi,  425. 

%Annal.  de  Chim.  et  de  Phys.,  XLIX,  306. 

|  Annal.  de  Chem.  u.  Pharm.,  xcvii,  216. 


§  160.]  BASES   OF  GROUP  IY.  645 

ing  some  time  in  the  cold,  but,  on  the  contrary,  becomes  rather 
more  turbid  than  otherwise,  the  reaction  may  be  considered 
completed.  When  this  point  has  been  attained,  heat  slowly 
to  boiling,  and  keep  in  ebullition  for  a  short  time  after 
the  carbonic  acid  has  been  entirely  expelled.  The  iron 
separates  as  a  basic  ferric  salt,  which  rapidly  settles 
if  the  solution  was  not  too  concentrated.  Pour  off  the 
hot  fluid  through  a  filter  and  wash  by  decantation  com- 
bined with  filtration  with  boiling  water  containing  a  little 
ammonium  chloride.  It  is  well  to  redissolve  the  precipitate  in 
hydrochloric  acid  and  throw  down  the  iron  with  ammonia. 
The  first  filtrate  should  be  mixed  with  excess  af  ammonia.  If 
a  small  portion  of  ferric  hydroxide  is  thrown  down  here,  filter 
it  off,  dissolve  in  hydrochloric  acid,  precipitate  with  ammonia 
and  thus  free  the  small  quantity  of  iron  entirely  from  the  strong 
basic  metals ;  if,  on  the  other  hand,  a  large  quantity  of  iron 
is  thrown  down,  this  is  a  sign  that  the  operation  has  been  con- 
ducted improperly,  and  the  hydrochloric  solution  of  the  pre- 
cipitate must  be  reprecipitated  as  above.  The  fluid  should 
not  contain  more  than  3  or  4  grm.  of  iron  in  the  litre,  and 
should  be  tolerably  free  from  sulphuric  acid,  as  when  this  is 
present  it  is  impossible  to  hit  the  exact  point  of  saturation. 

5.   FERROUS  IKON  FROM  FEBEIC  IRON. 

In  compounds  difficultly  soluble  in  hydrochloric  acid,  but  83 
which  are  decomposed  below  326°  by  moderately  concentrated 
sulphuric  acid,*  ScHEERERf  separates  ferric  from  ferrous 
iron  by  effecting  solution  in  an  atmosphere  of  carbonic  acid 
(which  is  maintained  during  the  entire  operation),  diluting  the 
solution  by  adding  pieces  of  air-free  ice,  adding  ammonium 
carbonate  until  the  acid  has  been  nearly  neutralized,  then  add- 
ing finely  triturated  magnesite  (not  the  artificial  magnesium 
carbonate),  and  boiling  for  10  to  15  minutes.  All  the  ferric 
iron  is  precipitated  by  this  process.  The  washing  is  carried 
out  as  in  76,  with  water  which,  after  admixture  of  some 
ammonium  sulphate,  has  been  boiled  and  allowed  to  cool  with 

*  On  boiling  ferrous  sulphate  is  oxidized,  the  sulphuric  acid  being  reduced 
to  sulphurous,     v.  KOBELL,  Annal.  d.  Chem.  u.  Pharm.,  xc,  244. 
\Pogg.  Annal.,  LXXXVI,  91,  and  xcm,  448. 


646  SEPARATION.  §  160. 

exclusion  of  air.  v.  KOBELL*  uses  as  a  solvent  a  mixture  of 
1  volume  concentrated  sulphuric  acid,  2  volumes  water,  and  1 
volume  strong  hydrochloric  acid.  Solution  may  generally  be 
effected  by  heating  with  hydrochloric  acid,  or  a  mixture  of  4: 
parts  concentrated  sulphuric  acid  and  1  part  of  water,  in 
sealed  tubes  heated  to  210°  (A.  MITSCHERLICH  f).  Silicates 
anay  be  easily  dissolved  by  hydrofluoric  and  hydrochloric  acids, 
or  by  hydrofluoric  acid  and  diluted  sulphuric  acid.  To  ex- 
clude the  air,  invert  a  plaster-of-Paris  cylinder  (which  may  be 
readily  made  in  the  laboratory)  provided  with  a  cover  over 
the  silicate  with  its  solvent  contained  in  a  platinum  dish. 
Through  a  hole  in  the  cover  conduct  a  moderate  current  of 
carbonic  acid,  taking  care  that  the  cylinder  is  completely  filled 
with  the  gas  before  applying  heat.  Similar  methods  and  appa- 
ratus have  been  described  by  WERTHER,:):  J.  P.  COOKE,§  and 
WILBUR  and  WHITTLESEY.  J  Care  must  be  taken  to  see  that 
the  hydrofluoric  acid  is  free  from  hydrogen  sulphide  and  sul- 
phurous acid. 

c.  FERRIC  IRON  FROM  ALUMINIUM. 

To  the  dilute  solution,  which  may  contain  aluminium  and  84 
iron  chlorides  or  even  sulphates,  add  sodium  carbonate  if 
necessary,  to  neutralize  any  too  large  quantity  of  free  acid  that 
may  be  present;  then  add  solution  of  sodium  thiosulphate 
until  all  ferric  iron  has  been  reduced  to  a  ferrous  state ;  now 
add  a  further  quantity  of  thiosulphate  and  boil  continuously 
until  every  trace  of  sulphurous-acid  odor  has  disappeared. 

The  precipitation  of  the  alumina  takes  place  according  to 
the  following  equation :  Ala(SO4)3  +  3NaaSaO,  +  3H2O  = 
Ala(OH)6  +  3JSTaaSO4  +  3SOa  +  3S.  Filter  off,  wash  the 
precipitate  well,  and  incinerate;  this  will  give  the  aluminium. 
Heat  the  filtrate  with  hydrochloric  acid  to  decompose  the  ex- 
cess of  sodium  thiosulphate,  filter  off  the  precipitated  sulphur, 
and  in  the  filtrate  estimate  the  iron  (CHANCEL  T). 

*  Annal.  d.  Chem.  u.  Pharm.,  xc,  244. 

f  Zeitschr.  f.  analyt.  Chem.,  i,  54.  J  Journ.f.  prakt.  Chem.,  xci,  329. 

%  Zeitschr.  f.  analyt.  Chem.,  vn,  99.          |  2b.,  x,  98. 

1  Compt.  Tend.,  XLVI,  987.     Compare  also  WERTHER,  Journ.f.  prakt.  Chem., 
XCI,  329,  and  GIBBS,  ZeitscJir.  f.  analyt.  Chem.,  m,  391.      I  have  felt  it  necea- 


§  160.]  BASES    OF    GROUP    IV.  Gl7 

i 

4.  Method  based  on  the  behavior  of  the  Acetates  at  a 
boiling  heat. 

FERRIC  IRON  AND  ALUMINIUM  FROM  MANGANESE,  ZINC, 
COBALT,  NICKEL,  AND  FERROUS  IRON. 

The  metals  should  be  present  in  the  form  of  chlorides.  The  85 
solution  should  be  in  a  flask.  If  much  free  acid  is  present  first 
nearly  neutralize  with  sodium  or  ammonium  carbonate  ;  the 
solution  should  remain  clear,  but  if  there  is  much  ferric  chloride 
it  should  be  of  a  deep  red  color.  Add  a  concentrated  solution 
of  neutral  sodium  or  ammonium  acetate,  not  in  large  excess,  and 
boil  for  a  short  time — long-continued  boiling  would  make  the 
precipitate  slimy.  "When  the  lamp  is  removed  the  precipitate 
should  settle  rapidly,  leaving'the  supernatant  fluid  clear.  Wash 
the  precipitate  immediately  by  decantation  and  filtration  with 
boiling  water  containing  a  little  sodium  or  ammonium  acetate. 
In  very  particular  analyses  it  would  be  well  after  washing  the 
precipitate  a  little  to  redissolve  it  in  hydrochloric  acid  and 
reprecipitate. 

In  separating  ferric  from  ferrous  iron  REICHARDT*  recom- 
mends a  slight  addition  of  ammonium  chloride  or  of  sodium 
chloride  to  prevent  oxidation  of  the  ferrous  salt. 

The  precipitate  of  basic  ferric  acetate  or  basic  aluminium 
acetate  is  best  dissolved  in  hydrochloric  acid,  in  order  to  precipi- 
tate the  basic  metals  from  this  solution  again  by  ammonia. 
This  method  is  more  suitable  to  the  separation  of  ferric  iron 
or  ferric  iron  and  aluminium  from  the  strong  basic  metals  than 
to  the  separation  of  aluminium  alone.  It  is  a  good  method 
and  is  very  generally  used. 

Instead  of  the  alkali  acetates,  the  formates  may  also  be 
used  with  excellent  results  (§  81, y). 

[The  results  obtained  by  this  method  depend  greatly  on  the 
proper  adjustment  of  free  acetic  acid  to  the  volume  of  the  solu- 
tion which  is  boiled.  The  solution  at  this  point  may  contain 


sary  to  give  this  frequently  recommended  method,  but  I  would  add  that  I  have 
not  found  it  to  be  perfectly  trustworthy. 
*  Zeitschr.  /.  analyt.  Chem.,  v,  64. 


648  SEPAKATION.  [§  160 

about  four  per  cent,  (by  volume)  of  acetic  acid  sp.  gr.  1-044 
(JEWETT*).  If  too  little  acetic  is  present,  zinc,  manganese, 
nickel,  and  cobalt  are  precipitated  in  notable  quantity  along  with 
the  iron.  If  too  much  is  present  the  precipitation  of  iron  is 
incomplete.  The  operator  may  control  the  amount  of  acid 
within  narrow  limits  by  proceeding  as  follows :  Add  the  alkali 
carbonate  to  the  cold  and  preferably  concentrated  solution  until 
a  slight  precipitate  forms  which  no  longer  redissolves  in  four 
or  five  minutes  with  occasional  shaking,  but  imparts  a  turbidity 
to  the  deep  red  solution ;  HC1  is  then  added  without  further 
delay,  slowly,  drop  by  drop,  until  the  fluid,  though  still  dark, 
becomes  clear.  Next  the'  amount  of  acetic  acid  required  to 
form  four  per  cent,  of  the  final  volume  is  added,  then  sodium 
acetate  (about  ten  times  as  much  of  the  crystallized  salt  as  there 
is  iron  present,  or  more  if  but  little  iron  is  present).  Dilute 
now  to  the  final  volume,  which  should  amount  to  at  least  100  c.c, 
per  O'l  grm.  iron  and  heat  to  boiling.  After  boiling  two  or 
three  minutes  only,  allow  the  iron  precipitate  to  settle.  Pour 
the  clear  liquid  through  a  filter,  then  bring  the  precipitate  upon 
the  filter  at  once  and  wash  as  above  directed.  The  iron  pre- 
cipitate contains  no  zinc  and  but  an  inappreciable  trace  of  man- 
ganese. Small  quantities  of  cobalt  and  still  more  nickel  will,, 
however,  be  precipitated  with  the  iron.  When  these  two  metals 
are  present  in  considerable  quantity  a  repetition  of  the  process, 
is  indispensable  when  accuracy  is  required.  Coprecipitation  of 
nickel  is  lessened  but  not  entirely  prevented  by  presence  of 
ammonium  chloride.f 

In  carrying  out  the  process  according  to  this  plan  great  care 
must  be  taken  in  the  preliminary  neutralization  with  alkali 
carbonate  to  leave  as  little  free  mineral  acid  as  possible  without 
formation  of  a  permanent  precipitate,  otherwise  this  free  acid 
will  liberate  enough  acetic  acid  from  the  soaium  acetate  to 
prevent  (with  that  intentionally  added)  the  precipitation  of  iron 
in  a  form  easy  to  wash. 

In  separating  large  quantities  of  iron  from  small  quantities 
of  manganese  the  addition  of  2  or  3  per  cent,  of  acetic  acid  will 
secure  a  separation  satisfactory  enough  for  most  purposes  (e.g. 
in  iron  and  iron  ores),  and  the  danger  that  the  acetic  acid  present 


*  Amer.  Chem.  Journ.,  i,  251.  -j-  Loc.  cit. 


§  160.]  BASES   OF  GKOUP   IV.  649 

may  accidentally  exceed   the  proper  limit  will  of   course  be 
lessened.] 

5.  Method  based  on  the  different  'behavior  of  the  Suo- 
cinates. 

FERRIC   IRON    (AND  ALUMINIUM)   FROM   ZINC,  MANGANESE, 
NICKEL,  AND  COBAT. 

The  solution  should  contain  no  considerable  quantity  of  sul-  86 
phuric  acid.  If  acid,  as  is  usually  the  case,  add  ammonia  till 
the  color  is  reddish  brown,  then  sodium  or  ammonium  acetate 
(H.  ROSE)  till  the  color  is  deep  red,  finally  precipitate  with 
neutral  alkali  succinate  at  a  gentle  heat,  and  when  cool  filter  the 
ferric  succinate  from  the  solution  which  contains  the  rest  of  the 
metals.  Wash  the  precipitate  first  with  cold  water,  then  with 
warm  ammonia,  which  removes  the  greater  part  of  the  acid, 
leaving  it  darker  in  color.  Dry  and  ignite,  moisten  with  a 
little  nitric  acid,  and  ignite  again.  With  proper  care  the  sepa- 
ration is  complete,  and  especially  to  be  recommended  when  a 
relatively  large  quantity  of  iron  is  present!  The  method  may 
also  be  used  in  the  presence  of  aluminium.  The  latter  falls 
down  completely  with  the  iron  (E.  MITSCHEKLICH,  PAGELS*). 

6.  Methods  based  upon  ih#  different  deportment  of  the 
several  Sulphides  with  Acids,  or  of  Acid  Solutions  with 
Hydrogen  Sulphide. 

a.  ZINC  FROM  ALUMINIUM  AND  MANGANESE. 

The  solution  of  the  acetates,  which  must  be  free  from  in-  87 
organic  acids,  and  must  contain  a  sufficient  excess  of  acetic  acid, 
is  precipitated  with  hydrogen  sulphide,  which  throws  down  the 
zinc  only  (§  .108,  I).  The  metals  are  usually  most  readily 
obtained  in  the  form  of  acetates,  by  converting  them  into 
sulphates,  and  adding  a  sufficient  quantity  of  barium  acetate. 
Hydrogen  sulphide  is  then  conducted,  without  application  of 
heat,  into  the  unfiltered  fiuid,  to  which,  if  necessary,  some  more 
acetic  acid  has  been  added.  The  precipitate,  which  consists  of 
a  mixture  of  zinc  sulphide  and  barium  sulphate,  is  washed 
with  water  containing  hydrogen  sulphide.  It  is  then  heated 
with  dilute  hydrochloric  acid,  the  solution  filtered,  and  the  zinc 

*  Jahresbericht  von  KOPF  imd  WILL,  1858,  617. 


650  SEPARATION.  ['§  160. 

in  the  filtrate  determined  as  directed  §  108,  a.  The  other 
metals  acre  determined  in  the  fluid  filtered  from  the  zinc  sul- 
phide, after  removal  of  the  barium  by  precipitation.  BKUNNER 
has  proposed  a  modification  of  this  process,  especially  for  the 
separation  of  zinc  from  nickel. 

b.  ZINC  FROM  NICKEL,  COBALT,  AND  MANGANESE. 

To  the  hydrochloric  solution  add  sodium  carbonate  till  a  88 
permanent  precipitate  just  forms,  and  then  a  drop  or  two  of 
hydrochloric  acid  to  redissolve  the  precipitate.  Now  pass 
hydrogen  sulphide  till  the  precipitate  of  zinc  sulphide  ceases  to 
increase.  Add  a  few  drops  of  a  very  dilute  solution  of 
sodium  acetate,  and  continue  passing  the  gas  for  some  time. 
When  all  the  zinc  is  precipitated,  allow  to  stand  for  twelve 
hours,  filter,  wash  with  hydrogen  sulphide  water,  and  determine 
the  nickel  and  cobalt  in  the  filtrate  (SMITH  and  BRUNNER*)  A 
good  method  ;  compare  KLAYE  and  DEUS.')'  The  method  is 
also  adapted  for  separating  zinc  from  manganese. 

[Precautions. — Bear  in  mind  that  Zn  can  be  precipitated 
from  solutions  containing  free  HC1,  but  only  in  case  the 
amount  of  the  latter  is  very  small.;):  When  ZnS  is  precipi- 
tated the  amount  of  HC1  set  free  may  be  sufficient  to  prevent 
complete  precipitation  of  the  Zn.  Addition  of  sodium  acetate 
converts  this  HC1  into  NaCl,  and  allows  the  formation  of  ZnS 
to  continue.  Care  must  be  taken  not  to  add  enough  sodium 
acetate  to  convert  all  the  HC1  into  Nad,  for  in  that  case  JSHS 
and  CoS  will  be  precipitated.] 

[Zinc  can  be  precipitated  by  hydrogen  sulphide  from  a  cold 
solution  containing  a  sufficient  amount  of  free  acetic  acid  to 
prevent  precipitation  of  nickel  and  cobalt.  To  effect  separation 
by  this  means  §  add  sodium  or  potassium  carbonate  to  the  solu- 
tion till  it  is  slightly  alkaline.  If  a  large  quantity  of  any  free 
volatile  acid  is  present  it  may  be  previously  remove'd  by 
evaporation.  Dissolve  the  precipitate  produced  by  the  alkali 
carbonate  (without  filtering)  in  acetic  acid,  and  add  a  large 
quantity  more  of  acetic  acid.  Precipitate  the  zinc  by  passing 

*  Dingler's  polyt.  Journ.,  CL,  369  ;   Ghem.  Centralbl.,  1859,  26. 
^Zeitschr.f.  analyt.  Chem.,  x,  200. 

j  STOKER  and  ELIOT,  Mem.  Am.  Acad.  Arts  and  Sciences,  vin,  95. 
§  ROSE  and  HINKENER,  Anal.  (Jhem.t  u,  129  and  143. 


§  160.]  BASES    OF   GKOUP   IV.  651 

IT2S  through  the  cold  moderately  diluted  solution.  "Wash  the 
sulphide  of  zinc  with  water  to  which  hydrogen  sulphide  and  a 
little  ammonium  acetate  has  heen  added.  The  zinc  sulphide 
should  not  be  dark-colored.  This  will  only  be  the  case  when 
not  enough  acetic  is  present  to  prevent  precipitation  of  nickel 
or  cobalt.  Cobalt  and  nickel  may  be  best  separated  from  the 
filtrate  by  evaporating  till  the  greater  part  of  the  acetic  acid  is 
removed,  then  adding  some  ammonium  chloride  and  ammonia 
to  slight  alkaline  reaction,  evaporating  further  till  the  reaction 
becomes  acid,  heating  finally  to  boiling,  and  passing  hydrogen 
sulphide  through  the  solution,  as  directed  in  §  110,  1,  b,  ft. — A 
good  method.] 

c,  COBALT  AND  NICKEL  FROM  MANGANESE  AND  IKON. 

Add  ammonia  to  the  nitric-acid  free  solution  to  neutralize  89 
any  other  free  acid  present,  precipitate  with  ammonium  sul- 
phide, then  add  very  dilute  hydrochloric  acid,  and  saturate  with 
hydrogen  sulphide  while  the  liquid  is  frequently  stirred.  By 
this  treatment  the  manganese  and  iron  sulphides  are  dissolved, 
while  the  cobalt  and  nickel  sulphides  remain  undissolved,  the 
latter,  it  is  true,  less  completely.  On  filtering,  precipita- 
ting the  filtrate  with  ammonia  and  ammonium  sulphide  and 
treating  the  precipitated  sulphides  also  as  above,  the  results 
are  quite  accurate.  Caution  requires,  however,  that  the 
weighed  cobalt  and  nickel  compounds  be  tested  for  manga- 
nese, and  also  more  particularly  for  iron. 

d.  COBALT  AND  NICKEL  FROM  MANGANESE. 

Add  first  an  excess  of  sodium  carbonate  to  the  acid  solu-  90 
tion,  then  considerable  of  an  excess  of  acetic  acid,  then  add  to 
the  clear  fluid  (containing,  say,  1  grm.  of  nickel  or  cobalt)  30 
to  50  c.  c.  of  a  sodium-acetate  solution  (1 :  10),  warm  the  fluid 
to  70°,  and  saturate  it  with  hydrogen  sulphide.  After  the 
precipitation  is  complete,  filter  off  the  precipitated  nickel  and 
cobalt  sulphides,  wash,  and  dry.  Concentrate  the  filtrate  by 
evaporation,  and  add  hydrogen  and  ammonium  sulphide  and 
an  excess  of  acetic  acid,  thus  obtaining  a  further  precipitate  of 
nickel  and  cobalt  sulphides.  For  the  sake  of  caution  test  the 
filtrate  once  more  in  a  similar  manner.  In  the  combined  pre- 
cipitates determine  the  nickel  or  cobalt  as  in  §  110,  1,  J,  <*, 


652  SEPARATION.  [§160. 

and  §  111,  1,  c\  in  the  filtrate  determine  the  manganese  as  in 
§  109,  2. 

T.   Methods  based  upon  the  deportment  of  the  several 
oxides  with  Hydrogen  Gas  at  a  red  heat. 

a.  FERRIC  IRON  FROM  ALUMINIUM  AND  CHROMIUM. 

RIVOT'S  method.*  Precipitate  with  ammonia,  heat,  filter,  91 
ignite,  and  weigh.  Triturate,  and  weigh  a  portion  in  a  small 
porcelain  boat,  which  insert  in  a  horizontal  porcelain  tube, 
through  which  is  passed  from  one  end  a  current  of  hydrogen 
dried  by  sulphuric  acid  and  calcium  chloride.  Close  the  other 
end  of  the  tube  with  a  stopper  bearing  a  narrow  open  glass  tube. 
After  all  the  air  has  been  expelled  from  the  apparatus,  gradu- 
ally heat  the  porcelain  tube  to  redness  and  maintain  it  at  this 
heat  so  long  as  water  still  forms  (about  one  hour) .  Let  the 
tube  cool  while  maintaining  the  current  of  hydrogen,  then  re- 
move the  boat  and  weigh  it.  The  loss  of  weight  gives  the 
oxygen  which  was  combined  to  form  ferric  iron.  To  deter- 
mine the  oxides  separately,  which  may  be  deemed  necessary 
when  but  little  ferric  iron  is  present,  treat  the  mixture  of 
aluminium,  chromium,  and  metallic  iron  with  a  mixture  of  1 
part  nitric  acid  and  30  to  40  parts  of  water  (or  with  water  to 
which  a  very  little  nitric  acid  is  added  from  time  to  time). 
The  iron  dissolves,  while  the  aluminium  and  chromium  re- 
main behind.  Weigh  these  direct ;  precipitate  the  iron  with 
ammonia  after  boiling  the  solution.  The  test  analyses  given 
by  RIVOT  were  very  satisfactory.  The  method  is  particularly 
to  be  recommended  when  much  aluminium,  etc.,  and  but  little 
iron  are  present. 

~b.   FERRIC  IRON  FROM  ALUMINIUM. 

After  reduction  by  hydrogen  (as  in  &),  DEVILLE  conducts  92 
a  current  of,  first,  hydrochloric-acid  gas,  then  again  hydrogen, 
through  the  tube.  The  aluminium  remains  behind,  while  the 
iron  volatilizes  as  ferrous  chloride,  and  is  determined  either 
from  the  loss  in  weight  or  direct.  In  the  latter  case,  dissolve 
all  the  ferrous  chloride  in  the  tubes  and  tubulated  receiver  by 
heating  with  diluted  hydrochloric  acid  to  boiling,  and  conduct 

. » 

*  Annal.  de  Ghim.  et  dePhys.,  xxx,  188;  Journ.  f.  prakt.  Chem.,  LI,  338. 


§  160.]  BASES    OF   GROUP   IV.  653 

the  vapors  into  the  porcelain  tube.  The  tubulure  of  the  re- 
ceiver is  directed  downwards  during  the  operation.  By  the  use 
of  a  platinum  tube  the  operation  is  greatly  facilitated  (COOKE  *). 

8.  Methods  based  upon  the  different  capacity  of  the 
several  Oxides  to  be  converted  by  Oxidizing  Agents 
into  higher  Oxides,  or  by  Chlorine  into  higher  Chlo- 
rides. 

a.  CHROMIUM  FROM  ALL  THE  METALS  OF  THE  FOURTH  GROUP, 
AND  FROM  ALUMINIUM. 

ex.  Fuse  the  oxides  with  potassium  nitrate  and  sodium  car-  93 
bonate  (comp.  59),  boil  the  mass  with  water,  add  a  small 
quantity  of  alcohol,  and  heat  gently  for  several  hours.  Filter 
and  determine  in  the  filtrate  the  chromium  as  directed  in 
§  130,  and  in  the  residue  the  metals  of  the  fourth  group* 
The  following  is  the  theory  of  this  process :  The  oxides  of 
zinc,  cobalt,  nickel,  iron,  and  partly  that  of  manganese,  sepa- 
rate upon  the  fusion,  whilst,  on  the  other  hand,  potassium 
manganate  (perhaps  also  some  ferrate)  and  chromate  are 
formed.  Upon  boiling  with  water,  part  of  the  manganic  acid 
of  the  potassium  manganate  is  converted  into  permanganic  acid 
at  the  expense  of  the  oxygen  of  another  part,  which  is  reduced 
to  the  state  of  bin  oxide ;  the  latter  separates,  whilst  the  potas- 
sium salts  are  dissolved.  The  addition  of  alcohol,  with  the 
application  of  a  gentle  heat,  effects  the  decomposition  of  the 
potassium  manganate  and  permanganate,  manganese  dioxide 
being  separated.  Upon  filtering  the  mixture,  we  have  there- 
fore now  the  whole  of  the  chromium  in  the  filtrate  as  alkali 
chromate,  and  all  the  oxides  of.  the  fourth  group  on  the  filter. 
Aluminium,  if  present,  will  be  found  partly  in  the  residue, 
partly  as  alkali  aluminate  in  the  filtrate;  proceed  with  the 
latter  according  to  59. 

If  you  have  to  deal  with  the  native  compound  of  sesqui- 
oxide  of  chromium  with  ferrous  oxide  (chromic  iron)  the  above 
method  does  not  answer.  In  this  case  proceed  according 
to  one  of  the  methods  detailed  in  the  Special  Part.  The 

*  Zeitschr.  /.  aiuilyt.  Chem.,  vi,  226. 


654  SEPARATION.  [§  160. 

substance  may  also  be  decomposed  by  fusion  witli  cryolite  and 
potassium  disulphate. 

.  The  radicals  to  be  separated  may  be  in  the  form  of  a  94 
solution  of  their  salts;  nearly  neutralize  the  solution,  add 
sodium  acetate,  heat,  and  convert  the  chromium  into  chromic 
acid  by  passing  chlorine.  Compare  61.  If  ferric  iron  and 
aluminium  are  present,  they  will  separate  during  boiling  by 
the  action  of  the  sodium  acetate,  while  the  chromic  acid  and 
any  zinc  will  remain  in  solution.  If  manganese,  nickel,  and 
cobalt  are  present,  the  method  loses  its  simplicity ;  the  manga- 
nese is  precipitated  as  hydrated  peroxide  with  a  portion  of  the 
cobalt,  almost  the  whole  of  the  nickel  and  some  zinc,  while 
the  chromic  acid  remains  in  solution  with  the  principal  amount 
of  the  zinc  and  the  rest  of  the  cobalt  and  nickel  (W.  GIBBS*). 

5.   COBALT  FROM  NICKEL. 

OL.  After  II.  RosE.f  Dilute  the  hydrochloric-acid  solu-  95 
tion,  contained  in  a  capacious  flask,  with  water  so  that  a  litre 
of  the  solution  will  contain  about  2  grm.  of  the  metal,  conduct 
chlorine  gas  into  the  fluid  until  the  latter  is  saturated  and  the 
upper  part  of  the  flask  is  entirely  filled  with  the  gas,  add  an 
excess  of  calcium-  or  barium  carbonate  shaken  up  with  water, 
shake  frequently  during  5  or  6  hours  in  the  cold,  and  filter  off 
the  precipitated  cobalt  hydroxide  from  the  liquid  containing 
the  nickel  in  solution.  Instead  of  chlorine,  HENRY  uses 
bromine.  DENHAM  SMITH  recommends  adding  a  dilute  solution 
of  chlorinated  lime  which  has  been  completely  decomposed 
with  sulphuric  acid,  so  as  to  leave  no  hypochlorite  present. 

According  to  FR.  GATJHE,;}:  HOSE'S  method  is  unsafe,  as  an 
insufficiently  prolonged  action  of  the  carbonates  of  the  alkaline 
earths  precipitates  the  cobalt  incompletely,  while  a  too  pro- 
longed action  precipitates  nickel  as  well.  The  method  may  be 
serviceable  by  observing  special  conditions,  and  when  applied 
with  great  experience ;  it  is  not  suitable  for  accurate  analyses. 

ft.  The  method  of  GIBBS,  elaborated  by  H.  ROSE  §  (boiling 
the  sulphuric -acid  solution  with  lead  peroxide),  gives  only 
approximate  results  also.  Compare  GAUHE  (loo.  cit.). 

*  GIBBS  and  CLARK,  Amer.  Jour.  Sci.,  2d  ser.,  XLVIII,  198. 

\Pogfj.  Annul ,  LXXI,  545,  and  Handb.  d.  analyt.  Chem.,  6.   Aufl.,  u,  143. 

%Zeitschr.f.  analyt.  Chem.,  v,  84. 

%Pogg.  Annal.,  ex,  413. 


§160.]  BASES    OF   GROUP   IV.  655 

9.   Method  based  upon  the  different  deportment  vf 
the  Nitrates. 

COBALT  FROM  NICKEL,  ALSO  FROM  MANGANESE  AND  ZINC. 

The  separation  of  cobalt  as  tripotassium  cobaltic  nitrite  was  96 
recommended  first  by  FISCHER,*  afterwards  by  A.  STROMEYER^ 
GENTH  and  GIBBS,  J  IL  ROSE,§  FR.  GAUHE,||  and  myself  (com- 
pare last  edition  of  this  work).  The  results  are  quite  satisfac- 
tory both  in  presence  of  much  cobalt  and  little  nickel,  and  in 
the  presence  of  little  cobalt  and  much  nickel;  but  the  process 
is  peculiarly  good  for  the  latter  case.  However,  it  is  absolutely 
necessary  that  barium,  strontium,  and  calcium  should  be  absent, 
as  in  their  presence  nickel  is  thrown  down  as  triple  nitrite  of 
nickel,  potassium,  and  alkali-earth  metal  (KUNZEL,  O.  L.  ERD- 
MANNl[).  The  best  way  of  proceeding  is  as  follows:  The 
solution  (from  which  any  iron  must  first  be  separated)  is  evapo- 
rated to  a  small  bulk,  and  then,  if  much  free  acid  is  present, 
neutralized  with  potassa.  Then  add  a  concentrated  solution  of 
potassium  nitrite  (previously  neutralized  with  acetic  acid  and 
filtered  from  any  flocks  of  silica  and  alumina  that  may  have 
separated)  in  sufficient  quantity,  and  finally  acetic  acid,  till  any 
flocculent  precipitate  that  may  have  formed  from  excess  of 
potassa  has  redissolved  and  the  fluid  is  decidedly  acid.  Allow 
it  to  stand  at  least  for  twenty-four  hours  in  a  warm  place,  take 
out  a  portion  of  the  supernatant  fluid  with  a  pipette,  mix  it  with 
more  potassium  nitrite,  and  observe  whether  a  further  precipita- 
tion takes  place  in  this  after  long  standing.  If  no  precipitate  is 
formed  the  whole  of  the  cobalt  has  fallen  down,  otherwise  the 
small  portion  must  be  returned  to  the  principal  solution,  some 
more  potassium  nitrite  added,  and  after  long  standing  the  same 
test  applied.  Thus,  and  thus  alone,  can  the  analyst  be  sure  of 
the  complete  precipitation  of  the  cobalt.  Finally  filter  and 
treat  the  precipitate  according  to  §  111,  1,  d.  Boil  the  filtrate 
with  excess  of  hydrochloric  acid,  precipitate  with  potash, 
redissolve  the  precipitate  in  hydrochloric  acid,  throw  down 
the  nickel  according  to  §  110,  1,  £,  7,  as  sulphide,  and  then 

*  Pogg,  Annal ,  LXXII,  477. 

\Annal.  d.  Chem.  u.  Pharm.,  xcvi,  218.          \  lb.,  civ,  309. 

§  Pogg.  Annal. .  ex,  412. 

||  Zeitschr.  /.  analyt.  Chem.,  v,  74. 

Tf  Jb.,  in.  161:  Journ.f.  prakt.  CTiem.,  xcvn,  387. 


656  SEPARATION.  [§  160, 

convert  into  metal.  In  this  manner  alone  can  the  nickel  be 
obtained  pure,  as  the  original  filtrate  contains  so  much  alkali 
salt  and  also  generally  alumina  and  silica. 

[When  nickel  and  cobalt  are  obtained  in  the  form  of 
sulphides  in  the  process  of  separation  from  other  metals,  the 
mixed  sulphides  may  be  converted  into  metals  without  previous 
separation,  by  the  same  process  that  is  described  for  nickel 
sulphide  §  110,  1,  &,  and  2.  Cobalt  may  then  be  separated 
from  a  nitric  acid  solution  of  the  two  metals  and  nickel  estimated 
by  difference.] 

10.  Methods  Seised  upon  the  different  deportment  with 
Potassium  Cyanide. 

a.  ALUMINIUM  FROM  ZINC,  COBALT,  AND  NICKEL. 

Mix  the  solution  with  sodium   carbonate,  add   potassium  97 
cyanide  in  sufficient  quantity,  and  digest  in  the  cold,  until  the 
precipitated  zinc,  cobalt,  and  nickel  carbonates  are  redissolved. 
Filter  off  the   undissolved   aluminium  precipitate,  wash,  and 
remove  the  alkali  which  it  contains,  by  resolution  in  hydro-  . 
chloric  acid  and  reprecipitation  by  ammonia  (FRESENIUS  and 
HAIDLEN  *). 

fr.  COBALT  FROM  NICKEL. 

LIEBIG'S  method,f  which  depends  upon  the  conversion  of  98 
the  cobalt  into  potassium  cobalticyanide,  and  of  the  nickel  into 
double  nickel  and  potassium  cyanide,  has  been  carefully  studied 
in  my  laboratory  by  FR.  GAUHE.^:  It  has  been  thus  found  that 
boiling  the  solution  containing  potassium  cyanide  and  hydro- 
cyanic acid  (LIEBIG'S  first  method)  does  not  completely  convert 
the  double  cobalt  and  potassium  cyanide  first  formed  into 
potassium  cobalticyanide,  but  that  passing  chlorine  (LIEBIG'S 
second  method)  effects  a  ready  and  thorough  conversion.  The 
method  then  gives  a  very  excellent  separation,  and  is  more  par- 
ticularly to  be  recommended  where  the  quantity  of  nickel  is 
small  in  proportion  to  the  cobalt.  We  proceed  as  follows, 
taking  a  hydrochloric  solution  of  the  metals  :  Remove  the 
greater  part  of  the  free  acid  by  evaporation  or  neutralize  it  by 
potash,  add  pure  potassium  cyanide  till  the  precipitate  first 

*  Annal.  d.  Chem.  u.  Pharm. ,  XLIII,  129.       f  2b.,  LXV,  244,  and  LXXXVII,  128. 
$  Zeitschr,  f.  analyt.  Chem.,  v,  75. 


§  160.]  BASES    OF   GROUP   IV.  657 

formed  has  redissolved-;  then  add  more  cyanide,  dilute,  boil  for 
some  time  or  not,  as  you  like,  pass  chlorine  through  the  cold 
fluid,  adding  potash  or  soda  occasionally,  so  that  the  fluid  may 
remain  strongly  alkaline  to  the  end.  Bromine  may  be  used 
instead  of  chlorine,  and  indeed  is  far  more  convenient.  In  the 
course  of  an  hour  the  whole  of  the  nickel  will  have  precipi- 
tated as  black  hydrate  of  the  sesquioxide.  Having  taken  out  a 
portion  and  satisfied  yourself  of  this  by  addition  of  a  further 
quantity  of  chlorine  or  bromine,  filter,  and  wash  with  boiling 
water.  The  precipitate  always  retains  alkali,  and  must  be  redis- 
solved in  hydrochloric  acid,  and  estimated  according  to  §  110, 
1,  a,  or  2. 

As  regards  the  cobalt,  it  is  most  convenient  to  estimate  it 
by  difference.  But  if  you  wish  to  make  a  direct  estimation,  it 
will  be  advisable,  in  consequence  of  the  large  quantity  of  salts 
present  in  solution,  first  to  evaporate  to  dryness  with  excess  of 
hydrochloric  acid,  to  take  up  the  residue  with  a  little  water, 
and  to  heat  in  a  large  platinum  dish,  with  the  addition  of 
excess  of  pure  concentrated  sulphuric  acid  till  the  greater  part 
of  the  sulphuric  acid  has  escaped.  The  red  mass,  consisting 
principally  of  alkali  disulphate,  is  then  treated  with  water,  and 
the  cobalt  estimated  according  to  §  111,  1,  c. 

Another  method  of  separating  nickel  and  cobalt  by  means 
of  potassium  cyanide  has  been  described  by  FLECK  ;  *  it  does 
not,  however,  appear  to  be  in  any  way  better.  The  method  is 
based  on  the  fact  that  cobalt  monosulphide,  as  well  as  nickel 
sulphide,  dissolves  readily  in  potassium -cyanide  solution,  but 
that  this  is  not  the  case  with  the  cobalt  sulphide  precipitated 
by  ammonium  sulphide  from  a  solution  of  cobalt  which  has 
been  treated  with  ammonia  in  excess  and  exposed  to  the  air 
until  its  color  no  longer  changes. 

c.   COBALT  FROM  ZINC. 

Add  to  the  solution  of  the  two  metals,  which  must  con-  99 
ttim  some  free  hydrochloric  acid,  common  potassium  cyanide 
(prepared  by  LIEBIG'S  method)  in  sufficient  quantity  to  redis- 
solve  the  precipitate  of  cobalt  cyanide  and  zinc  cyanide  which 

*  Journ.  f.  prakt.  Chem.,  xcvn,  303,  Zeitsclir.f.  analyt.  CTiem.,  v,  399. 


658  SEPARATION.  [§  16(X 

forms  at  first;  then  add  a  little  more  potassium  cyanide  and 
boil  some  time,  adding  occasionally  one  or  two  drops  of  hydro- 
chloric acid,  but  not  in  sufficient  quantity  to  make  the  solu- 
tion acid.  After  cooling,  add  some  chlorine  or  bromine,  and 
digest  for  some  time  to  complete  the  conversion  of  the  cobalt 
into  potassium  cobalticyanide.  Mix  the  solution  with  hydro- 
chloric acid  in  an  obliquely  placed  flask  and  boil  until  the  zinc 
cobalticyanide  which  precipitates  at  first  is  redissolved,  and  the 
hydrocyanic  acid  is  completely  expelled.  Add  solution  of  soda 
or  potassa  in  excess  and  boil  until  the  fluid  is  clear ;  the  solu- 
tion may  now  be  assumed  to  contain  all  the  cobalt  as  potas- 
sium cobalticyanide,  and  all  the  zinc  as  a  compound  of  zinc 
oxide  and  alkali.  Precipitate  the  zinc  by  hydrogen  sulphide 
(§  108).  Filter,  and  determine  the  cobalt  in  the  filtrate  as  in 
98.  The  process  is  simple  and  the  separation  complete  (FKE- 
SENIUS  and  HAIDLEN). 

d.   NICKEL  FROM  ZINC. 

Add  to  the  concentrated  solution  of  the  two  rnetals  an  100 
excess  of  pure  concentrated  potassa  lye,  then  sufficient 
aqueous  hydrocyanic  acid  to  completely  redissolve  the  precipi- 
tate, add  a  solution  potassium  monosulphide  (not  ammonium 
sulphide),  let  the  precipitated  zinc  sulphide  deposit  at  a  gentle 
heat,  filter,  wash  the  sulphide  with  a  dilute  potassium-sul- 
phide solution,  treat  the  precipitate  with  hydrochloric  acid, 
and  from  the  solution  precipitate  the  zinc  with  sodium  car- 
bonate, as  in  §  108,  1,  a.  In  the  filtrate  estimate  the  nickel 
by  heating  for  some  time  with  fuming  hydrochloric  and 
nitric  acids,  or  instead  of  the  latter,  potassium  chlorate,  evap- 
orating, and  finally  precipitating  with  potassa  lye  (WOHLER  *). 
KLAYE  and  DEus,f  who  tested  the  process  in  my  laboratory , 
found  that  instead  of  potassa  lye  and  hydrocyanic  acid,  * 
potassium  cyanide  could  be  used  if  perfectly  pure  and 
recently  dissolved.  If  the  solution  of  the  cyanide  contains 
ammonium  carbonate  or  formate,  or  potassium  cyanate  (as 
is  the  case  even  on  short  exposure),  the  complete  precipita- 
tion of  the  zinc  as  sulphide  is  greatly  interfered  with.  On 

*  AnnaL  de  Chem.  u.  Pkarm.,  LXXXIX,  376. 
•\Zeitschr.f.  analyt.  Chem.,    x,  197. 


§  160.]  BASES   OF   GROUP   IV.  659 

finally  washing  the  precipitated  zinc  sulphide  completely  with, 
water  containing  hydrogen  sulphide,  the  zinc  may  be  esti- 
mated according  to  §  108,  2. 

e.  COBALT  AND  NICKEL  FROM  MANGANESE  AND  ZINC 
(W.  GIBBS  *). 

Add  sodium  acetate  to  the  solution  of  the  chlorides  and  101 
pass  in  hydrocyanic-acid  gas.  Zinc  cyanide  is  immediately 
more  or  less  completely  precipitated  as  a  white  powder.  Now 
add  sodium  sulphide,  which  converts  the  zinc  and  manganese 
into  sulphides,  while  the  cobalt  and  nickel  remain  in  solution 
as  double  cyanides,  and  may  be  separated  as  in  98.  The 
employment  of  gaseous  hydrocyanic  acid  renders  the  method 
very  unpleasant. 

Another  method  for  separating  cobalt  and  nickel  from 
manganese  is  as  follows:  To  the  acid  solution  add  sodium 
carbonate  in  excess,  then  acetic  acid  in  liberal  excess,  then  to 
the  clear  fluid,  containing  say  1  grm.  of  nickel  or  cobalt,  30  to 
40  c.  c.  of  sodium-acetate  solution  (1  in  10),  and  pass  hydro- 
gen sulphide  to  saturation,  keeping  at  70°.  Filter  off  the 
precipitated  nickel  or  cobalt  sulphide,  wash  and  dry  it.  Con- 
centrate the  filtrate  by  evaporation,  add  ammonium  sulphide 
and  then  acetic  acid,  thus  obtaining  a  second  precipitate  of 
nickel  or  cobalt  sulphide.  Test  the  filtrate  again  in  the  same 
manner.  In  the  united  precipitates  determine  the  nickel  or 
cobalt  according  to  §  110,  1,  J,  or  §  111,  1,  c\  in  the  fil- 
trate, the  manganese  according  to  §  109,  2. 

11.  Methods  Abased  on  the  Volatility  of  Zinc, 
a.   COBALT  AND  NICKEL  FROM  ZINC. 

BERZELIFS  f  gives  the  following  method  for  the  absolute  102 
separation  of  cobalt  and  nickel  from  zinc :  Precipitate  the 
solution  with  an  excess  of  potassa  lye,  boil,  and  filter  the 
fluid  containing  the  greater  portion  of  the  zinc  oxide  dis- 
solved in  the  potassa  solution  from  the  precipitated  nickel  and 
cobalt  hydroxides  also  containing  much  zinc,  wash  completely 

*  Zeitschr.  f.  analyt.  Chem.,  in,  832. 
f  BEIIZELIUS'  Jahresber.,  xxi,  144. 


660  SEPARATION.  [§  160. 

with  boiling  water,  and  determine  the  zinc  in  the  filtrate  (see 
§  108).  Dry  the  precipitate,  ignite,  and  weigh ;  then  mix 
with  pure  sugar  (recrystallized  from  alcohol)  in  a  porcelain 
crucible  and  heat  slowly  until  all  the  sugar  is  completely 
-carbonized.  Then  place  the  crucible,  covered  with  its  lid, 
in  a  bath  of  magnesia  within  a  covered  crucible  of  larger 
isize,  and  heat  in  a  wind  furnace  for  one  hour  at  the  highest 
temperature  obtainable.  By  this  treatment  the  metals  are 
reduced,  the  nickel  and  cobalt  mixed  with  carbon  remaining 
behind,  while  the  zinc  is  volatilized.  Treat  the  residue  with 
nitric  acid,  and  determine  the  metals  by  precipitating  with 
potassa  solution  and  weighing  the  precipitate.  The  differ- 
ence between  this  weight  and  that  obtained  before  gives  the 
weight  of  the  zinc  which  had  been  conjointly  precipitated. 
KLAYE  and  DEUS,*  who  tested  the  method  in  my  laboratory, 
obtained  good  results  with  it.  They  recommend  replacing 
the  sugar  by  sugar-carbon,  as  the  former  causes  much  intu- 
mescence. An  attempt  to  use  the  gas  blowpipe,  as  given  by 
BERZELIUS,  gave  poor  results. 

lf>.  ZINC  FROM  IRON  IN   ALLOYS. 

According  to  BOBIERRE  such  alloys  may  be  readily  and  103 
accurately  analyzed  by  ignition  in  a  current  of  hydrogen. 

12.  Methods  based  upon  the  Volumetric  Determi- 
nation of  one  of  the  Metals,  and  the  finding  of  the 
other  from  the  difference. 

a.  FERRIC  IRON  FROM  ALUMINIUM. 

Precipitate  both  metals  with  ammonia  (§  105,  <z,  and  104 
§  113,  1).  Dissolve  the  weighed  residue,  or  an  aliquot  part 
of  it,  by  digestion  with  concentrated  hydrochloric  acid,  or  by 
fusion  with  potassium  bisulphate  and  treatment  with  water 
containing  sulphuric  acid,  and  determine  the  iron  volumetri- 
cally  as  directed  in  §  113,  3,  a  or  b.  The  alumina  is  found 
from  the  difference.  This  is  an  excellent  method,  and  to  be 
recommended  more  particularly  in  cases  where  the  relative 
amount  of  iron  is  small.  If  you  have  enough  substance,  it  is 

*  Zeitschr.f.  analyt.  Chem.,  x,  192. 


§  160.]  BASES    OF   GROUP  IV.  661 

of  course  much  more  convenient  to  divide  the  solution,  by 
weighing  or  measuring,  into  2  portions,  and  determine  in  the 
one  the  sesquioxide  of  iron  +  alumina,  in  the  other  the 
iron.  Instead  of  titrating  the  iron,  this  may  be  precipitated 
with  ammonium  sulphide  after  the  addition  of  tartaric  acid 
and  ammonia  (77). 

J.   FEKRIC  IRON  FROM  FERROUS  IRON  (ZiNc  AND  NICKEL). 

of.  Determine  in  a  portion  of  the  substance  the  total  105 
amount  of  the  iron  as  sesquioxide,  or  by  the  volumetric  way. 
Dissolve  another  portion  by  warming  with  sulphuric  acid  in 
a  flask  through  which  carbonic  acid  is  conducted,  to  exclude 
the  air;  dilute  the  solution  and  determine  the  ferrous 
iron  volumetrically  (§  112,  2,  a).  The  difference  gives 
the  quantity  of  the  ferric  iron.  Or,  dissolve  the  com- 
pound in  like  manner  in  hydrochloric  acid,  and  determine 
the  ferric  chloride  with  sodium  thiosulphate  according  to 
§  113,  3,  b.  In  this  case  the  difference  gives  the  ferrous 
iron.  If  it  is  desired  to  determine  the  ferrous  chloride  in 
the  hydrochloric-acid  solution  directly,  it  will  be  well  to  use 
PENNY'S  methods  (§  112,  2,  £).  If  the  compound  in  which 
the  ferrous  and  ferric  basic  radicals  are  to  be  estimated  is  de- 
composed by  acids  with  difficulty,  heat  it  with  a  mixture  of  4 
parts  sulphuric  acid  and  1  part  water  (or  with  hydrochloric 
acid)  in  a  sealed  tube  for  2  hours  at  210°  (MITSCHERLICH  *) 
(see  page  521).  Or,  if  this  is  not  enough,  fuse  it  with  borax 
(1  part  mineral,  5  to  6  vitrified  borax)  in  a  small  retort  con- 
nected with  a  flask  containing  nitrogen  (produced  by  com- 
bustion of  phosphorus  in  air) ;  an  atmosphere  of  carbonic  acid 
is  less  suitable.  Triturate  the  fused  mass  with  the  glass, 
and  dissolve  in  boiling  hydrochloric  acid  in  an  atmosphere 
of  carbonic  acid  (HERMANN  v.  KOBELL).  Or,  as  will  gener- 
ally be  the  best  way,  you  may  dissolve  the  substance  in  a 
mixture  of  hydrofluoric  and  hydrochloric  or  hydrofluoric  and 
sulphuric  acids  with  exclusion  of  air  (83).  CooKEf  dissolves 
silicates  in  a  mixture  of  sulphuric  and  hydrofluoric  acids  in 

*Journ.f.  prakt.  C7iem.,  LXXXI,  108,  and  LXXXIII,  455. 
\  Amer.  Journ.  of  Science,  3d  ser.,  LXIV,  347. 


662 


SEPARATION. 


[§  160. 


an    atmosphere   of   steam  and   carbonic  acid,  and  determines 
the  ferrous  iron  by  means  of  potassium  permanganate. 

Fig.  114  exhibits  his  apparatus.     To  the  sides  of  a  copper 
water-bath  are  attached  three  tubes.     The  tube  on  the  left  COD- 


Fig.  114. 

nects  with  a  MAKIOTTE'S  flask  to  maintain  the  water  at  a  constant 
level.  The  upper  tube  on  the  right  connects  with  a  carbonic- 
acid  gas  generator,  while  the  third  tube  carries  off  any  over- 
flow of  water  to  the  sink. 

On  the  cover  of  the  water-bath  close  to  the  rim  is  a  circular 
groove  which  receives  the  edge  of  an  inverted  glass  funnel. 
When  the  apparatus  is  in  use  this  groove  is  kept  full  of  water 
by  the  spray  from  the  boiling  liquid,  and  thus  forms  a  perfect 
water- joint ;  but  in  order  to  secure  this  result  the  bath  must  be 
kept  nearly  full  of  water,  and  holes  for  the  ready  escape  of  the 
steam  and  spray  should  be  provided  in  the  rings,  which  cover 
the  bath  and  adapt  it  for  vessels  of  various  sizes.  By  this 
arrangement  the  funnel  may  be  kept  filled  with  an  atmosphere 
of  steam  or  of  carbonic  acid  for  an  indefinite  period.  More- 
over, we  can  either  pour  in  fresh  quantities  of  solvent,  or  we 
can  stir  up  the  material,  in  the  vessel  within,  introducing  a 
tube-funnel  or  stirrer  through  the  spout  of  the  covering  funnel. 

The  finely  pulverized  substance  (•§•  to  1  grm.)  is  placed  in  a 
large  platinum  crucible.  Upon  it  pour  a  mixture  of  dilute 
sulphuric  acid  (sp.  gr.  1-5)  with  as  little  hydrofluoric  acid  as 
experience  may  show  is  required  to  dissolve  or  decompose  the 
substance,  stirring  up  the  material  with  a  platinum  spatula. 


§   160.]  BASES   OF  GROUP  IV.  663 

The  crucible  is  next  transferred  to  the  water-bath,  the  covering 
funnel  put  in  place,  water  poured  into  the  groove,  the  interior 
filled  with  carbonic  acid,  and  the  lamp  lighted.  As  soon  as  the 
water  boils,  the  supply  of  carbonic  acid  is  stopped  ;  and  if  the 
water-level  has  been  properly  adjusted,  the  apparatus  will  take 
care  of  itself,  the  groove  will  be  kept  full  of  water,  and  the 
interior  of  the  funnel  full  of  steam.  If  the  materials  cake  on  the 
bottom  of  the  crucible,  as  is  not  unfrequently  the  case  when  a 
large  amount  of  insoluble  sulphate  is  formed,  the  lamp  may  be 
removed,  the  apparatus  again  filled  with  carbonic  acid,  and  the 
contents  of  the  crucible  stirred  up  by  aid  of  a  stout  platinum 
wire  about  two  inches  long,  fused  to  the  end  of  a  glass  tube. 
Anything  adhering  to  the  rod  can  easily  be  washed  back  into 
the  crucible  by  directing  the  jet  from  the  wash-bottle  down  the 
throat  of  the  covering  funnel.  The  lamp  may  then  be  replaced, 
the  current  of  carbonic  acid  interrupted,  and  the  process  of 
digestion  continued.  When  the  decomposition  is  complete,  the  * 
current  of  carbonic  acid  gas  is  re-established,  the  lamp  extin- 
guished, and  the  air-tube  of  the  Mariotte's  flaek  raised  until  its 
lower  end  is  above  the  level  of  the  overflow.  A  slow  current 
of  water  is  thus  caused  to  flow  through  the  bath,  whjch  soon 
cools  down  the  whole  apparatus.  The  crucible  may  now  be 
removed,  its  contents  washed  into  a  beaker-glass,  and  the  solu- 
tion diluted  with  pure  water  until  the  volume  is  about  500  c.c., 
when  the  amount  of  ferrous  iron  present  can  be  determined 
with  a  solution  of  potassium  permanganate  in  the  usual  way. 

Many  iron  compounds  in  fine  powder  are  completely  decom- 
posed by  boiling  a  few  minutes  only  with  the  mixed  acids 
above  mentioned.  If  a  preliminary  experiment  shows  this  to 
be  the  case,  a  simple  and  satisfactory  way  of  effecting  a  solu- 
tion is  to  boil  the  substance  with  the  solvent  acids  in  a  platinum 
crucible  of  40  to  50  c.c.  capacity,  provided  with  a  well-fitting 
concave  cover.  By  watching  the  escaping  vapor,  one  can  regu- 
late the  boiling  so  as  to  prevent  access  of  air  without  appreciable 
mechanical  loss.  If 'on  removing  the  cover  the  decomposition 
is  complete,  the  operation  may  be  considered  successful.  Put 
the  crucible  and  its  contents  at  once  into  cold  water  in  a  beaker 
and  titrate  with  permanganate  (or  thiosulphate  if  IIC1  has  been 
used). 

Iron  may  also  be  determined  volumetrically  in  presence  of  106 


664 


SEPARATION. 


[ 


zinc,  nickel,  etc.  It  is,  indeed,  often  the  better  way,  instead 
of  effecting  the  actual  separation  of  the  oxides,  to  determine 
in  one  portion  of  the  solution  the  iron  -f-  zinc  or  -f-  nickel,  in 
another  portion  the  iron  alone,  and  to  find  the  quantity  of  the 
other  metal  by  the  difference .  However,  this  can  be  done  only 
in  cases  where  the  quantity  of  iron  is  relatively  small. 

fi.   FERRIC  IRON  FROM  FERROUS  IRON  (BUNSEN). 

Fill    the   flask   d,  Fig.  89,  two-thirds  full    with   fuming  107 
hydrochloric  acid'  and  replace  the  air  in  the  flask  by  carbon 
dioxide  by  throwing  a  few  fragments  of  sodium  carbonate  into 

the  acid.  Then  immediately  in- 
troduce the  substance  previously 
weighed  off  in  a  small  tube  and 
add  a  slight  excess  of  potassium 
dichromate,  also  contained  in  a 
similar  tube,  attach  the  evolution 
tube,  and  proceed  for  the  rest  as- 
in  §  130,  e,  ft.  Of  course  less 
free  iodine  is  obtained  than  had 
no  potassium  dichromate  been  dis- 
solved with  the  ferrous  salt,  as  a 
portion  of  the  chlorine  evolved  is 
used  up  in  converting  the  ferrous 
into  ferric  chloride.  In  fact  the 
difference  between  the  iodine 
corresponding  to  the  dichromate 
used  and  that  actually  obtained 
corresponds  to  the  ferrous  iron  present,  hence  1  eq.  of  iodine 
=  1  eq.  of  ferrous  chloride. 

To  determine  the  total  quantity  of  iron  present,  dissolve 
another  portion  of  the  sample  as  before  in  acid  in  the  flask, 
and  effect  the  reduction  of  the  ferric  to  ferrous  iron  by  intro- 
ducing a  ball  of  chemically  pure  zinc  cast  on  the  end  of  a  fine 
platinum  wire.  To  exclude  all  access  of  air,  connect  die  flask 
during  the  boiling  with  the  apparatus  bl>'  shown  in  Fig.  115. 
As  soon  as  the  reduction  is  complete,  and  which  may  be 
recognized  by  the  colorless  appearance  of  the  fluid,  cool  the 
flask  by  immersing  it  in  cold  water,  lift  the  upper  stopper, 


•  Fig.  115. 


§  160.]  BASES   OF  GROUP   IV.  665 

throw  a  few  fragments  of  sodium  carbonate  into  the  acid, 
draw  the  zinc  ball  up  the  tube  5,  wash  off  the  fluid  adhering 
to  the  ball  into  the  flask,  and  remove  W.  Now  quickly  add 
the  weighed  potassium  dichromate  and  proceed  as  above 
directed. 

c.  MANGANESE  FEOM  ALUMINIUM  AND  IKON  (KRIEGER  *). 

Precipitate  with  sodium  carbonate,  digest  the  precipitate  108 
for  some  time  with  the  fluid,  wash  first  by  decantation,  then  on 
the  filter,  and  as  thoroughly  as  possible,  dry,  ignite,  and  deter- 
mine the  manganese  in  a  portion  according  to  72.  Care  must 
be  taken  that  the  precipitate  contains  the  manganese  as  Mn,O4 , 
and  also  that,  in  the  case  of  highly  accurate  analyses,  the  small 
quantity  of  manganese  passing  in  the  filtrate  be  not  disregarded 
(§  109,  1,  a).  The  bases  may  also  be  precipitated  with 
ammonium  carbonate  instead  of  with  sodium  carbonate  (65) ; 
in  fact  it  deserves  the  preference. 

d.  MANGANESE  FROM  ZINC  (KEIEGEK). 

Precipitate  boiling  with  sodium  carbonate,  wash  the  pre-  109 
cipitate  with  boiling  water,  dry,  and  ignite.  If  sufficient 
zinc  is  present,  the  precipitate  will  consist  of  ZnO  -)-  a?Mn,O,. 
Weigh  off  a  portion  of  the  precipitate  and  in  it  determine  the 
manganese  as  in  72.  If  insufficient  zinc  is  present,  proceed 
as  in  72  N.  B.  Eegarding  the  small  quantity  of  manganese 
passing  into  the  filtrate  see  §  109,  1  a. 

e.  COBALT  FROM  NICKEL. 

Determine  both  metals  as  in  §  110,  1,  &,  and  2,  and  §  111,  110 
1,  &,   dissolve  the  reduced  metals  in  hydrochloric  acid  with 
the  addition  of  some  nitric  acid,  evaporate  the  solution  repeat- 
edly with  hydrochloric  acid  to  dryness  until  all  the  nitric  acid 
has  been  expelled,  and  in  the  solution  of  the  chlorides  then 
determine  the  cobalt  according  to  §  111,  3,  the  nickel  being 
found  by  difference.      The  method  is  applicable  only   in  the 
presence  of  small  quantities  of  nickel,  and  gives  only  fair 
results. 
• 

*  Annal.  de  Chem.  u.  Pharm.,  LXXXVII,  261. 


666  SEPARATION.  [§161. 

13.  Indirect  Method. 

FERRIC  IRON  FROM  FERROUS  IRON. 

Of  the  many  indirect  methods  proposed,  but  which  are  111 
now  seldom  resorted  to  since  the  introduction  of  volumetric 
methods,  I  will  give  only  the  following :  Dissolve  in  hydro- 
chloric acid  in  a  current  of  carbonic  acid,  add  an  excess  of 
gold  and  sodium  chloride,  stopper  the  flask,  and  allow  the 
precipitated  gold  to  subside.  Then  filter,  and  determine  the 
gold  as  in  §  123.  Determine  the  total  quantity  of  iron  in  the 
filtrate  or  in  another  portion  of  the  substance.  The  calcula- 
tion is  readily  made  if  it  be  remembered  that  2  eq.  of  precipi- 
tated gold  are  the  equivalent  of  6  eq.  of  ferrous  chloride  (or 
oxide)  thus :  6FeCla+  2AuCl9  =  2Au  +  3FeQCl8  (H.  EOSE). 

IV.   SEPARATION  OF  IRON,  ALUMINIUM,  MANGANESE,  CAL- 
CIUM, MAGNESIUM,  POTASSIUM,  AND  SODIUM. 

§  161. 

As  these  metals  are  found  together  in  the  analysis  of  most 
silicates,  and  also  in  many  other  cases,  I  devote  a  separate  para- 
graph to  the  description  of  the  methods  which  are  employed 
to  effect  their  separation. 

1.  Method  based  upon  the  employment  of  Barium  Car- 
bonate (particularly  applicable  in  cases  where  the  mixture  con- 
tains only  a  small  proportion  of  calcium). 

The  solution  should  contain  no  free  chlorine,  'and  the  iron  112 
should  be  all  in  the  form  of  ferric  salt.  Precipitate  the  iron 
and  aluminium  by  barium  carbonate  *  (54  and  76),  dissolve  the 
precipitate  in  hydrochloric  acid,  throw  down  the  barium  with 
sulphuric  acid,  filter,  and  estimate  the  iron  and  aluminium 
according  to  one  of  the  methods  given  in  §  160,  by  preference 
104,  at  least  when  the  quantity  of  aluminium  is  not  too  small. 

To  the  filtrate  from  the  barium -carbonate  precipitate  add 
hydrochloric  acid,  heat,  throw  down  the  barium  with  sulphuric 

*  Before  adding  the  barium  carbonate,  it  is  absolutely  indispensable  to  ascer- 
tain whether  a  solution  of  it  in  hydrochloric  acid  is  completely  precipitated  by 
sulphuric  acid,  so  that  the  filtrate  leaves  no  residue  upon  evaporation  in  a 
platinum  dish. 


$  161.]  BASES    OF   GROUP   IV.  667 

acid,  added  just  in  excess.  Filter  off  the  precipitate,  wash  till  • 
free  from  soluble  sulphate,  concentrate  if  necessary,  precipitate, 
and  determine  the  manganese  as  sulphide  (§  109,  2).  To  the 
filtrate  add  hydrochloric  acid,  heat,  filter  oft'  the  sulphur,  pre- 
cipitate the  lime  with  oxalate  of  ammonia,  and  finally  separate 
the  magnesia  from  the  alkalies  by  one  of  the  methods  p-iven  § 
153. 

2.  3frthod  based  upon  the  application  of  Alkali  Acetates 
or  Formcvtes. 

Eemove  by  evaporation  any  very  considerable  excess  of  acid  113 
which  may  be  present,  dilute,  add  sodium  carbonate  *  until  the 
fiuid  is  nearly  neutral,  then  sodium  acetate  (or  sodium  formate) 
and  precipitate  iron  and  aluminium,  observing  all  directions 
given  in  85.  Wash  the  precipitate  well,  dissolve  in  hydrochloric 
acid,  precipitate  the  solution  with  ammonia  (45),  dry,  ignite, 
and  weigh.  Dissolve  in  concentrated  hydrochloric  acid  and 
determine  the  iron  volumetrically  with  stannous  chloride,  as 
in  §  113,  3,  5,  or  digest  it  with  16  times  its  weight  of  a 
mixture  of  8  parts  sulphuric  acid  and  3  parts  water,  or  fuse 
it  for  a  long  time  with  potassium  bisulphate,  dissolve  in 
water,  and  determine  the  iron  volumetrically.  as  in  §  113, 
3,  a.  The  difference  gives  the  quantity  of  the  aluminium. 
If  any  silicic  acid  remains  behind  on  dissolving  the  pre- 
cipitate, it  is  to  be  collected  on  a  filter,  ignited,  weighed, 
and  deducted  from  the  alumina.  The  filtrate  contains  the 
manganese,  the  alkali- earth  rnetals,  and  the  alkalies.  Pre- 
cipitate the  manganese  with  ammonium  sulphide  (§  109,  2), 
boil  with  hydrochloric  acid  and  filter  off  the  sulphur,  precipi- 
tate the  calcium,  after  addition  of  ammonia,  with  ammonium 
oxalate,  and  lastly,  after  removing  the  ammonium  salts  by  igni- 
tion, precipitate  the  magnesium  from  the  hydrochloric  acid 
solution  of  the  residue  with  ammonium  sodium  phosphate. 
However,  if  it  is  intended  to  estimate  the  alkalies,  the  magne- 
sium must  be  separated  by  one  of  the  processes  in  jj  1  •'»;],  -1-.  This 

*  In  cases  where  it  is  intended  to  estimate  the  alkalies  in  the  filtrate,  ammo- 
nium salts  must  be  used  instead  of  the  sodium  salts.  If,  however,  it  is 
intended  to  precipitate  manganese  subsequently  with  bromine,  ammonium 
salts  must  not  be  introduced  into  the  solution. 


668  SEPARATION  [§  161. 

•method  is  convenient,  mid  gives  good  results,  especially  in  the 
presence  of  much  iron  and  little  aluminium.  Since  aluminium 
is  not  precipitated  by  alkali  acetates  or  formates  with  the  same 
certainty  as  iron,  it  is  necessary  to  test  the  weighed  manganese 
sulphide  for  aluminium. 

[This  method  is  to  be  recommended  when  manganese  is  pres- 
ent with  iron,  or  with  iron  and  a  moderate  proportion  of  alumin- 
ium. If,  however,  the  amount  of  aluminium  is  large  in  propor- 
tion to  the  iron,  it  is  difficult  to  precipitate  it  completely  with 
sodium  acetate.  Instead  of  precipitating  manganese  with 
ammonium  sulphide  it  may  be  separated  from  calcium  and 
magnesium  by  precipitation  with  bromine.  Add  aqueous  solu- 
tion of  bromine  to  the  filtrate  from  the  iron  precipitate  with- 
out previous  concentration  of  the  filtrate,  unless  its  volume 
exceeds  600  or  700  c.c.,  and  proceed  according  to  §  159,  72,  d. 

3.  Method  based  upon  the  application  of  Ammonium  Sul- 
phide. 

Mix  the  fluid  in  a  flask  with  ammonium  chloride,  then  with  114 
ammonia,  until  a  precipitate  just  begins  to  form,  then  with 
yellow  ammonium  sulphide,  fill  the  flask  nearly  up  to  the  top 
with  water,  cork  it,  allow  to  settle  in  a  warm  place,  filter,  and 
wash  the  precipitate — consisting  of  iron  and  manganese  sulphides 
and  aluminium  hydroxide — without  interruption  with  water 
containing  ammonium  sulphide.  Separate  the  calcium,  magne- 
sium, and  alkalies  in  the  filtrate  as  in  113.  Dissolve  the  precipi- 
tate in  hydrochloric  acid,  and  separate  the  aluminium  from 
the  iron  and  manganese  according  to  77  or  78,  and  then  the 
iron  from  the  manganese,  say  by  82  or  85. 

The  following  method  is  particularly  suitable  in  cases 
where  no  manganese  is  present,  or  only  inappreciable  traces: 

4.  Method  based  upon  the  application  of  Ammonia. 

a.  The  solution  must  contain  all  the  iron  in  the  state  of  a  115 
ferric  salt.  Add  a  relatively  large  quantity  of  ammonium 
chloride,  and — observing  the  precautions  indicated  in  45 — 
precipitate  with  ammonia.  The  precipitate  contains  the  whole 
of  the  iron  and  aluminium  ;  at  most  an  inappreciable  amount 
of  the  latter  remains  in  solution  if  the  free  ammonia  has  been 
almost  but  not  entirely  driven  off  by  heat,  if  the  solution  was 


§  161.]  BASES    OF   GROUP  IV.  669 

diluted  sufficiently,  and  if  enough  ammonium  chloride  was 
present.  It  may  also  contain  small  quantities  of  calcium  and 
magnesium  and  a  little  manganese.  It  is  well,  therefore, 
usually  to  redissolve  the  washed  precipitate  in  hydrochloric 
acid,  and  reprecipitate  with  ammonia.  In  this  way  the  pre- 
cipitate will  be  obtained  free  from  alkali-earths  and  manganese. 
Wash  the  precipitate  completely,  dry,  ignite,  and  treat  accord- 
ing to  113.  If  silicic  acid  remains  undissolved,  it  is  to  be 
determined  and  deducted.  The  solution  filtered  from  the 
aluminium  and  ferric  hydroxide  is  concentrated  by  evaporation, 
and  the  manganese  is  precipitated  and  determined  according 
to  §  109,  2,  as  sulphide;  the  alkali-earth  metals  and  alkalies 
in  the  filtrate  are  determined  according  to  113.  The  weighed 
sulphide  of  manganese  is  digested  with  dilute  hydrochloric 
acid ;  any  residue  that  may  remain  is  fused  with  potassium 
bisulphate,  dissolved  in  water,  and  tested  for  aluminia. 

5.  Precipitate  the  aluminium,  iron,  and  calcium  by  add- 116 
ing  ammonia  and  ammonium  carbonate  and  oxalate,  decant, 
and  filter.  Dissolve  the  precipitate  in  hydrochloric  acid,  add 
pure  tartaric  acid  to  prevent  the  aluminium  and  iron  from 
being  precipitated,  and  then  precipitate  the  calcium  with 
ammonia  as  an  oxalate.  In  the  solution  separate  the  iron  and 
aluminium  as  in  77 ;  and  in  the  first  filtrate  the  magnesium 
and  alkalies  according  to  18.  Should  sulphuric  acid  be  pres- 
ent in  the  first  filtrate,  remove  it  by  means  of  barium  chloride, 
then  separate  the  alkali-earths  from  the  alkalies  by  evaporat- 
ing with  oxalic  acid,  igniting,  and  treating  the  residue  with 
boiling  water,  and  finally  separate  the  barium  from  the  mag- 
nesium as  in  29  (E.  MITSCHERLICH  ;  LEWTNSTEIN  *).  As  alu- 
minium in  the  presence  of  ammonium  oxalate  is  only  grad- 
ually precipitated  on  warming  (PISANI),  the  liquid  must  be 
digested  for  some  time  with  heat  before  the  first  filtration; 
and  as  the  precipitate  always  contains  a  portion  of  the  magne- 
sium, I  would  advise  that,  after  separating  the  iron  from  the 
aluminium,  the  filtrate  from  the  latter,  as  well  as  the  alumina 
itself,  be  tested  for  magnesia.  If  weighable  quantities  of 
manganese  are  present,  the  method  is  inapplicable. 

*  Jourji.  f.  prakt.  Chem.,  LVIII,  99. 


670  SEPARATION.  [§  161. 

c.  Precipitate  with  ammonia,  digest  for  some  time  with  117 
heat,  and  until  the  greater  part  of  the  excess  of  ammonia  has 
been  expelled,  filter,  carefully  and  thoroughly  wash  the  pre- 
cipitate, ignite,  and  add  to  the  residue,  without  reducing  it  to 
powder,  at  least  ten  times  its  quantity  of  anhydrous  sodium 
carbonate,  cover  the  crucible  and  heat  the  mixture  in  a  blast- 
lamp  or  other  suitable  flame  (an  alcohol-lamp  with  double 
draught  is  not  sufficiently  powerful)  until  no  further  decom- 
position of  the  sodium  carbonate  is  observed,  for  at  least  45 
minutes.  Now  add  some  caustic  potassa  to  the  fused  mass,  and 
boil  it  with  water  (heat  in  a  silver  dish)  until  thoroughly  ex- 
tracted ;  if  a  green  color  indicates  that  sodium  manganate  is 
present  add  a  few  drops  of  alcohol,  wash  the  precipitate  by 
decantation  and  filtration,  first  with  water  containing  potassa, 
then  with  pure  water.  Dissolve  the  precipitate  in  hydro- 
chloric acid,  add  a  few  drops  of  alcohol,  and  heat  in  order  to 
more  readily  reduce  the  manganese  chloride,  and  finally  add 
ammonium  acetate  to  separate  the  iron  from  the  portions  of 
manganese,  calcium,  and  magnesium  which  were  contained  in 
the  ammonia  precipitate,  and  which  may  be  estimated  sepa- 
rately or  together  with  the  main  quantities  according  to  113. 
The  aluminium  is  determined  in  the  alkaline  solution  as  in  78. 
(R.  KIOHTEB.*) 

5.   Method  based  on  the  Decomposition   of  the  Ni- 
trates (DEVILLE). 

This  method  assumes  that  the  bases  are  present  as  nitrates  118 
only.  Proceed  first  as  in  46.  The  nitrous  acid  evolved  dur- 
ing the  heating  of  the  nitrate  is  no  indication  of  the  total  de- 
composition of  the  ferric  or  aluminium  nitrate,  because  these 
vapors  may  also  be  due  to  the  conversion  of  manganous  nitrate 
into  manganese  dioxide.  When  all  vapors  cease  to  be  evolved, 
and  the  substance  acquires  a  uniform  black  color,  interrupt  the 
heat.  After  treatment  with  ammonium  nitrate  there  remains 
in  solution  the  nitrates  of  calcium,  magnesium,  and  the  alka- 
lies, while  the  residue  will  contain  aluminium,  iron,  manga- 
nese dioxide,  and — if  much  manganese  is  present — small  quan- 

*  Journ.  /.  prakt.  Chem.,  LXIV,  378. 


§  161.]  BASES   OF   GROUP   IV.  671 

titles  of  alkaline  earths.  (That  under  certain  circumstances 
some  manganese  dissolves  has  already  been  stated  in  71 ;  this 
trace  is  found  with  the  magnesium,  from  which  it  is  finally 
separated.) 

DEVILLE    recommends    the  following  methods  to  further 
effect  the  separation : 

a.  Heat  the  precipitate  with  moderately  strong  nitric  acid 
until  the  iron  and  aluminium  are  dissolved,  leaving  the  man- 
ganese dioxide  as  a  pure- black  residue,  which  is  ignited,  and 
the  sesquioxide  then  weighed.     Evaporate  the  solution  in  a 
platinum  crucible,  ignite  the  residue,  and  weigh  the  mixture 
of  ferric  oxide,  alumina  (and  possibly  some  manganese  sesqui- 
oxide).    Now  treat  a  portion  according  to  91,  and  thus  find 
the  alumina.     If  manganese  was  present,  the  iron  cannot  be 
determined  by  difference.     DEVILLE,  therefore,  evaporates  the 
solution   of  the    chlorides    (92)    with    sulphuric   acid,  ignites 
gently,   and  treats  the  residual  mixture  of  ferric  oxide  and 
manganous  sulphate  with  water  to  remove  the  manganese  salt. 
(In  case  too  strong  a  heat  has  been  applied,  in  which  case  the 
manganous  sulphate  may  also  have  been  decomposed,  moisten 
the  residue  with  a  mixture  of  oxalic  and  nitric  acids,  add  a 
little  sulphuric  acid  and  repeat  the  ignition.) 

b.  From    the   filtrate  precipitate   first    the    calcium  with 
ammonium  oxalate  and  then  separate  the  magnesium    as    in 
§  153,  4.    In  the  presence  of  manganese  this  method  is  not  to 
be  recommended. 

6.   Method  which  combines  4-  and  5. 

Precipitate  with  ammonia  (45),  decant,  filter,  wash,  119 
remove  the  still  moist  precipitate  so  far  as  possible  from  the 
filter,  dissolve  the  remainder  in  nitric  acid  and  transfer  this  to 
the  dish  to  effect  solution  of  the  bulk  of  the  precipitate,  pro- 
ceed according  to  118,  and  mix  the  fluid  separated  from  the 
ferric  oxide  and  alumina  (and  which  contains  small  quantities 
of  magnesium,  possibly  also  traces  of  calcium)  with  the  main 
filtrate.  This  method  is  to  be  recommended  when  manganese 
is  absent.  The  estimation  of  aluminium  is  best  effected  by 
determining  the  total  weight  of  the  ferric  oxide  and  alumin- 
ium, and  then  determining  the  iron  volumetrically  (104).  If 


67:2  SEPARATION.  [§  161. 

on  dissolving  the  precipitate  of  ferric  oxide  and  alumina  there 
remains  any  silica,  this  must  be  deducted. 


Supplement  to  the  Fourth  Group. 
To  §§  158,  159,  160. 

SEPAKATION  OF  UEANIUM  FROM  THE  OTHER  METALS  OF 
GROUPS  I. — IY. 

It  has  already  been  stated,  in  §  114,  that  uranium  in  uranyl  120 
compounds  cannot  be  completely  separated  from  the  alkalies 
by  means  of  ammonia,  as  the  precipitated  ammonium  uranate 
is  likely  to  contain  also  fixed  alkalies.  The  precipitate  should 
therefore  be  dissolved  in  hydrochloric  acid,  the  solution  evapo- 
rated in  the  platinum  crucible,  the  residue  gently  ignited  in  a 
current  of  hydrogen  gas  (Fig.  83),  the  chlorides  of  the  alkali 
metals  extracted  with  water,  and  the  uranous  oxide  (UOa) 
ignited  in  hydrogen  in  order  to  weigh  it  as  UO2 ,  or  in  the 
air,  whereby  it  is  converted  into  uranous  uranate,  TJ(UO4)a. 
Instead  of  dissolving  the  precipitate  in  hydrochloric  acid 
and  treating  the  solution  as  directed,  you  may  .heat  the 
precipitate  cautiously  *  with  ammonium  chloride  and  treat  the 
residue  with  water  (H.  HOSE).  Uranium  may  be  completely 
separated  from  the  alkalies  also  by  ammonium  sulphide, 
as  H.  ROSE  found.  REMELE  f  has  examined  this  subject 
with  great  care  and  recommends  the  following  method  of  pre- 
cipitation :  The  solution  being  neutral  or  slightly  acid,  add  an 
excess  of  yellow  ammonium  sulphide  and  keep  nearly  boiling 
for  an  hour  to  convert  the  first-formed  precipitate  of  uranium 
oxysulphide  entirely  into  a  mixture  of  uranous  oxide  and  sul- 
phur. The  fluid,  at  first  dark  from  presence  of  dissolved 
uranium,  will  now  appear  yellow  and  transparent.  Filter  off 
the  precipitate  containing  all  the  uranium  and  wash  it  with 
cold  or  warm  water,  first  by  decantation,  finally  on  the  filter. 
It  is  well  to  mix  a  little  ammonium  sulphide  or  chloride  with 


*  Strong  ignition  would  occasion  the  volatilization  of  uranium  chloride. 
f  Zeiischr.f.  analyt.  Chem.,  iv,  379. 


§  161.]  BASES    OF   GROUP   IV.  673 

the  water,  as  when  pure  water  is  used  the  last  filtrate  is  apt  to 
be  turbid.  The  dried  precipitate  is  roasted  and  then  converted 
into  uranons  uranate  by  ignition  in  the  air,  or  into  uranous 
oxide  by  ignition  in  hydrogen  (§  114). 

FR.  STOLBA  *  recommends  separating  uranyl  from  alkalies  121 
by  means  of  hydrosilicofluoric  acid  with  the  addition  of  alcohol. 
Treat  the  substance  with  a  sufficient  quantity  of  3-  to  5-per 
cent,  aqueous  silicon1  uoric  acid  and  warm  gently.  As  soon  as 
the  yellow  powder  has  disappeared,  allow  to  cool,  add  3  to  4 
volumes  of  75-  to  80-per  cent,  alcohol,  mix,  allow  to  settle  in 
the  dark,  or  at  least  in  a  place  not  exposed  to  direct  sunlight, 
filter,  wash  with  alcohol  until  the  washings  are -absolutely  free 
from  acidity,  and  determine  the  alkali  volumetrically  accord- 
ing to  §  97.  5.  Direcj:  sunlight  renders  the  alcoholic  solution 
cloudy,  an  insoluble  uranium  silicofluoride  precipitating.  If 
the  uranyl  is  to  be  estimated  also,  evaporate  the  alcoholic  liquid, 
heat  the  residue  with  an  excess  of  sulphuric  acid  to  expel 
the  hydrosilicofluoric  acid,  dissolve  the  residue  in  water  with 
the  addition  of  some  nitric  acid,  filter,  and  in  the  filtrate 
determine  the  uranyl  according  to  §  114. 

This  method  is  also  applicable  for  the  analysis  of  uranyl- 
alkali  salts  soluble  in  alcohol.  It  should  be  remarked  here 
that  moderate  quantities  of  hydrochloric  or  nitric  acid  do  not 
noticeably  interfere,  while  sulphuric  acid,  by  causing  a  pre- 
cipitation of  alkali  sulphates,  gives  too  low  an  alkali  value. 

From  barium,  uranyl  may  be  separated  by  sulphuric  acid ;  122 
from  strontium  and  calcium,  by  sulphuric  acid  and  alcohol. 
Ammonia  fails  to  effect  complete  separation  of  uranyl  from 
the  alkali-earth  metals,  the  precipitate  always  containing  not 
inconsiderable  quantities  of  the  latter.  In  such  precipitates, 
however,  the  uranium  and  the  alkali-earth  metals  may  like- 
wise be  separated  by  gentle  ignition  with  ammonium  chloride 
and  treatment  of  the  residue  with  water. 

Uranyl  may  be  separated  from  strontium  and  calcium  also  123 
by  precipitation  with  ammonium  sulphide  by  the  method  given 
above  in  the  separation  from  the  alkalies.    As  carbonates  of  the 
alkali-earth  metals  may  be  coprecipitated,  treat  the  washed  pre- 

*Zcitsc7ir.  f.  analyt.  Chem.,  in,  71. 


074  SEPARATION.  [§  161, 

cipitate  of  uranous  oxide  and  sulplmr  in  the  cold  with  dilute 
hydrochloric  acid  which  will  not  dissolve  uranous  oxide. 
Ammonium  sulphide  will  not  answer  for  the  separation  of 
uranium  from  barium  (REMELE*). 

Magnesium  may  be  separated  from  uranyl  not  only  by  124 
ammonium  sulphide  in  presence  of  ammonium  chloride,  but 
also  by  ammonia.  Add  enough  ammonium  chloride  to  the 
solution,  heat  to  boiling,  supersaturate  with  ammonia,  continue 
boiling  till  the  odor  of  ammonia  is  but  slight,  filter  the  hot  fluid, 
and  wash  the  precipitate,  which  is  free  from  magnesium,  with 
hot  water  containing  ammonia  (H.  ROSE).  It  is  always  well  to 
test  the  uranous  oxide  obtained  by  ignition  in  hydrogen  for 
magnesium  by  treating  with  dilute  hydrochloric  acid. 

Aluminium  is  best  separated  from  uranyl  by  mixing  the 
somewhat  acid  fluid"  with  ammonium  carborfate  in  excess.  The 
uranyl  passes  completely  into  solution,  while  the  aluminium 
remains  absolutely  undissolved.  Filter,  evaporate,  add  hydro- 
chloric acid  to  resolution  of  the  precipitate  produced,  heat  till 
all  the  carbonic  acid  is  expelled,  and  precipitate  with  ammonia 
(§  1U). 

Uranyl  is  best  separated  from  chromium  (W.  GiBBsf)  by 
adding  to  the  solution  soda  in  slight  excess,  heating  to'  boiling 
and  adding  bromine  water,  when  the  chromium  is  rapidly 
converted  into  chromic  acid.  Filter  the  solution  containing 
sodium  chromate  from  the  precipitate  which  has  a  deep  orange- 
red  color  and  consists  of  a  compound  of  soda  and  uranic  oxide 
mixed  with  some  uranyl  chromate.  Wash  the  precipitate  with 
hot  water  containing  a  little  soda,  dissolve  it  in  hot  nitric  acid, 
boil  the  solution  a  few  minutes  to  drive  off  any  nitrous  acid,  and 
precipitate  the  chromic  acid  according  to  §  130.  L,  «-,  ft  with 
mercurous  nitrate  (according  to  GIBBS  at  a  boiling  heat).  The 
filtrate  now  contains  the  whole  of  the  uranium,  of  course  in 
presence  of  mercury. 

The  separation  of  uranyl  from  the  metals  of  the  fourth  125 
group  may  be  based  simply  on  the  fact  that  ammonium  carbonate 
prevents  the  precipitation  of  uranyl,  but  not  that  of  the  other 
metals    by    ammonium    sulphide.     Mix    the    solution    with    a 
mixture  of  ammonium  carbonate  and  ammonium  sulphide,  allow 


*  Zeitschr.f.  analyt.  Chem.,  iv,  383.  ^  Ib.t  xn,  310. 


§  161.]  BASES   OF   GROUP  IY.  675 

to  subside  in  a  closed  flask,  and  wash  the  precipitate  with  water 
containing  ammonium  carbonate  and  ammonium  sulphide. 

Remove  the  greater  part  of  the  excess  of  ammonium  car- 
bonate from  the  filtrate  by  a  very  gentle  heat,  acidify  with 
hydrochloric  acid,  warm,  filter  off  the  separated  sulphur,  and 
throw  down  the  uranium  either  by  ammonium  sulphide  (see 
above,  Separation  of  Uranium  from  the  Alkalies]  or  by 
heating  with  nitric  acid  and  then  adding  ammonia  (H.  ROSE,* 
REMELEf).  The  method  is  not  so  suitable  in  presence  of  nickel, 
as  a  little  of  this  metal  is  very  liable  to  pass  into  the  filtrate 
on  precipitation  with  ammonium  carbonate  and  ammonium 
sulphide. 

Ferric  iron  may  be  also^eparated  from  uranyl  by  means  of 
an  excess  of  ammonium  carbonate.  The  small  quantity  of-  iron 
which  passes  with  the  uranium  into  solution  will  fall  down  on 
allowing  the  solution  to  stand  for  several  hours,  or  it  may  be 
precipitated  with  ammonium  sulphide,  before  the  uranium  is 
thrown  down  (PiSANiJ). 

From  nickel,  cobalt,  manganese,  zinc,  and  magnesium 
the  uranyl  may  also  be  separated  by  barium  carbonate.  The 
fluid,  which  should  contain  a  little  free  acid,  is  mixed  with  the 
precipitant  in  excess,  and  allowed  to  stand  in  the  cold  for  24 
hours  with  frequent  shaking  (76). 

From  cobalt,  nickel,  and  zinc,  uranyl  may  also  be  separated  126 
(GIBBS  and  PERKINS§)  by  taking  the  neutral  or  slightly  acid 
solutions  of  the  chlorides,  adding  sodium  acetate  in  excess  and 
a  few  drops  of  acetic  acid,  and  passing  a  rapid  current  of  hydro- 
gen sulphide  for  half  an  hour  through  the  boiling  fluid.  The 
uranium  remains  dissolved  while  the  other  metals  are  precipi- 
tated. I  should  advise  testing  the  filtrate  with  a  mixture  of 
ammonium  carbonate  and  ammonium  sulphide  to  see  if  any 
nickel,  cobalt,  or  zinc  remain  in  solution. 


*  Zeitsctir.  f.  analyt.  Chem.,  I,  412.         \  Compt.  rend.,  LII,  106. 

f/6.,  iv,  385.  §Zeit8chr.f.  analyt.  Chem.,  in,  334. 


676  SEPARATION.  [§  162. 

Fifth  Group. 

SILVER MERCURY  (iN  MERCUROUS  AND  MERCURIC  COMPOUNDS) LEAD 

—BISMUTH — COPPER CADMIUM. 

I.  SEPARATION  OF  THE  METALS  OF  THE  FIFTH  GROUP  FROM  THOSE 

OF    THE    FIRST    FOUR    GROUPS. 

§162. 

INDEX.     (The  numbers  refer  to  those  in  the  margin.) 

Silver  from  the  metals  of  Groups  L—  IY.,  127,  128. 
Mercury  (in  mercurous  and  mercuric  compounds)  from  the 

metais  of  Groups  I.— IV.,  127,  129. 
Lead  from  the  metais  of  Groups  L— £V.,  127,  130. 

"          manganese,  142. 
Binmuth  from  the  metais  of  Groups  I. — IV.,  127,  140. 

"  manganese,  142. 

Copper  from  the  metals  of  Groups  I.— IV.,  127,  131,  132, 

133,  134,  135. 

Copper  from  zinc,  136,  137. 
"         manganese,  142. 

iron,  138. 
"         nickel,  139. 

Cadmium  from  the  metals  of  Groups  I. — IV.,  127. 
"  zinc,  105. 

"  manganese,  142. 

A.     General  Method. 

ALL  THE  METALS  OF  THE  FIFTH  GROUP  FROM  THOSE  OF 
THE  FIRST  FOUR  GROUPS. 

Principle :  Hydrogen  Sulphide  precipitates  from  Acid 
Solutions  the  Metals  of  the  Fifth  Group,  but  not  those  of  the 
first  Four  Groups. 

The  following  points  require  especial  attention  in  the  execu- 
tion of  the  process : 

OL.  To  effect  the  separation  of  the  metals  of  the  fifth  group  127 
from  those  of  the  first  three  groups,  by  means  of  hydrogen 
sulphide,  it  is  necessary  simply  that  the  reaction  of  the  solution 
should  be  acid,  the  nature  of  the  acid  to  which  the  reaction  is 
due  being  of  no  consequence.  But,  to  effect  the  separation  of 
the  metals  of  the  fifth  group  from  those  of  the  fourth,  the 
presence  of  a  free  mineral  acid  is  indispensable  ;  otherwise  zinc 
and,  under  certain  circumstances,  also  cobalt  and  nickel  may  be 
coprecipitated. 


§  162.]  BASES   OF  GROUP  V.  677 

(3.  But  even  the  addition  of  hydrochloric  acid  to  the  fluid 
will  not  always  entirely  prevent  the  coprecipitation  of  the  zinc. 
RIVOT  and  BOUQUET  *  declare  a  complete  separation  of  copper 
from  zinc  by  means  of  hydrogen  sulphide  altogether  imprac- 
ticable. CALVERT  f  states  that  he  has  arrived  at  the  same  con- 
clusion. On  the  other  hand,  SPIRGATIS  J  concurs  with  H.  ROSE 
in  maintaining  that  the  complete  separation  of  copper  from 
zinc  may  be  effected  by  means  of  hydrogen  sulphide  in  presence 
of  a  sufficient  quantity  of  free  acid. 

In  this  conflict  of  opinions,  I  thought  it  necessary  to  subject 
this  method  once  more  to  a  searching  investigation.  I  there- 
fore had  R.  GRUNDMANN  make  a  series  of  experiments  in  the 
matter  in  my  laboratory  with  a  view  to  settling  the  question.f 
The  following  process  is  founded  on  the  results  which  we 
obtained  : 

Add  to  the  COPPER  and  ZINC  solution  a  large  amount  of 
hydrochloric  acid  (e.g., to  0'4  grm.  oxide  of  copper  in  250  c.c.  of 
solution,  30  c.c.  hydrochloric  acid  of  1-1  sp.  gr.),  conduct  into 
the  fluid  at  about  70°  hydrogen  sulphide  largely  in  excess,  filter 
before  the  excess  of  hydrogen  sulphide  has  had  time  to  escape 
01-  become  decomposed,  wash  with  hydrogen  sulphide  water, 
dry,  roast,  redissolve  in  nitrohydrochloric  acid,  evaporate  nearly 
to  dryness,  add  water  and  hydrochloric  acid  as  above,  and  pre- 
cipitate again  with  hydrogen  sulphide.  This  second  precipi- 
tate is  free  from  zinc ;  it  is  treated  as  directed  in  §  119,  3. 

If  CADMIUM  is  present,  it  is  well  to  have  less  acid  present, 
e.</.,  to  0-4  grm.  oxide  of  cadmium  in  250  c.c.  of  solution  add  10 
c.c.  hydrochloric  acid  of  1*1  sp.  gr.  If  the  quantity  of  zinc  is 
considerable,  dissolve  the  first  precipitate  of  cadmium  sulphide 
in  hot  hydrochloric  acid,  evaporate  nearly  to  dryness,  add  10 
c.c.  hydrochloric  acid  and  about  250  c.c.  water,  and  precipitate 
again.  In  this  way  the  results  are  quite  satisfactory. 

y.  The  other  metals  of  the  fifth  group  comport  themselves 
in  this  respect  similarly  to  cadmium,  i.e.,  they  are  not  com- 
pletely precipitated  by  hydrogen  sulphide  in  presence  of  too 
much  free  acid  in  a  concentrated  solution.  Lead  requires  the 
least  amount  of  free  acid  to  be  retained  in  solution  ;  then  follow 
in  order  of  succession,  cadmium,  nk-rcury,  bismuth,  copper,  sil- 
ver (M.  MARTIN  |).  A 'portion  of  the  filtrate  should,  if  neces- 


*  Annul  d  ('hem.  u.  Pharm,,  LXXX,  304.     \Journ.f.pvaLt.  (  In  in.,  i.xxi,  155. 
\  lit.,  LVJI,  184.  £  //>.,  LXXIII  241.  |  Ib  ,  i. xvn,  371. 


678  SEPARATION.  [§  162. 

sary,  be  tested  by  addition  of  a  large  quantity  of  hydrogen  sul- 
phide to  see  if  the  precipitation  of  the  fifth  group  was  com- 
plete. 

#.  If  hydrochloric  acid  produces  no  precipitate  in  the 
••solution,  it  is  preferred  as  acidifying  agent,  otherwise  sulphuric 
or  nitric  acid  must  be  used.  In  the  latter  case  the  fluid  must 
be  rather  largely  diluted.  ELIOT  and  STOKER  *  arrived  at  the 
•same  conclusion  as  ourselves,  and  showed  that  the  cause  of 
CALVERT'S  unfavorable  results  was  the  too  large  dilution  of  his 
solutions.  For,  to  prevent  the  precipitation  of  zinc  you  have 
not  merely  to  preserve  a  certain  proportion  between  the  zinc 
and  the  free  acid,  but  also  a  certain  degree  of  dilution. 
Although  I  agree  with  the  above-named  chemists  in  the  opinion 
that  it  is  possible  to  produce  a  condition  of  the  fluid,  under 
which  one  precipitation  will  effect  complete  separation,  still  it 
appears  to  me  better,  for  practical  purposes,  to  precipitate  twice, 
as  this  is  sure  to  lead  to  the  desired  result. 

e.  A  somewhat  extended  experience  in  the  separation  of  COP- 
PER from  NICKEL  (and  COBALT)  which  so  frequently  occurs  has 
led  me  to  the  opinion  that  a  double  precipitation  is  unnecessary. 
If  the  solution  which  is  to  be  treated  with  hydrogen  sulphide 
contains  enough  free  hydrochloric  acid  and  riot  too  much  water, 
the  copper  falls  down  absolutely  free  from  nickel,  while,  on  the 
other  hand,  if  the  quantity  of  free  acid  is  not  too  large,  the  fil- 
trate will  be  quite  free  from  copper.  The  method  of  sepa- 
rating copper  from  zinc  given  in  j3  is  also  to  be  recommended 
in  this  case. 

C-  CADMIUM  and  ZINC  may,  according  to  FoLLENius,f  also  be 
completely  separated  by  a  single  precipitation,  if  the  metals  are 
present  in  a  sulphuric  acid  solution  containing  25  or  30  per 
cent,  of  dilute  acid  of  1*19  sp.  gr.  Precipitate  with  hydrogen 
sulphide  at  70°.  Collect  the  precipitate  on  a  weighed  asbestos 
filter,  dry  in  a  current  of  heated  air,  ignite  gently  in  a  stream 
of  pure  hydrogen  sulphide  (to  convert  small  quantities  of  cad- 
mium sulphate  into  sulphide),  remove  the  small  quantity  of 
.•separated  sulphur  by  gentle  ignition  in  a  current  of  air,  and 
weigh. 

*  On  the  Impurities  of  Commercial  Zinc,  etc.— Memoirs  of  the  American 
Academy  of  Arts  and  Sciences.     New  Series.     Vol.  8. 
f  Zeitschr.f.  analyt.  Chem.,  xm,  Part  4. 


§  162.]  BASES    OF   GROUP  V.  679 

B.  Special  Methods. 

SINGLE  METALS  OF  THE  FIFTH  GROUP  FROM  SINGLE  OB 
MIXED  METALS  OF  THE  FIRST  FOUR  GROUPS. 

1.  SILVER  is  most  simply  and  completely  separated  from  the  128 
METALS  OF  THE  FIRST  FOUR  GROUPS  by  means  of  hydrochloric 
acid.     The  hydrochloric  acid  must  not  be  used  too  largely  in 
excess,  and  the  fluid  must  be  sufficiently  dilute ;  otherwise  a 
portion  of  the  silver  will  remain  in  solution.     Care  must  be 
taken  also  not  to  omit  the  addition  of  nitric  acid,  which  pro- 
motes the  separation  of  the  silver  chloride.     The  latter  should 

be  treated  according  to  §  115,  1,  a. 

2.  The  separation  of  MERCURY  from  the  METALS  OF  THE  129 
FIRST  FOUR  GROUPS  may  be  effected  also  by  ignition,  which  will 
cause  the  volatilization  of  the  mercury  or  the  mercurial  com- 
pound, leaving  the  non-volatile  bodies  behind.     The  method  is 
applicable  in  many  cases  to  alloys,  in  others  to  oxides,  chlorides, 

or  sulphides.  If  the  mercury  is  estimated  only  from  the  loss, 
the  operation  is  conducted  in  a  crucible ;  otherwise  in  a  bulb- 
tube,  or  a  wide  glass  tube  with  porcelain  boat.  In  the  latter 
case  it  is  well  to  use  a  current  of  hydrogen  (compare  §  118, 
1,  a ;  also  Examination  of  Mercury  Ores  in  the  Special  Part). 
The  precipitation  of  mercury  as  mercurous  chloride  with 
phosphorous  acid,  according  to  §  118,  2,  is  also  well  adapted  for 
its  separation  from  metals  of  the  first  four  groups.  If  the  mer- 
cury is  already  present  as  a  mercurous  salt,  it  may  be  separated 
and  determined  in  a  simple  manner,  by  precipitation  with 
hydrochloric  acid  (§  117,  1). 

3.  FROM  THOSE  BASIC  RADICALS  WHICH  FORM  SOLUBLE  SALTS  130 
WITH  SULPHURIC  ACID,  LEAD  may  be  readily  separated  by  that 
acid.     The  results  are  very  satisfactory,  if  the  rules  given  in 

§  116,  3  are  strictly  adhered  to. 

If  you  have  lead  in  presence  of  barium,  both  in  form  of 
sulphn'es,  digest  the  precipitate  with  a  solution  of  ordinary 
ammonium  sesquicarbonate,  without  application  of  heat.  This 
decomposes  the  lead  salt,  leaving  the  barium  salt  unaltered. 
Wash,  first  with  solution  of  ammonium  carbonate,  then  with 
w;itc>r,  and  separate  finally  the  lead  carbonate  from  the  barium 
sulphate,  by  acetic  acid  or  dilute  nitric  acid  (II.  HOSE*).  The 

*Journ.f.  praL-t.  Clicm.,  LXVI,  166. 


GSO  SEPARATION.  [§  162. 

same  object  may  also  be  attained  by  suspending  the  washed 
insoluble  salts  in  water  and  digesting  with  a  clear  concentrated 
solution  of  sodium  thiosulphate  at  15—20°  (not  higher).     The 
barium  sulphate  remains  undissolved,  the  lead  sulphate   dis- 
solves.    Determine  the  lead  in  the  filtrate  (after  §  116,  2)  as 
lead   sulphide  (J.  LOWE  *).      The   method   recommended,  by 
EJVOT,  BEUDANT,  and  DAGUIN  f  for  separating  lead  by  adding 
sodium  acetate  to  the  solution,  heating,  and  passing  in  chlo- 
rine gas,  requires  to  be  carried  out  with  great  caution,  ac- 
cording to  H.  RosE,J  since  portions  of  other  metals,  even 
such  as  are  not  converted  into  higher  oxides — e.g.,  zinc — are 
very  likely  to  be  precipitated  with  the  lead. 

4.    COPPER  FROM  ALL  METALS  OF  THE  FIRST  FoiJR  GROUPS. 

a.  Free  the  solution  as  far  as  possible  from  hydrochloric  131 
and  nitric  acids  by  evaporation  with  sulphuric  acid.  Dilute  if 
necessary,  boil,  and  add  sodium  thiosulphate  §  as  long  as  a  black 
precipitate  continues  to  form.  As  soon  as  this  has  deposited, 
and  the  supernatant  fluid  contains  only  suspended  sulphur, 
the  whole  of  the  copper  is  precipitated.  The  precipitate  is 
cuprous  sulphide  (Cu2S),  and  may  be  readily  washed  without 
suffering  oxidation.  Convert  it  into  anhydrous  cuprous  sul- 
phide by  ignition  in  hydrogen  (§  119,  3).  The  other  metals 
are  in  the  filtrate  and  washings.  Evaporate  with  some  nitric 
acid,  filter,  and  determine  the  metals  in  the  filtrate.  ||  Results 
good.  The  method  requires  practice,  as  the  end  of  the  pre- 
cipitation of  the  copper  is  not  so  easy  to  hit  as  when  hydro- 
gen sulphide  is  employed. 

If  the  solution  contained  hydrochloric  or  nitric  acid,  and 
this  was  not  first  removed  before  the  addition  of  the  thiosul- 
phate, the  precipitant  would  be  required  in  much  larger  quan- 
tity ;  in  the  presence  of  hydrochloric  acid  because  the  cuprous 

*Journ.  f.  prakt.  Chem.,  LXXVIT,  75.  f76.,  LXT.  136. 

\  Pogg.  Annal,  ox,  417. 

§  The  commercial  salt  is  often  not  sufficiently  pure,  in  which  case  some 
^odium  carbonate  must  be  added  to  its  solution  and  the  mixture  filtered, 

*||  As  far  back  as  1842.  C.  HIMLY  made  the  first  proposal  to  employ  sodium 
thiosulphate  for  the  precipitation  of  many  metals  as  sulphides  (Annal.  d  Chem. 
u.  Pharm.,  XLIII,  150).  The  question,  after  long  neglect,  was  afterwards  taken 
up  again  by  VOHL  (Annal.  d.  Chem.  u.  Pharm.,  xcvi,  237),  and  SLATER  (Chem,. 
Gaz.,  1855,  369)  FLAJOLOT,  however,  made  the  first  quantitative  experiment 
(Annal  des  Mines,  1853,  641;  Journ.  f.  prakt.  Chem.,  LXI,  105).  The  results 
obtained  by  him  are  perfectly  satisfactory, 


§  162.]  BASES    OF   GROUP   V.  681 

chloride  produced  is  only  decomposed  by  a  large  excess  of 
tliiosulphate,  in  the  presence  of  nitric  acid  because  the  thio- 
sulphate  does  not  begin  to  act  on  the  copper  salt  till  all  the 
nitric  acid  is  decomposed. 

~b.  Precipitate  the  copper  as  cuprous  sulphocyanate  132 
according  to  §  119,  3,  &,  or  §  119,  4,  e\  the  other  metals 
remain  in  solution  (Kivox).  If  alkalies  were  present  and  it 
were  desired  to  determine  them  in  the  filtrate,  ammonium 
sulphocyanate  must  be  used  instead  of  the  potassium  salt 
usually  employed.  This  method  is  particularly  well  adapted 
for  the  separation  of  copper  from  zinc.  The  zinc  can  be 
precipitated  at  once  from  the  filtrate  by  sodium  carbonate. 
The  method  is  also  suitable  for  separating  copper  from  iron 
(H.  ROSE  *) ;  in  this  case  it  is  unnecessary  that  ferric  salts  be 
completely  reduced  by  the  sulphurous  acid  added ;  the  sepa- 
ration may  be  effected,  even  if  the  solution  becomes  blood- 
red  on  the  addition  of  the  precipitant. 

c.  The  method  proposed  by  FLAJOLOT,f  and  which  has   133 
been  so  frequently  recommended,  consisting  in  precipitating 

the  copper  by  adding  a  solution  of  iodine  in  aqueous  sul- 
phurous acid,  after  removing  the  greater  part  of  the  free  acid 
present  and  adding  sulphurous  acid,  gives  inaccurate  results, 
according  to  II.  EOSE,^  because  a  not  inconsiderable  quantity 
of  copper  remains  dissolved  in  the  sulphurous  liquid.  This 
difficulty  may  be  avoided  by  adding  to  the  hydrochloric-acid 
solution,  containing  a  slight  excess  of  acid,  an  excess  of 
stannous  chloride,  ammonium  chloride,  and  potassium  iodide, 
until  this  last  just  predominates  (E.  FLEISCHER  §).  As,  how- 
ever, the  excess  of  stannous  chloride  in  the  filtrate  and  the 
stannic  chloride  formed  must  first  be  removed  before  the 
bases  of  groups  1  to  4  can  be  determined,  this  method  offers 
no  advantages. 

d.  If  the  solution  is  not  too  dilute,  the  bases  being  pres-   134 
ent   as   sulphates,   while  hydrochloric    and   nitric    acids   are 
absent,  the  copper  may  also  be  completely  precipitated  by 

*  Pogrj.  Annal.,  ex,  424 

f  Annal.  des  Mines,  1853,  641;  Journ.f.  prakt.  Chem.,  LXI,  105. 

\Pogg.  Annal.,  ex,  425. 

§Zeil8chr.f.  analyt.  Chem.,  ix,  256. 


682  SEPAKATION.  [§  162. 

means  of  an  alkali  hypophosphite.  At  about  70°  copper 
hydride  is  precipitated,  and  this,  on  heating  to  a  still  higher 
temperature,  which  should  not,  however,  exceed  90°,  decom- 
poses into  copper  and  hydrogen.  The  precipitation  is  com- 
plete when  a  drop  of  the  liquid  is  no  longer  colored  brown 
by  hydrogen  sulphide.  "Wash  the  spongy  copper  by  decan- 
tatiori,  dry,  and  ignite  in  a  current  of  hydrogen.  The  sepa- 
ration is  complete  (W.  GIBBS  and  R.  CHAUVENET*).  The 
method  is  adapted  particularly  for  separating  copper  from 
the  metals  of  group  4,  which  may  be  precipitated  from  the 
filtrate  by  ammonium  sulphide. 

e.  The  solution  should  be  free  from  hydrochloric  acid,  135 
and  should  contain  a  certain  quantity  of  free  nitric  acid  (20 
c.  c.  nitric  acid  of  1-2  sp.  gr.  to  200  c.  c.)  and  some  sul- 
phuric acid.  Throw  down  the  copper  by  a  galvanic  current, 
so  that  the  metal  may  be  firmly  deposited  on  a  platinum  ves- 
sel (preferably  a  platinum  cone),  which  forms  the  negative 
pole.  Take  care  that  the  current  is  strong  enough,  and, 
without  interrupting  it,  remove  the  cone  from  the  fluid 
occasionally  to  see  when  the  copper  is  all  precipitated.  With 
proper  execution  the  separation  of  copper  from  all  metals  of 
groups  1—4  is  thorough.  All  metals  of  groups  1-4  remain  dis- 
solved, except  manganese,  which  separates  as  dioxide  at  the 
positive  pole.  The  method  requires  practice  and  strict  atten- 
tion to  the  conditions  which  have  been  determined  by  a  long 
course  of  experiments.  It  is  particularly  suited  for  mining 
assays  and  manufacturers.  The  electrolytic  method  of  sepa- 
rating copper  was,  I  believe,  first  recommended  by  GIBBS, f 
and  afterwards  improved  by  LUCKOW.  J  LECOQ  DE  BOISBAU- 
DRAN,§  ULLGREN,  I  and  MERRICK^[  hate  also  written  on  this 
subject.  Finally  the  method  was  very  accurately  and 
minutely  described  by  the  Mansfelder  Ober-Berg-  und  Hut- 
tend  irection  at  Eisleben,**  who,  after  giving  a  prize  to 
LUCKOW 's  method,  afterwards  adopted  it,  and  still  further 

*  Zeitschr.f.  analyt.  Chem.,  vn,  256.  f  75.,  m,  334. 

Dingier  s  polyt.  Journ.,  CLXXVII,  296,  and  (in  detail)  Zeilschr,  f.  analyt. 

.,  vin,  25. 

%Zeitschr.  f.  analyt.  Chem.,  vn,  253,  and  ix,  102.  |  75.,  7,  255. 

^American  Chemist,  n,  136.  ** Zeitschr.  /.  analyt.  Chem.,  xi,  1 


§  162.]  BASES    OF   GROUP  V.  683 

improved  it.     I  must  refer  the  reader  for  details  to  the  last 
mentioned  memoir  and  LUCKOW'S  paper. 

5.  COPPER  FROM  ZINC. 

a.  BOBIERRE  *  employed  the  following  method  with  satis-  136 
factory  results  in  the  analysis  of  many  alloys  of  zinc  and 
copper :  The  alloy  is  put  into  a  porcelain  boat  lying  in  a  por- 
celain tube,  and  heated  to  redness  for  three-quarters  of  an 
hour  at  the  most,  a  rapid  stream  of  hydrogen  gas  being  con- 
ducted over  it  during  the  process.  The  zinc  volatilizes,  the 
copper  remains  behind.  If  the  alloy  contains  a  little  lead 
(under  2  to  3  per  cent.)  this  goes  off  entirely  with  the  zinc, 
and  is  partly  deposited  in  the  porcelain  tube  in  front  of  the 
boat ;  if  more  lead  is  present  part  only  is  volatilized,  the  rest 
remaining  with  the  copper  (M.  BURSTYN  f). 

I.  A.  W.  HOFMANN'S  method  given  below  (159)  for  137 
separating  copper  from  cadmium  (boiling  the  precipitated 
sulphides  with  dilute  sulphuric  acid,  whereby  the  cadmium 
sulphide  is  dissolved  while  copper  sulphide  remains  behind), 
is  also  adapted  for  separating  copper  from  zinc  (G.  C. 
WITTSTEIN  J). 

6.  COPPER  FROM  IRON. 

One  of  the  oldest  methods  of  separating  the  oxides  con-  138 
sists  in  precipitating  the  solution  with  ammonia  and  filtering 
off  the  precipitated  iron  hydroxide  from  the  amrnoniacal 
copper  solution.  To  obtain  accurate  results  by  this  method, 
however,  the  precipitation  must  be  repeated  according  to  the 
quantity  of  copper  present,  two  or  even  three  times,  or  until 
the  filtrate  is  no  longer  blue,  otherwise  the  iron  hydroxide 
will  contain  copper. 

7.  COPPER  FROM  NICKEL. 

Evaporate  the  nitric-acid  solution,  if  it  be  such  a  one,   139 
with  hydrochloric  acid  to  dry  ness,  dissolve  the  chlorides  in 
water,  add  about  twice  as  much  pure  potassium  tartrate  as 
there  are  metals  present,  warm  slightly  to  facilitate  solution, 


*  Compt.  rend.,  xxxvi,  224;  Journ.f.  pi*akt.  Chem.,  LVIII,  380. 
"\Zeitschr.f.  analyt.  Chem.,  xi,  175. 

\  VwrteljahresscJir.  f.  prakt.  Pharm.,  xvn,  461 ;  Zeitschr.  f  analyt.  Chem., 
vni,  202. 


684  SEPARATION.  [§  162. 

and  then  add  gradually  alcoholic  solution  of  potassa  until  the 
hydrated  oxides  precipitated  redissolve.  After  cooling,  add  a 
solution  of  pure  grape  sugar,  and  boil  for  one  or  two  minutes. 
The  copper  is  precipitated  as  cuprous  oxide.  Make  certain 
that  precipitation  is  complete  by  adding  a  drop  of  grape- 
sugar  solution  to  the  clear  liquid,  then  filter,  and  determine 
the  copper  as  oxide,  either  by  ignition  (treating  with  nitric 
acid  and  reigniting),  or  as  cuprous  sulphide  (§  119,  3,  c),  or 
volumetrically  (§  119,  4,  e).  Evaporate  the  liquid  contain- 
ing the  nickel  to  dryness,  ignite  the  residue,  remove  the 
potassium  carbonate  by  washing,  reignite,  dissolve  the  resi- 
due in  nitrohydrochloric  acid,  and  precipitate  the  nickel  with 
potassa  solution,,  as  in  §  110,  1,  a  (DEWILDE*).  The  cu- 
prous oxide  must  be  rapidly  filtered  off  and  washed,  otherwise 
a  part  will  redissolve.  The  method  is  inconvenient,  and  is 
by  no  means  more  accurate  than  the  separation  by  hydrogen 
sulphide. 

8.  BISMUTH  FKOM   THE    METALS    OF    THE    FIBST  FOUR 
GROUPS,  WITH  THE  EXCEPTION  OF  FERRIC  IRON. 

Precipitate  the  bismuth  according  to  §  120,  4,  as  bismuth  140 
oxychloride,  and  determine  it  as  metal ;   all  the  other  basic 
metals  remain  completely  in  solution.     Results  very  satis- 
factory (H.  KosEf). 

9.  CADMIUM  FROM  ZINC. 

Prepare  a  hydrochloric-  or  nitric-acid  solution  of  the  two  141 
oxides  as  neutral  as  possible,  add.  a  sufficient  quantity  of  tar- 
taric  acid,  then  solution  of  potassa  or  soda,  until  the  reaction 
of  the  clear  fluid  is  distinctly  alkaline.  Dilute  now  with  a 
sufficient  quantity  of  water,  and  boil  for  1^-2  hours.  All 
the  cadmium  precipitates  as  hydroxide,  free  from  alkali  (to  be 
determined  as  directed  in  §  121),  whilst  the  whole  of  the  zinc 
remains  in  solution ;  the  latter  metal  is  determined  as  directed 
in  §  108,  1,  I  (AuBEL  and  KAMDOHK^:).  The  test-analyses 
communicated  are  satisfactory.  As  the  separation  only  suc- 
ceeds when  the  substances  are  present  in  correct  proportions, 

*  Chem.  News,  1863,  vn,  49;  Zeitschr.  f.  analyt.  Chem.,  n,  72. 

f  Pogg.  AnnaL,  ex,  429.  \Annal.  d.  Chem.  u.  Pharm.,  cm,  33. 


§  163.]  BASES    OF   GROUP   V.  685 

I  will  give  the  quantities  employed  by  AUBEL  and  HAMDOHK 
with  especially  good  effect.  About  1  grm.  oxide  of  zinc 
and  1  grm.  oxide  of  cadmium  were  dissolved  in  hydrochloric 
acid,  30  grm.  solution  of  tartaric  acid  (containing  0*23  grrn. 
acid  in  1  grm.),  50  grm.  soda  solution  of  1*16  sp.  gr.,  and 
120  grm.  water  added,  and  the  whole  boiled  2  hours.  (The 
boiling  must  on  no  account  be  done  in  glass;  a  platinum  or 
.silver  dish  should  be  used.) 

10.  LEAD,  BISMUTH,  CADMIUM,  AND   COPPER  FROM   MAN- 

OANESE, 

If  the  solution  contains  a  manganous  salt  and  one  of  the  142 
other  bases,  precipitate  the  hot  solution  with  sodium  carbonate, 
wash  the  precipitate  first  by  decantation,  then  on  the  filter, 
with  boiling  water,  dry,  ignite  for  some  time,  weigh,  and  in 
a  portion  of  the  residue  estimate  the  manganese  volumetric- 
ally  (72).  If  sufficient  bismuth,  lead,  cadmium,  or  copper  is 
present,  the  residue  will  have  the  formula  Mn3Os  -\-  a?MO, 
or  MnO,  -j-  ^Bi2O,  (KKIEGER*).  Never  neglect  testing  the 
filtrate  by  adding  ammonium  sulphide,  to  ascertain  whether 
the  metals  have  been  completely  precipitated  by  the  sodium 
carbonate.  When  precipitating  copper  by  alkali  carbonates, 
dilute  the  liquid  so  that  it  contains  about  1  grm.  per  litre, 
add  the  alkali  carbonate  in  very  slight  excess,  and  boil  the 
mixture  for  about  half  an  hour,  whereby  the  bluish-green 
basic  carbonate  becomes  dark,  granular,  and  more  easily 
washed  ("W.  GIBBS  and  E.  R.  TAYLOR  f). 

II.  SEPARATION    or    THE    METALS    OF  THE    FIFTH    GROUP 

FROM   EACH   OTHER,  if 

§  163. 

INDEX.     (The  numbers  refer  to  those  in  the  margin.) 
Silver  from  copper,  143,  148,  150,  164,  165. 
"          cadmium,  143,  148,  150. 

bismuth,  143,  147,  150,  161. 
"          mercuricuni,!  143,  148,  150,  158,  160. 
lead,  143,  146.  147,  150,  155,  164,  165. 


*  Arwal.  de  Chem.  u.  Pharm.,  i/xxxvir,  264. 
\Zeit8chr.f.  analyt.  CJiem.,  vn,  258. 

t  For  the  sake  of  brevity  the  terms  "  inercuriciim  "  and  "  inercurosum  "  are 
used  to  designate  respectively  mercury  in  mercuric  and  mercurous  compounds. 


686  SEPARATION.  [§  163. 

Mercuricum*  from  silver,  143,  148,  150,  158,  160. 
"  mercurosum,*  144. 

«'  lead,  145,  146,  147,  150,  158,  160. 

bismuth,  145,  147,  150,  151,  158. 
copper,  145,  149,  150,  158,  160. 
cadmium,  145,  150,  158. 
Mercurosum*  from  inercaricum,  144. 
44  copper,  144. 

<4  cadmium,  144. 

lead,  144,  146. 

Compare  also  uiercuricuin  from  other  metals. 
Lead  from  silver,  143,  147,  150,  155,  164,  165. 

mercuricum,  145,  146,  147,  150,  158,  160. 
"          mercurosum,  144,  146. 
"          copper,  146,  147,  150,  152. 
"          bismuth,  146,  147,  152,  161,  162. 
"          cadmium,  146,  147,  150. 
Bismuth  from  silver,  143,  147,  150,  161. 

lead,  146,  147,  152,  161,  162. 
copper,  147,  150,  151,  153,  161. 
cadmium,  147,  150,  151,  152,  157. 
mercuricum,  145,  147,  150,  151,  158. 
Copper  from  silver,  143,  148,  150,  164,  165. 
lead,  146,  147,  150,  152. 
bismuth,  147,  150,  151,  153,  161. 
mercuricum,  145,  149,  150,  158,  160. 
"  mercurosum,  144. 

"          cadmium,  149,  150,  152,  154,  156,  159. 
Copper  in  cupric  from  copper  in  cuprous  compounds,  163,  165. 
Cadmium  from  silver,  143,  148,  150. 
lead,  146,  147,  150. 
bismuth,  147,  150,  151,  152,  157. 
copper,  149,  150,  152,  154,  156,  159. 
"         mercuricum,  145,  150,  158. 
mercurosum,  144. 

1.  Methods  based  upon  the  Insolubility  of  certain  of  the 
Chlorides  in  Water  or  Alcohol. 

a.  SILVER  FROM  COPPER,  CADMIUM,  BISMUTH,  MERCURICUM,  AND 
LEAD. 

a.  To  separate  silver  from  copper,  cadmium,  and  bismuth,  143 
add  to  the  nitric  acid  solution  containing  excess  of  nitric  acid, 
hydrochloric  acid  as  long  as  a  precipitate  forms,  and  separate 
the  precipitated  silver  chloride  from  the  solution  which  con- 
tains the  other  metals,  as  directed  §  115,  1,  a.     In  the  presence 

'"'  For  the  sake  of  brevity  the  terms  "mercuricum"  and  "mercurosum  "  are 
used  to  designate  respectively  mercury  in  mercuric  and  mercurous  compounds. 


§  163.]  BASES   OF   GROUP  V.  687 

of  bismuth,  after  pouring  off  the  supernatant  fluid,  heat  again 
with  nitric  acid,  and  wash  with  dilute  nitric  acid  before  wash- 
ing with  water. 

ft.  If  you  wish  to  separate  mercuricum  from  silver  by 
hydrochloric  acid,  special  precautions  must  be  taken,  as  a  solu- 
tion of  mercuric  nitrate  possesses  the  property  of  dissolving 
silver  chloride  (WACKENRODER,  v.  LIEBIG,*  H«.  DEBRAYf). 
Although  the  silver  chloride  in  solution  for  the  most  part 
separates  on  the  addition  of  enough  hydrochloric  acid  to  con- 
vert the  mercuric  nitrate  into  chloride,  or  on  addition  of 
sodium  acetate,  still  we  cannot  depend  upon  the  complete  pre- 
cipitation of  the  silver.  On  this  account,  mix  the  nitric  acid 
solution — which  must  not  contain  any  mercurous  salt,  and  is 
to  be  in  a  sufficiently  dilute  condition  and  acidified  with  nitric 
acid — writh  hydrochloric  acid,  as  long  as  a  precipitate  forms. 
Allow  to  deposit,  filter  off  the  clear  fluid,  heat  the  precipitate 
—to  'free  it  from  any  possibly  coprecipitated  basic  mercuric 
salts — with  a  little  nitric  acid,  add  water,  then  a  few  drops  of 
hydrochloric  acid,  and  filter  off  the  silver  chloride.  In  the 
filtrate  determine  the  mercury  as  sulphide  (§  118,  3),  and 
finally  test  this  for  silver,  by  ignition  in  a  stream  of  hydrogen 
— any  silver  that  may  happen  to  be  present  will  remain  behind 
in  the  metallic  state. 

y.  In  the  separation  of  silver  from  lead,  the  precipitation 
is  advantageously  preceded  by  addition  of  sodium  acetate. 
The  fluid  must  be  hot  and -the  hydrochloric  acid  rather  dilute; 
no  more  must  be  added  of  the  latter  than  is  just  necessary. 
In  this  manner,  the  separation  may  be  readily  effected,  since 
lead  chloride  dissolves  in  sodium  acetate  (ANTHON).  The  sil- 
ver chloride  is  washed  with  hot  water.  The  lead  is  thrown 
down  from  the  filtrate  with  hydrogen  sulphide.  If  you  desire 
to  prevent  the  occasionally  injurious  influence  of  sodium  ace- 
tate, great  care  must  be  given  to  the  washing  of  the  silver 
chloride.  It  is  also  well  to  reduce  the  weighed  chloride  by 
gentle  ignition  in  a  current  of  hydrogen,  and  to  test  the  silver 
obtained  for  lead.  Regarding  the  estimation  of  very  small 
quantities  of  silver  in  lead,  compare  "  Analysis  of  Refined 
Lead  ' "  in  the  Special  Part. 

*  Annal.  d.  Chem.  u.  Pharm.,  LXXXI,  128. 

f  Compt.  rend.,  LXX,  847  ;  Zeitschr.f.  analyt.  G'hem.,  xiu,  349. 


688  SEPARATION.  [§  163. 

tf .  The  volumetric  method  (§  115,  5)  is  usally  resorted  to  in 
mints  to  determine  the  silver  in  alloys.  In  presence  of  a  mer- 
curic salt,  sodium  acetate  is  mixed  with  the  fluid,  immediately 
before  the  addition  of  the  solution  of  chloride  of  sodium.  In 
the  East  India  mint  the  silver  is  separated  and  weighed  as 
chloride.* 

b.  MERCUROSUM  FROM  MERCURICTJM,  COPPER,  CADMIUM,  AND 
LEAD. 

Mix  the  very  dilute  cold  solution  with  hydrochloric  acid  144 
as  long  as  a  precipitate  (mercurous  chloride)  forms  ;  allow  this 
to  deposit,  filter  on  a  weighed  filter,  dry  at  100°,  and  weigh. 
The  filtrate  contains  the  other  metals.  If  you  have  to  analyze 
a  solid  body,  insoluble  in  water,  either  treat  directly,  in  the 
cold,  with  dilute  hydrochloric  acid,  or  dissolve  in  highly  dilute 
nitric  acid,  and  mix  the  solution  with  a  large  quantity  of  water 
before  proceeding  to  precipitate.  Care  must  always  be  taken 
that  the  mode  of  solution  is  such  as  not  to  convert  mercurous 
into  mercuric  compounds.  If  lead  is  present  the  washing  of 
the  mercurous  chloride  must  be  executed  with  special  care 
with  water  of  60 — 70°,  till  the  filtrate  ceases  to  be  colored  with 
hydrogen  sulphide.  As  an  additional  security,  it  is  well  to 
test  at  last  whether  the  weighed  mercurous  chloride  leaves  no 
lead  sulphide  behind  on  cautious  ignition  with  sulphur  in  a 
stream  of  hydrogen. 

c.  MERCUROSUM  AND  MERCURICUM  FROM  COPPER,  CADMIUM, 
AND  (but  less  well)  FROM  BISMUTH  AND  LEAD. 

If  mercury  is  present  as  a  mercuric  compound,  or  partly  145 
in  a  mercuric  and  partly  in  a  mercurous  compound,  it  is  pre- 
cipitated according  to  §  118,  2,  by  means  of  hydrochloric  acid 
and  phosphorous  acid  as  mercurous  chloride.  The  precipitate, 
particularly  when  bismuth  is  present,  is  first  washed  with  water 
containing  hydrochloric  acid,  then  with  pure  water,  till  the 
washings  are  no  longer  colored  with  hydrogen  sulphide  (H. 
RosEf).  In  the  presence  of  lead,  the  remarks  in  144  must 
be  attented  to. 


*  Ckem.  CentralbL,  1872,  202. 
\Pogg.  Annal.,  ex,  534. 


§  163.]  BASES    OF   GROUP   V.  689 

d.  The  method  of  separating  lead  from  silver,  copper, 
and  bismuth  by  highly  concentrating  the  nitric-acid  solution, 
adding  hydrochloric  acid  and  alcohol,  and  washing  the  lead 
chloride  with  alcohol,  is  not  to  be  recommended.  It  is  inferior 
to  146  in  accuracy. 

2.   Methods  based  upon  the    Insolubility  of  Lead 
Sulphate. 

LEAD  FROM  ALL  OTHER  METALS  OF  THE  FIFTH  GROUP. 

Mix  the  nitric  acid  solution  with  pure  sulphuric  acid  in  not  146 
too  slight  excess,  evaporate  until  the  sulphuric  acid  begins  to 
volatilize,  allow  the  fluid  to  cool,  add  water  (in  which,  if  there 
is  a  sufficient  quantity  of  free  sulphuric  acid  present,  the  mer- 
curic and  bismuth  sulphates  dissolve  completely),  and  then 
filter  the  solution,  which  contains  the  other  metals,  without 
delay  from  the  undissolved  lead  sulphate.  If  it  is  feared  that 
the  residue  no  longer  contains  enough  free  sulphuric  acid,  add 
some  dilute  acid  to  it  before  adding  the  water.  Wash  the 
precipitate  with  water  containing  sulphuric  acid,  displace  the 
latter  with  alcohol,  dry,  and  weigh  (§  116,  3).  Precipitate 
the  other  metals  from  the  nitrate  by  hydrogen  sulphide.  If 
silver  is  present  in  any  notable  quantity,  this  method  cannot 
be  recommended,  as  the  silver  sulphate  is  not  soluble  enough. 
In  this  case  you  may  follow  ELIOT  and  STORER,*  viz.,  mix  the 
solution  with  ammonium  nitrate,  warm,  precipitate  the  greater 
portion  of  the  silver  with  ammonium  chloride,  evaporate  the 
filtrate,  remove  the  ammonium  salts  by  ignition,  and  in  the 
residue  separate  the  small  remainder  of  the  silver  from  the 
lead  with  sulphuric  acid  as  just  directed.  For  the  separation 
of  lead  from  bismuth,  on  the  above  principle,  H.  ROSE  t  gives 
the  following  process  as  the  best.  If  both  oxides  are  in  dilute 
nitric  acid  solution,  as  is  usually  the  case,  evaporate  to  small 
bulk,  and  add  enough  hydrochloric  acid  to  dissolve  all  the 
bismuth;  the  lead  separates  partially  as  chloride.  Should  a 
portion  of  the  clear  fluid  poured  off  become  turbid  on  the 
addition  of  a  drop  of  water,  you  must  add  some  more  hydro- 

*  Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  Sept.  11, 
i860,  p.  52  ;  Zeitschr.f.  analyl.  Chem.,  i,  389. 
f  Pogg.  AnnaL,  ex,  432. 


690  SEPARATION.  [§  163. 

chloric  acid,  till  no  permanent  turbidity  is  produced  unless 
several  drops  of  water  are  added.  The  turbid  fluids  should 
all  be  returned,  and  the  glasses  rinsed  with  alcohol.  Add 
now  dilute  sulphuric  acid,  allow  to  stand  some  time  with  stir- 
ring, add  alcohol  of  0'8  sp.  gr.,  stir  well,  allow  to  settle 
for  a  long  time,  filter,  wash  the  lead  sulphate  first  with  alco- 
hol mixed  with  a  small  quantity  of  hydrochloric  acid,  then 
with  pure  alcohol.  Determine  it  after  §  116,  3.  Mix  the 
filtrate  at  once  with  a  large  quantity  of  water,  and  proceed 
with  the  precipitated  basic  bismuth  chloride  according  to 
§  120,  4. 

3.    Methods  lased  upon  different  deportment  with 
Cyanide  of  Potassium  (FRESENIUS  and  HAIDLEN*). 

a.   LEAD  AND  BISMUTH  FROM  ALL  OTHER  METALS  OF  THE 
FIFTH  GROUP. 

Mix  the  dilute  solution  with  sodium  carbonate  in  slight  14T 
excess,  add  solution  of  potassium  cyanide  (free  from  sulphide), 
heat  gently  for  some  time,  filter  and  wash.  On  the  filter  you 
have  lead  and  bismuth  carbonates  (containing  alkali) ;  the  fil- 
trate contains  the  other  metals  as  cyanides  in  combination  with 
potassium  cyanide.  The  method  of  effecting  their  further 
separation  will  be  learnt  from  what  follows.  In  very  accurate 
analyses  bear  in  mind  that  the  filtrate  generally  contains  traces 
of  bismuth,  which  may  be  precipitated  by  ammonium  sulphide. 

1}.  SILVER  FROM  MERCURICUM,  COPPER,  AND  CADMIUM. 

Add  to  the  solution,  which,  if  it  contains  much  free  acid,  148 
must  previously  be  nearly  neutralized  with  soda,  potassium 
cyanide  until  the  precipitate  which  forms  at  first  is  redissolved. 
The  solution  contains  the  cyanides  of  the  metals  in  combina- 
tion with  potassium  cyanide  as  soluble  double  salts.  Add 
dilute  nitric  acid  in  excess,  which  effects  the  decomposition  of 
the  double  cyanides ;  the  insoluble  silver  cyanide  precipitates 
permanently,  whilst  the  mercuric  cyanide  remains  in  solution, 
and  the  cyanides  of  copper  and  cadmium  redissolve  in  the 
excess  of  nitric  acid.  Treat  the  silver  cyanide  as  directed 
§  115,  3.  If  the  filtrate  contains  only  mercury  and  cadmium, 
precipitate  at  once  with  hydrogen  sulphide,  which  completely 

*  Annal.  d.  Chem.  u.  Pharm.,  XLIII,  129. 


§  163.]  BASES   OF   GROUP   V.  691 

throws  down  the  sulphides  of  the  two  metals ;  but  if  it  con- 
tains copper,  you  must  first  heat  with  sulphuric  acid,  until  the 
odor  of  hydrocyanic  acid  is  no  longer  perceptible,  and  then 
precipitate  with  hydrogen  sulphide  (§  119,  3). 

c.  COPPER  FROM  MERCURICUM  AND  CADMIUM. 

Mix  the  solution,  as  in  J,  with  potassium  cyanide  until  the  149 
precipitate  wrhich  is  first  thrown  down  redissolves  ;  add  some 
more  potassium  cyanide,  then  hydrogen  sulphide  water  or 
ammonium  sulphide,  as  long  as  a  precipitate  forms.  The 
cadmium  and  mercury  sulphides  are  completely  thrown  down, 
wliilst  the  copper  remains  in  solution,  as  sulphide  dissolved  in 
potassium  cyanide.  Allow  the  precipitate  to  subside,  decant 
repeatedly,  treat  the  precipitate,  for  security,  once  more  with 
solution  of  potassium  cyanide,  heat  gently,  filter,  and  wash 
the  sulphides  of  the  metals.  To  determine  the  copper  in  the 
filtrate,  evaporate  the  latter,  with  addition  of  nitric  and  sul- 
puric  acids,  until  there  is  no  longer  any  odor  of  hydrocyanic 
acid,  and  then  precipitate  with  hydrogen  sulphide  (§  119,  3). 

d.  ALL  THE  METALS  OF  THE  FIFTH   GROUP  FROM  EACH 
OTHER. 

Mix  the  dilute  solution  with  sodium  carbonate,  then  with  150 
potassium  cyanide  in  excess,  digest  some  time  at  a  gentle  heat, 
and  filter,,  On  the  filter  you  have  lead  carbonate  and  bismuth 
carbonate  (containing  alkali);  separate  the  two  metals  by  a 
suitable  method.  Add  to  the  filtrate  dilute  nitric  acid  in 
excess,  warm  gently  till  the  cuprous  sulphocyanate  first  pre- 
cipitated with  the  silver  cyanide  has  redissolved,  and  filter 
off  the  undissolved  silver  salt,  wThich  is  to  be  determined  as 
directed  §  115,  3.  Neutralize  the  filtrate  with  sodium  car- 
bonate, add  potassium  cyanide,  and  pass  hydrogen  sulphide  in 
excess.  Add  now  some  more  potassium  cyanide,  to  redissolve 
the  copper  sulphide  which  may  have  fallen  down,  and  filter 
the  fluid,  which  contains  the  whole  of  the  copper,  from  the 
precipitated  sulphides  of  mercury  and  cadmium.  Determine 
the  copper  as  directed  in  c,  and  separate  the  mercury  and  cad- 
mium as  in  145  or  158. 


692  SEPARATION.  [§  163e 

4.  Methods  based  on  tfie  Formation  and  Separation 
of  insoluble  Basic  Salts. 

a.  BISMUTH  FROM  COPPER,  CADMIUM,  AND  MERCURICUM  (also 
from  the  basic  radicals  of  tlie  first  four  groups,  with  the  excep- 
tion of  ferric  iron). 

Precipitate  the  bismuth  as  basic  chloride  according  to  §  120,  151 
4,  and  throw  down  the  copper,  &c.,  in  the  nitrate  by  hydro- 
gen sulphide.     Results  thoroughly  satisfactory  (II.  ROSE*). 

b.  BISMUTH  FROM  LEAD  AND  CADMIUM. 

Separate  the  bismuth  according  to  §  120,  1,  <?,  as  basic  152 
nitrate,  and  precipitate  the  lead  and  cadmium  in  the  filtrate 
by  hydrogen  sulphide.     Results  very  satisfactory  ( J.  LowEf). 

c.  BISMUTH  AND  COPPER  FROM  LEAD  AND  CADMIUM. 
Separate  the  bismuth  after  §  120,  1,  c,  as  basic  nitrate,  then 

heat  the  dish  on  the  water-bath  till  the  normal  copper  nitrate 
is  completely  converted  into  bluish-green  basic  salt  and  no 
blue  solution  is  produced  on  addition  of  water.  Allow  to  cool, 
treat  with  an  aqueous  solution  of  ammonium  nitrate  (1  in  500), 
filter,  wash  with  the  same  solution,  and  separate  in  the  solution 
lead  from  cadmium;  in  the  residue  copper  from  bismuth. 
Results  very  satisfactory  (J.  LOWE,  loc.  cit.\ 

5.  Method  based  upon  the  Precipitation  of  some  of 
the  Metals  by  Ammonia  or  Ammonium  Carbonate. 

COPPER  FROM  BISMUTH. 

a.  Mix  the  (nitric  acid)  solution  with  ammonium  carbonate  153 
in  excess,  and  warm  gently.  The  bismuth  separates  as  car- 
bonate, whilst  the  copper  carbonate  is  redissolved  by  the  excess 
of  ammonium  carbonate.  As  the  precipitate,  however,  gen- 
erally retains  a  little  copper,  it  is  necessary  to  redissolve  it, 
after  washing,  in  nitric  acid,  and  precipitate  again  with  ammo- 
nium carbonate ;  the  same  operation  must  be  repeated  a  third  - 
time  if  required.  Some  solution  of  ammonium  carbonate  may 
be  added  to  the  water  used  for  washing.  Apply  heat  to  the 
:filtrate  that  the  ammonium  carbonate  may  volatilize,  acidify 
cautiously  with  hydrochloric  acid,  and  determine  the  copper 
as  cuprous  sulphide  (§  119,  3).  The  oxide  of  bismuth  thus 

*  Pogg.  Annal.,  ex,  430.  \  Journ.f.  prakt.  Chem.,  LXXIV,  345. 


§163.]  BASES    OF   GROUP    V.  693 

obtained  is  quite  copper-free,  Jbut  a  little  bismuth  passes  into 
the  copper  solution,  hence  the  separation  does  not  give  such 
exact  results  as  that  in  114  (H,  ROSE*). 

p.  Add  some  ammonium  chloride  to  the  solution  and 
drop  the  latter  gradually  into  dilute  ammonia.  The  bismuth 
is  hereby  precipitated  as  bismuth  oxychloride,  while  the 
copper  remains  in  solution  as  an  ammoniacal  double  salt 
(BERZELIUS).  Wash  the  precipitated  bismuth  salt  with  dilute 
ammonia,  dissolve  it  in  dilute  nitric  acid,  and  determine 
according  to  §  120.  The  copper  is  determined  in  the  ammo- 
niacal solution.  In  this  method,  also,  it  is  advisable  to  pre- 
cipitate twice,  as  in  a. 

b.  COPPEK  FKOM  CADMIUM. 

Add  an  excess  of  ammonium  carbonate.  Cadmium  car-  154 
bonate  precipitates,  while  copper  remains  in  solution  with 
some  cadmium.  On  exposure  to  air,  now,  the  dissolved  cad- 
mium precipitates,  while  the  copper  still  remains  in  solution 
(STROMEYER).  The  solution  is  to  be  treated  as  in  153. 
The  separation  is  more  convenient,  but  less  accurate  than  in 
149  or  159. 

c.  LEAD  AND   SILVER  CHLORIDES  may  be  separated  by  155 
•ammonia,   which   dissolves  the  latter,   but  not  the  former. 
Care  must  be  taken  that  the  silver  chloride  be  freshly  pre- 
cipitated with  exclusion  of  light.    From  the  ammoniacal  solu- 
tion precipitate  the  silver  by  nitric  acid.     It  is  necessary  to 
test    the    filtrate   from   the   silver    chloride    with    hydrogen 
sulphide  to  ascertain  whether    any  weighable  quantities   of 
silver  have  been  retained  in  solution  by  the  agency  of  the 
ammonium  salts. 

6.   Method  based  on  the  Precipitation  of  the  Copper 
as  Cuprous  Sulpliocyanate. 

COPPER  FROM  CADMIUM  (and  the  metals  of  Groups  I. -IV., 
comp.  132). 

Pecipitate  the  copper  according  to  §  119,  3,  5,  as  cuprous  156 
sulphocyanate  (RivoT),  and  the  cadmium  from  the  filtrate  as 

*Pogg.  A  rmal.,  ex,  430. 


694  SEPARATION.  [§  163. 

sulphide.     Results  good  (H.  ROSE).     Palladium  may  also  be 
separated  from  copper  in  this  way  (WOHLER  *). 

7.  Method  ~based  upon  the  different  deportment  of 
the  Chromates. 

BISMUTH  FROM  CADMIUM. 

Precipitate  the  bismuth  as  directed  §  120,  2.     The  filtrate  157 
contains  the  whole  of  the  cadmium.     Concentrate  by  evapora- 
tion, and  then  precipitate  the  cadmium  by  the  cautious  addi- 
tion of  sodium  carbonate,  as  directed  §121,  1,  a  (J.  L6wE,f 
W.  PEARSON  :f).     The  results  given  are  satisfactory 

8.  Method  based  upon  the  different  deportment  of 
the  Sulphides  with  Acids. 

a.  MERCURICUM  FROM  SILVER,  BISMUTH,  COPPER,  CADMIUM, 
AND  (but  less  well)  FROM  LEAD. 

Boil  the  thoroughly  washed  precipitated  sulphides  with  158 
perfectly  pure  moderately  dilute  nitric  acid.  The  mercuric 
sulphide  is  left  undissolved,  the  other  sulphides  are  dissolved. 
No  chlorine  may  be  present,  and  it  is  necessary  that  the  mer- 
curic sulphide  should  be  pure,  that  is,  free  from  finely  divided 
mercury,  which,  as  is  well  known,  is  precipitated  when  mer- 
curous  salts  are  treated  with  hydrogen  sulphide.  G.  v.  RATH  § 
employed  this  method,  which  is 'so  universally  used  in  qualita- 
tive analysis,  with  perfect  success  for  the  separation  of  mer- 
cury from  bismuth. 

b.  COPPER  FROM  CADMIUM. 

Boil  the  well- washed  precipitate  of  the  sulphides  with  159 
dilute  sulphuric  acid  (1  part  concentrated  acid  and  5  parts 
water),  and,  after  some  time,  filter  the  undissolved  copper  sul- 
phide, to  be  determined  according  to  §  119,  3,  from  the  solu 
tion  containing  the  whole  of  the  cadmium  (A.W.HoFMANN  ]) 

9.  Methods  based  upon  the  Volatility  of  some  of  itW 
Metals,  Oxides,  Chlorides,  or  Sulphides  at  a  high  '-  om 
perature. 

a.  MERCURY  FROM  SILVER,  LEAD,  COPPER  (in  geneid  ii-om 
the  metals  forming  non-volatile  chlorides). 

*  Annal.  d.  Chem.  u.  Pharm  ,  CXL,   144  ;  Zeitschr.  f.  analy ,    0/wm     v.  403. 

\Journ.f.prakt.  Chem.,  i.vir,  469.  \Phil.  Mag.    -a    sOi. 

§Pogg.  Annal. ,  xcvi,  322. 

|  Annal.  d.  Chem.  u.  Pharm.,  cxv,  286. 


163.] 


BASES    OF   GROUP   V. 


695 


Precipitate  with  hydrogen  sulphide,  collect  the  precipi-  160 
tated  sulphides  on  a  weighed  filter,  dry  at  100°,  weigh,  and 
mix  uniformly.      Introduce  an  aliquot  part  into  the  bulb  D 


Fig.  116. 

(Fig.  110),  pass  a  slow  stream  of  chlorine  gas,  and  apply  a 
gentle  heat  to  the  bulb,  increasing  this  gradually  to  faint 
redness.     The  excess  of  chlorine  escaping  from  E  during 
the  operation  may  be  conducted  into  a  flue  or  into  a  carboy 
containing  rnoist  slaked  lime  by  connecting  the  carboy  with 
G.     First  sulphur  chloride  distils  over,  which  decomposes 
with    the  water  in  E  and  F  \    then  the  mercuric  chloride 
formed  volatilizes,  condensing  partly  in  E,  partly  in  the  hind 
part  of  0.     Cut  off  that  part  of  the  tube,  rinse  the  sublimate 
with  water  into  E,  and  mix  the  contents  of  the  latter  with 
the  water  in  F.     Mix  the  solution  with  excess  of  ammonia, 
warm  gently  till  110  more  nitrogen  is  evolved,  acidify  with 
hydrochloric  acid,  and  then  determine  in  the  fluid  filtered 
from  the  sulphur,  which  may  still  remain  undissolved,  the 
mercury  as  directed  in  §118,  3.     If  the  residue  consists  of 
silver  chloride  alone,  or  lead  chloride  alone,  you  may  weigh 
it  at  once ;  but  if  it  contains  several  metals,  you  must  reduce 
the  chlorides  by  ignition  in  a  stream  of  hydrogen,  and  dis- 
solve the  reduced  metals  in  nitric  acid,   for  their   ulterior 
separation.     Bear  in  mind  that,  in  presence  of  lead,  the  sul- 
phides and  the  chlorides  must  be  heated  gently  in  the  chlo- 


696  SEPARATION.  [§  163. 

rine  and  hydrogen  respectively,  otherwise  some  lead  chloride 
might  volatilize. 

If  it  is  intended  to  determine  the  mercury  by  the  difference 
instead  of  directly,  the  apparatus  may  be  greatly  simplified, 
but  in  this  case  great  care  must  be  exercised  in  drying  the 
sulphides  at  100°.  It  is  hence  advisable  to  use  the  method 
only  when  the  mercury  contains  a  very  small  quantity  of 
another  metal.  Weigh  the  dried  precipitate  every  half -hour 
and  consider  the  lowest  weight  as  the  correct  one.  Then 
ignite  an  aliquot  part  of  the  precipitate  in  a  current  of  hydro- 
gen in  a  crucible  provided  with  a  perforated  cover,  or  in  a 
tube  with  a  porcelain  boat.  The  method  is  applicable  when 
only  one  other  metal  is  present  besides  mercury.  Calculate 
from  the  residue  in  the  crucible  or  boat  how  much  the  entire 
precipitate,  dried  at  100°,  would  have  yielded ;  then  calcu- 
late the  result  into  sulphide — in  which  form  the  substance 
was  contained  in  the  dried  precipitate — and  from  the  differ- 
ence find  the  mercury  sulphide.  On  ignition  in  hydrogen, 
silver  sulphide  yields  metallic  silver  and  cupric  and  cuprous 
sulphides.  If  lead  is  present  the  latter  method  is  inappli- 
cable, because  lead  sulphide  too  readily  loses  weight  in  a  cur- 
rent of  hydrogen  (§  83,  /), 

In  alloys  or  mixtures  of  oxides  the  mercury  may  usually 
be  determined  with  simplicity  from  the  loss  on  ignition  in  the 
air  or  in  hydrogen. 

b.  BISMUTH  FROM  SILVER,  LEAD,  AND  COPPER. 

The  separation  is  effected  exactly  in  the  same  way  as  that  161 
of  mercury  from  the  same  metals  (160).  The  method  is  more 
especially  convenient  for  the  separation  of  the  metals  in 
alloys.  Care  must  be  taken  not  to  heat  too  strongly,  as  other- 
wise lead  chloride  might  volatilize ;  nor  to  discontinue  the 
application  of  heat  too  soon,  as  otherwise  bismuth  would 
remain  in  the  residue.  AUG.  VOGEL  *  gives  360°  to  370°  as 
the  best  temperature.  Put  water  containing  hydrochloric 
acid  in  U- tubes,  which  serve  as  receivers  (Fig.  116),  and 
determine  the  bismuth  therein  according  to  §  120. 


*  Zeilschr.  f.  analyt.  Chem.,  xni,  61. 


§  163.]  KASES   OF   GKOUP    V.  697 

10.   Precipitation  of  one  Metal  in  the  Metallic 
State  l)y  another  or  the  lower  Oxide  of  another. 

a.  LEAD  FROM  BISMUTH. 

Precipitate  the  solution  with  ammonium  carbonate  (§  116,  162 
1,  a  and  §  120,  1,  #),  wash  the  precipitated  carbonates,  and 
dissolve  in  acetic  acid  in  a  flask  ;  place  a  weighed  rod  of  pure 
lead  in  the  solution  and  nearly  fill  up  with  water,  so  that  the 
rod  may  be  entirely  covered  by  the  fluid  ;  close  the  flask  and 
let  it  stand  for  about  12  hours,  with  occasional  shaking. 
Wash  the  precipitated  bismuth  off  from  the  lead  rod,  collect 
on  a  filter,  wrash,  and  dissolve  in  nitric  acid  ;  evaporate  the 
solution  and  determine  the  bismuth  as  directed  in  §  120.  De- 
termine the  lead  in  the  filtrate  as  directed  in  §  116.  Dry  the 
leaden  rod  and  weigh  ;  substract  the  loss  of  weight  which 
the  rod  has  suffered  in  the  process  from  the  amount  of  the 
lead  obtained  from  the  filtrate  (ULLGREN*).  PATER  A  f 
recommends  precipitating  from  dilute  nitric  solution,  wash- 
ing the  precipitated  bismuth  first  with  water,  then  with 
alcohol,  transferring  to  a  small  filter,  drying  and  weighing. 
If  it  is  feared  that  the  finely  divided  bismuth  has  undergone 
oxidation,  it  is  well  to  fuse  it  with  potassium  cyanide 
(§120,4). 

b.  CUPROUS  COPPER  FROM  CUPRIC  COPPER. 

Cuprous  copper  may  be  accurately  determined  in  the  163 
presence  of  cupric  copper  by  means  of  a  solution  of  silver 
nitrate.  The  action  of  this  solution  on  cuprous  oxide  was 
first  studied  by  H.  EOSE.^  According  to  HAMPE,§  who 
further  studied  the  subject  with  the  greatest  care,  the  action 
proceeds  as  follows,  sufficient  dilution  and  a  very  gentle  heat 
being  implied  : 


3Cu2O  +  4AgNO,  +  a?H,O   =   (4CuO*N,OB  +  3HSO)  + 
2CuM),  +  4Ag  +  (x  -  3)H,0. 

Add  to  the  mixture  of  the  very  finely  divided  oxides  200 
times  its  weight  of  water  and  an  excess  of  perfectly  pure  and 

*BERZELIUS'  Jahresber.,  xxi,  148.         }  ZeilscJir.  f.  analyt.  Clem.,  v,  226. 
\  Journ.f.  prakt.  Chem.,  LXXI,  412.     §  Zeitschr.f.  analyt.  Chem.,  xin,  207. 


698  SEPARATION.  [§  163. 

neutral  silver  nitrate,  heat  to  40°,  allow  to  stand  for  three 
days,  filter,  wash,  dissolve  in  nitric  acid,  and  determine  the 
silver  as  a  chloride.  Every  4  eq.  of  silver  found  represent 
6  eq.  of  cuprous  copper  present.  Now  determine  the  total 
copper  in  a  second  portion  of  the  substance,  and  thus  ascer- 
tain the  cupric  copper  by  the  difference. 

11.   Separation  of  Silver  by  Cupellation. 

CUPELLATION  was  formerly  the  universal  method  of  deter-  164 
mining  SILVER  in  alloys  with  COPPEK,  LEAD,  etc.  The  alloy  is 
fused  with  a  sufficient  quantity  of  pure  lead  to  give  to  1  part 
of  silver  16  to  20  parts  of  lead,  and  the  fused  mass  is  heated, 
in  a  muffle,  in  a  small  cupel  made  of  compressed  bone-ash. 
Lead  and  copper  are  oxidized,  and  the  oxides  absorbed  by  the 
cupel,  the  silver  being  left  behind  in  a  state  of  purity.  One 
part  by  weight  of  the  cupel  absorbs  the  oxide  of  about  2  parts 
of  lead;  the  quantity  of  the  sample  to  be  used  in  the  experi- 
ment may  be  estimated  accordingly.  This  method  is  only 
rarely  employed  in  laboratories ;  *  I  have  given  it  a  place 
here,  however,  because  it  is  one  of  the  safest  processes  to 
determine  very  small  quantities  of  silver  in  alloys,  f  Regard- 
tile  details  of  the  method,  I  refer  to  the  Special  Part, 
"Determination  of  Silver  in  Galena." 

12.  Methods  depending  on  the  Volumetric  Estima- 
tion of  one  3£etal. 

a.  COPPEK  OF  CUPKOUS  COMPOUNDS  IN  PKESENCE  OF  CUPKIC 
COMPOUNDS.:]: 

Dissolve  the  substance,  if  necessary  in  a  current  of  car-  165 
bonic  acid,  in  hydrochloric  acid,  and  determine  the  cupric 
chloride  by  means  of  stannous  chloride  as  in  §  119,  4,  d\  in 
a  second  portion  of  the  substance  determine  the  total  copper 

*For  details  of  this  process  consult  Bodemann  and  KerVs  Assaying, 
translated  by  GOODYEAR  ;  or  Notes  on  Assaying,  by  P.  D'B  RICKETTS. 

f  Compare  MALAGUTI  and  DUROCHER,  Comp.  rend.,  xxix,  689  ;  DINGLERS 
cxv,  276.  Also  W.  HAMPE,  Zeitschr.  /.  analyt.  Chem.,  xi,  221. 

|  The  method  of  COMMAILLE  (Comp.  rend.,  LVI,  309)  can  no  longer  be 
relied  upon,  since  STAS  (  Untersucliungen  uber  die  Geselze  der  chemisclien  Propor- 
tioned, von  J.  S.  STAS,  translated  by  ARONSTETN,  Leipzic,  1867,  p.  36)  has 
shown  that  the  finely  divided  silver  thrown  down  by  ammoniacal  solution  of 
cuprous  chloride  dissolves  largely  in  ammonia  with  access  of  air. 


§  164.]  METALS   OF   GROUP   VI.  699 

according  to  one  of  the  methods  given  in  §  119.  It  is  of 
course  evident,  from  the  statements  on  page  380,  that  cu- 
prous copper  may  be  determined  in  the  presence  of  cupric 
copper  by  means  of  ferric  chloride. 

5.   SILVER  IN  PRESENCE  OF  LEAD  AND  COPPER. 

Small  quantities  of  silver  may  be  estimated  by  PISANI'S 
method,  §  115,  II. 

Sixth  Group. 

GOLD — PLATINUM — TIN — ANTIMONY — (ANTIMONIC    ACLT>) — ARSEN- 
OUS    ACID — AKSENIC    ACID. 

L  SEPAKATION  OF  THE  METALS  OF  THE  SIXTH  GROUP  FROM 

THOSE   OF   THE    FIRST   FlVE    GROUPS. 

§164. 

INDEX.     (The  numbers  refer  to  those  in  the  margin.) 

Gold  from  the  metals  of  Groups  III. — I.,  166,  171. 
Group  IV.,  166,  169,  171. 
"          silver,  169,  188. 
"         mercury,  169,  182. 
"         lead,  169,  194. 
"         copper,  169,  171. 
«•         bismuth,  169,  171,  194. 
"          cadmium,  169,  171. 

Platinum  from  the  metals  of  Groups  I. — III. ,  166,  172. 

Group  IV.,  166,  170,  172. 
«  silver,  170,  188. 

"  mercury,  170,  172. 

lead,  170. 

«•  copper,  170,  172. 

"  bismuth,  170,  172. 

"  cadmium,  170,  172. 

Tin  from  the  metals  of  Groups  I.  and  II.,  166,  175,  181. 
"  "  Group  III.,  166,  175. 

"  zinc,  166,  168,  173,  175. 

••  manganese,  166,  168,  175. 

nickel  and  cobalt,  166,  168,  173,  175,  180. 
"  iron,  166,  168. 

silver,  167,  168,  173,  180. 
«  mercury,  167,  168,  173. 

"          lead,  167,  168,  173.  180. 
"  copper,  107,  168,  178,  175,  180. 

"  bismuth,  167,  168. 

"  cadmium,  167,  168,  173,  175. 


700  SEPARATION.  [§  164. 

Antimony  from  the  metals  of  Groups  I.  and  II.,  166,  178. 
"  "  Group  III.,  166. 

zinc,  166,  168,  174. 
"          manganese,  166,  168. 

nickel  and  cobalt,  166,  168,  174,  179,  180. 
iron,  166.  168,  178. 
silver,  167,  168,  174,  180. 
mercury,  167,  168,  174,  176,  189. 

lead,  167,  168,  174,  180,  191.  t 

copper,  167,  168,  174,  178,  180,  192. 
bismuth,  167,  168. 
cadmium,  167,  168,  174. 
Arsenic  from  the  metals  of  Group  I.,  166,  178,  184,  186,  187. 

"  "  II., 166,  177,  178,  184,  186,  187,  190. 

III.,  166,  185,  186. 

zinc,  166,  168,  177,  183,  184,  186, 187. 
manganese,  166,  168,  177,  183,  185,  186,  187. 
nickel  and  cobalt,  166,  168,  177,  179,  180,  183,  184,  185, 

186,  187. 

iron,  166,  168,  177,  178,  183,  185,  186. 
silver,  167,  168,177,  180,  186. 
mercury,  167,  168,  186,  189. 
lead,  167,  168,  177,  180,  183,  184,  186,  190. 
copper,  167,  168,  177,  178,  180,  183,  184,  185,  186,  192, 

193. 

bismuth,  167,  168,  177,  186. 
cadmium,  167,  168,  177,  184,  185,  186. 

A.    General  Methods. 

1 .  Method  based  upon  the  Precipitation  of  Metals 
of  the  Sixth  Group  from  Acid  Solutions  ~by  Hydro- 
gen Sulphide. 

ALL  METALS  or  THE  SIXTH  GROUP  FROM  THOSE  OF  THE 
FIRST  FOUR  GROUPS. 

Conduct   into   the   acid*   solution  hydrogen   sulphide  in   166 
excess,  and  filter  off  the  precipitated  sulphides  (correspond- 
ing to  the  oxides  of  the  sixth  group). 

The  points  mentioned  127,  a,  f3,  and  y,  must  also  be 
attended  to  here.  As  regards  7,  antimony  and  tin  are  to  be 
inserted  between  cadmium  and  mercury,  in  the  order  of 
metals  there  given.  With  respect  to  the  particular  conditions- 
required  to  secure  the  proper  precipitation  of  certain  metals 
of  the  sixth  group,  I  refer  to  Section  IY.  I  have  to  remark 
in  addition : 

*  Hydrochloric  acid  answers  best  as  acidifying  agent. 


§  164.]  METALS   OF   GKOUP  VI.  701 

a.  That  hydrogen  sulphide  fails  to  separate  arsenic  acid 
from  .zinc,  as,  even  in  presence  of  a  large  excess  of  acid, 
the  whole  or  at  least  a  portion  of  the  zinc  precipitates  with 
the  arsenic  (WOHLER).  To  secure  the  separation  of  the  two 
bodies  in  a  solution,  the  arsenic  acid  must  first  be  converted 
into  arsenous  acid,  by  heating  with  sulphurous  acid,  before 
the  hydrogen  sulphide  is  conducted  into  the  fluid. 

ft.  That  in  presence  of  antimony,  tartaric  acid  should  be 
added,  as  otherwise  the  sulphide  of  antimony  will  contain 
chloride  ;  and  that  sulphide  of  antimony,  when  thrown  down 
from  a  boiling  solution  by  hydrogen  sulphide,  becomes  black 
after  a  time,  and  so  dense  that  it  is  deposited  like  sand, 
whereby  the  filtration  and  washing  are  much  facilitated  (S.  P. 

SCHAFELER  *). 

2.  Method  based  upon  the  Solubility  of  the  Sulphides 
of  the  Metals  of  the  Sixth  Group  in  Sulphides  of  the 
Alkali  Metals. 

a.  THE  METALS  OF  GROUP  VI.  (with  the  exception  of  Gold 
and  Platinum)  FROM  THOSE  OF  GROUP  V. 

Precipitate  the  acid  solution  with  hydrogen  sulphide,  pay-  167 
ing  due  attention  to  the  directions  given  in  Section  IV.  under 
the  heads  of  the  several  metals,  and  also  to  the  remarks  in 
166.  The  precipitate  consists  of  the  sulphides  of  the  metals 
of  Groups  V.  and  VI.  Wash,  and  treat  at  once  with  yellow 
ammonium  sulphide  in  excess.  (It  is  usually  best  to  spread 
out  the  filter  in  a  porcelain  dish,  add  the  ammonium  sulphide, 
•cover  writh  a  large  watch-glass,  and  place  on  a  heated  water- 
bath.  Unnecessary  exposure  to  air  should  be  avoided.)  Add 
some  water,  filter  off  the  clear  fluid,  treat  the  residue  again 
with  ammonium  sulphide,  digest  a  short  time,  repeat  the  same 
operation,  if  necessary,  a  third  and  fourth  time,  filter,  and 
wash  the  residuary  sulpliides  of  Group  V.  with  water  contain- 
ing ammonium  sulphide.  If  stannous  sulphide  is  present, 
some  flowers  of  sulphur  must  be  added  to  the  ammonium  sul- 
phide, unless  the  latter  be  very  yellow.  In  presence  of  copper, 

*  BericJite  der  deutschen  chem.  Gesellsch.,  1871,  279.   I  have  myself  confirmed 
these  observations. 


702  SEPARATION.  [§  164 

the  sulphide  of  which  is  a  little  soluble  in  ammonium  sulphide, 
sodium  sulphide  should  be  used  instead.  However,  this  sub- 
stitution can  be  made  only  in  the  absence  of  mercury,  since 
the  sulphide  of  that  metal  is  soluble  in  sodium  sulphide. 

Add  to  the  alkaline  nitrate,  gradually,  hydrochloric  acid  in 
small  portions,  until  the  acid  predominates  ;  allow  to  subside, 
and  then  filter  off  the  sulphides  of  the  metals  of  the  sixth 
group,  which  are  mixed  with  sulphur. 

SCHNEIDER*  states  that  he  was  unsuccessful  in  effecting 
complete  separation  of  bismuth  disulphide.and  stannic  sulphide 
by  digestion  with  potassium  sulphide,  but  did  succeed  by  con- 
ducting hydrogen  sulphide  into  the  potassium-hydroxide  solu- 
tion of  bismuthic  tartrate  and  stannous  oxide  (which  decom- 
pose into  bismuthous  oxide  and  stannic  oxide). 

If  a  solution  contains  much  arsenic  acid  in  presence  of 
small  quantities  of  copper,  bismuth,  &c.,  it  is  convenient  to 
precipitate  these  metals  (together  with  a  very  small  amount  of 
arsenous  sulphide)  by  a  brief  treatment  with  hydrogen  sul- 
phide. Filter,  extract  the  precipitate  with  ammonium  sulphide 
(or  potassium  sulphide),  acidify  the  solution  obtained,  mix  it 
with  the  former  filtrate  containing  the  principal  quantity  of 
the  arsenic,  and  proceed  to  treat  further  with  hydrogen  sul- 
phide (§  127,  4,  J). 

~b.  THE  METALS  OF  GROUP  VI.  (with  the  exception  of  Gold 
and  Platinum)  FROM  THOSE  OF  GROUPS  IY.  AND  Y. 

a.  Neutralize  the  solution  with  ammonia,  add  ammonium  168 
chloride,  if  necessary,  and  then  yellow  ammonium  sulphide  in 
excess ;  digest  in  a  closed  flask,  for  some  time  at  a  moderate 
heat,  and  then  proceed  as  in  167.  Kepeated  digestion  with 
fresh  quantities  of  ammonium  sulphide  is  indispensable.  On 
the  filter,  you  have  the  sulphides  of  the  metals  of  Groups  TV 
and  V.  Wash  with  water  containing  ammonium  sulphide. 
In  presence  of  nickel,  this  method  offers  peculiar  difficul- 
ties ;  traces  of  mercuric  sulphide,  too,  are  liable  to  pass  into 
the  filtrate.  In  presence  of  copper  (and  absence  of  mer- 
cury), soda  and  sodium  sulphide  are  substituted  for  ammonia 
and  ammonium  sulphide. f 

*  Annal.  d.  Chem,  u.  Pharm.,  ci.  64. 

t  The  accuracy  of  this  method  has  been  called  in  question  by  BLOXAM  (Quart. 
Journ.  Chem.  Soc.,  v,  119).     That  chemist  found  that  ammonium  sulphide  fails 


§  164.]  METALS    OF   GROUP   VI.  703 

ft.  In  the  analysis  of  solid  compounds  (oxides  or  salts),  it 
is  in  most  cases  preferable  to  fuse  the  substance  with  3  parts 
of  dry  sodium  carbonate  and  3  of  sulphur,  in  a  covered  porce- 
lain crucible.  When  the  contents  are  completely  fused,  and 
the  excess  of  sulphur  is  volatilized,  the  mass  is  allowed  to  cool, 
and  then  treated  with  water,  which  dissolves  the  sulphosalts 
of  the  metals  of  the  sixth  group,  leaving  the  sulphides  of 
Groups  IV.  and  Y.  undissolved.  By  this  means,  even  ignited 
stannic  oxide  may  be  readily  tested  for  iron,  &c.,  and  the 
amount  of  the  admixture  determined  (H.  ROSE).  The  solu- 
tion of-  the  sulphosalts  is  treated  as  in  167.  In  the  presence 
of  copper,  traces  of  the  sulphide  may  be  dissolved  with  the 
sulphides  of  Group  YI.  Occasionally  a  little  ferrous  sulphide 
dissolves,  coloring  the  solution  green.  In  that  case  add  some 
ammonium  chloride,  and  digest  till  the  solution  has  turned 
yellow.  Instead  of  the  mixture  of  sodium  carbonate  and  sul- 
phur you.  may  also  use  already  prepared  hepar  sulphuris,  or, 
as  FROHDE*  says,  you  may  fuse  the  substance  with  4  or  5  parts 
of  sodium  thiosulphate. 

E.  Special  Methods. 

1.  Methods  based  upon  the  Insolubility  of  some 
Metals  of  the  Sixth  Group  in  Acids. 

a.  GOLD  FROM  METALS  OF  GROUPS  IY.  AND  Y.  IN  ALLOYS. 

a.  Boil  the  alloy  with  pure  nitric  acid  (not  too  concen-  169 
.trated),  or,  according  to  circumstances,  with  hydrochloric  acid. 
The  other  metals  dissolve,  the  gold  is  left.  The  alloy  must 
be  reduced  to  filings,  or  rolled  out  into  a  thin  sheet.  If  the 
alloy  were  treated  with  concentrated  nitric  acid,  and  at  a  tem- 
perature below  boiling,  a  little  gold  might  dissolve  in  conse- 
quence of  the  co-operation  of  nitrous  acid.  In  the  presence 
of  silver  and  lead,  this  method  is  only  applicable  when  they 

to  separate  small  quantities  of  stannic  sulphide  from  large  quantities  of  mercuric 
sulphide  or  cadmium  sulphide  (1  :  100);  and  thai  more  especially  the  separation 
of  copper  from  tin  and  antimony  (also  from  arsenic)  by  this  method  is  a  failure, 
as  nearly  the  whole  of  the  tin  remains  with  the  copper.  The  latter  statement  I 
cannot  confirm,  for  Mr.  Lucius,  in  my  laboratory,  has  succeeded  in  separating 
copper  fFom  tin  by  means  of  yellowish  sodium  sulphide  completely;  but  it  is 
indispensable  to  digest  three  or  four  times  with  sufficiently  large  quantities  of 
the  solvent,  as  stated  in  the  text. 

*  Zeilschr.f.  analyt.  C/tem.,  v,  405. 


704  SEPARATION.  [§  164 

amount  to  more  than  80 -per  cent.,  since  otherwise  they  are 
not  completely  dissolved.  Alloys  of  silver  and  gold  contain- 
ing less  than  80  per  cent,  of  silver  are  therefore  fused  with  3 
parts  of  lead,  before  they  are  treated  with  nitric  acid.  The 
residuary  gold  is  weighed ;  but  its  purity  must  be  ascertained, 
by  dissolving  in  cold  dilute  nitrohydrochloric  acid,  not  in  con- 
centrated hot  acid,  as  silver  chloride  also  is  soluble  in  the  latter. 
In  the  presence  of  silver,  a  small  quantity  of  its  chloride  is 
usually  obtained  here.  If  it  can  be  weighed,  it  should  be 
reduced  and  deducted. 

At  the  Mint  Conference  held  at  Vienna  in  1857,  the  fol- 
lowing process  was  agreed  upon  for  the  mints  in  the  several 
states  of  Germany.  Add  to  1  part  of  gold,  supposed  to  be 
present,  2-J  parts  of  pure  silver ;  wrap  both  the  alloy  and  the 
silver  in  a  paper  together,  and  introduce  into  a  cupel  in  which 
the  requisite  amount  of  lead  is  just  fusing.*  After  the  lead 
has  been  absorbed, f  the  button  is  flattened  by  hammering  or 
rolling,  then  ignited  and  rolled.  The  rolls  are  treated  first 
with  nitric  acid  of  1*2  sp.  gr.,  afterwards  with  nitric  acid  of 
1*3  sp.  gr.,  rinsed,  ignited,  and  weighed.^  Even  after  boiling 
again  with  nitric  a"cid  of  1*3  sp.  gr.,  they  retain  0-75  to  O'OOl 
of  silver  which  will  remain  as  chloride  if  the  rolls  are  treated 
with  cold  dilute  aqua  regia  (H.  ROSSLEK,  loc.  cit.). 

(3.  Heat  the  alloy  (previously  filed  or  rolled)  in  a  capacious 
platinum  dish  with  a  mixture  of  2  parts  pure  concentrated 
sulphuric  acid  and  1  part  water,  until  the  evolution  of  gas  has 
ceased  and  the  sulphuric  acid  begins  to  volatilize  ;  or  fuse  the 
alloy  with  potassium  disulphate  (H.  ROSE).  Separate  the  gold 
from  the  sulphates  of  the  other  metals,  by  treating  the  mass 
with  water  which  should  finally  be  boiling.  It  is  advisable  to 
repeat  the  operation  with  the  separated  gold,  and  ultimately 

*If  the  weighed  sample,  say  0'25  grm.,  contains  98-92$  gold,  3  grm.  of  lead 
are  required;  if  92-87'5,  4  grm.;  if  87*5-75,  5  grm.;  if  75-60,  6  grm.;  if  60-35, 
7  grm. ;  if  less  than  35,  8  grm. 

f  A  small  quantity  of  gold— from  one  to  three  thousandths — is  always  lost 
in  cupellation.  The  loss  increases  .with  the  amount  of  lead,  and  is  also  depend- 
ent on  the  proportion  of  silver  to  gold.  The  more  silver  present  the  less  is  the 
loss  of  gold.  In  large  buttons  the  loss  is  less  than  in  small  ones  (H.  ROSSLEK, 
Ding,  poly  1.  Journ  ,  ccvi,  185;  Zeitschr.  f.  analyt.  Chem.,  xin/87). 

t  Kunsl-  und  Gewerbeblatt  f.  Baiern,  1857,  151;  Chem.  CentralbL,  1857,  307; 
Polyt.  CentralbL,  1857,  1151,  1471,  1639. 


§  164.]  METALS   OF   GKOTJP   VI.  706 

test  the  purity  of  the  latter.     In  presence  of  lead  this  method 
is  not  good. 

y.  The  methods  given  in  a.  and  fi  may  be  united,  i.e.,  the 
cupelled  and  thinly-rolled  metal  may  be  first  warmed  with 
nitric  acid  of  1*2  sp.  gr.,  then  thoroughly  washed,  the  gold 
boiled  5  minutes  with  concentrated  sulphuric  acid,  washed 
again,  and  ignited  (MASCAZZINI,  BUGATTI). 

I).  PLATINUM   FROM   METALS   OF    GROUPS   IY.    AND  Y.  IN 
ALLOYS. 

The  separation  is  effected  by  heating  the  alloy  in  filings  170 
or  foil  with  pure  concentrated  sulphuric  acid,  with  addition  of 
a  little  water,  or  by  fusing  with  potassium  disulphate  (169,  ft); 
but  not  with  nitric  acid,  as  platinum  in  alloys  will,  under  cer- 
tain circumstances,  dissolve  in  that  acid. 

2.  Method  based  upon  the  Separation  of  Gold  in 
the  metallic  state. 

GOLD  FROM  ALL  METALS  OF  GROUPS  I. — Y.,  with  the  excep- 
tion of  LEAD,  MERCURY,  AND  SILVER. 

Precipitate  the  hydrochloric  acid  solution  with  oxalic  acid  171 
as  directed  §  123  J,  y,  or  with  ferrous  sulphate,  §  123,  £»,  a, 
.and  filter  off  the  gold  when  it  has  completely  separated.  Take 
•care  to  add  a  sufficient  quantity  of  hydrochloric  acid  after  the 
reduction  to  insure  solution  of  any  oxalates.  In  the  presence 
of  copper  the  addition  of  hydrochloric  acid  does  not  suffice, 
since  the  coprecipitated  cupric  oxalate  will  dissolve  with  diffi- 
culty in  this  acid.  E.  PURGOTTI*  recommends  in  this  case, 
after  precipitation,  adding  potash  cautiously  to  the  boiling  hot 
fluid  till  it  is  neutral,  and  then  if  necessary  some  normal 
potassium  oxalate.  Double  oxalate  of  copper  and  potash  will 
be  formed  which  dissolves  with  a  blue  color.  The  gold  after 
washing  will  now  be  pure. 

3.  Method  based  upon  the   Precipitation  of  Pla- 
ti until'  <(*  Potassium  Platinic,  or  Ammonium  Platinic 
Chi  <»>  id'. 

PLATINUM  FROM  THE  METALS  OF  GROUPS  IY.  AND  V., 
with  the  exception  of  MERCURY  IN  MERCUROUS  COMPOUNDS, 
LEAD,  AND  SILVER. 

Precipitate    the    platinum    with    potassium    chloride    or  172 

*  Zeitschr.  f.  analyL  Chem.,  ix,  128. 


706  SEPARATION.  [§  164. 

ammonium  chloride  as  directed  §  124,  and  wash  the  precipi- 
tate thoroughly  with  alcohol.  The  platinum  prepared  from 
the  precipitated  ammonium- or  potassium  salt  is  to  be  tested 
after  being  weighed,  to  see  whether  it  yields  any  metal 
(especially  iron)  to  fusing  potassium  disulphate. 

4.  Methods  based  upon  the  Separation  of  Oxides 
insoluble  in  Nitric  Acid. 

a.  TIN  FROM  METALS  OF  GROUPS  IY.  AND  Y.  (not  from 
Bismuth,  Iron,  or  Manganese*)  IN  ALLOYS. 

Treat  the  finely  divided  alloy,  or  the  metallic  powder  17$ 
obtained  by  reducing  the  oxides  in  a  stream  of  hydrogen  with 
nitric  acid,  as  directed  §  126,  1,  a.  The  filtrate  contains  the 
other  metals  as  nitrates.  As  stannic  oxide  is  liable  to  retain 
traces  of  copper  arid  lead  and  iron,  you  must,  in  an  accurate 
analysis,  test  an  aliquot  part  of  it  for  these  bodies,  and  determine 
their  amount  as  directed  168,  /?. 

BRUNNER  recommends  the  following  course  of  proceeding,, 
by  which  the  presence  of  copper  in  the  tin  may  be  effectually 
guarded  against.  Dissolve  the  alloy  in  a  mixture  of  1  part  of 
nitric  acid,  4  parts  of  hydrochloric  acid,  and  5  parts  of  water ; 
dilute  the  solution  largely  with  water,  and  heat  gently.  Add 
crystals  of  sodium  carbonate  until  a  distinct  precipitate  has- 
formed,  and  boil.  (In  presence  of  copper,  the  precipitate 
must,  in  this  operation,  change  from  its  original  bluish-green 
to  a  brown  or  black  tint.)  When  the  fluid  has  been  in  ebulli- 
tion some  10  or  15  minutes,  allow  it  to  cool,  and  then  add 
nitric  acid,  drop  by  drop,  until  the  reaction  is  distinctly  acid ; 
digest  ilie  precipitate  for  several  hours,  when  it  should  have 
acquired  a  pure  white  color.  The  stannic  oxide  thus  obtained 
is  free  from  copper ;  but  it  may  contain  some  iron,  which  can 
be  removed  as  directed  in  168,  (3. 

Before  the  stannic  oxide  can  be  considered  pure,  it  must 
be  tested  also  for  silicic  acid,  as  it  frequently  contains  traces  of 
this  substance.  To  this  end,  an  aliquot  part  is  fused  in  plati- 


*  If  the  alloy  of  tin  contains  bismuth  or  manganese,  there  remains  with  the 
stannic  oxide,  bismuth  trioxide  or  manganese  sesquioxide,  which  cannot  be 
extracted  by  nitric  acid;  if  it  contains  iron,  on  the  contrary,  some  stannic  oxide 
always  dissolves  with  the  iron,  and  cannot  be  separated  even  by  repeated  evapo- 
ration (H.  ROSE,  Pogg.  Annal.,  cxn,  1G9,  170,  172) 


§  164.]  METALS   OF  GKOUP   VI.  707 

num  with  3 — 4  parts  of  sodium  and  potassium  carbonate,  the 
fused  mass  boiled  with  water,  and  the  solution  filtered  ;  hydro- 
chloric acid  is  then  added  to  the  filtrate,  and,  should  silicic  acid 
separate,  the  fluid  is  filtered  off  from  this  substance.  The  tin 
is  then  precipitated  by  hydrogen  sulphide,  and  the  silicic  acid 
still  remaining  in  the  filtrate  is  determined  in  the  usual  way 
(§  140).  If  hydrochloric  acid  has  produced  a  precipitate  of 
silicic  acid,  the  last  filtration  is  effected  on  the  same  filter 
(KHITTEL*). 

1}.  ANTIMONY  FROM  THE  METALS  OF  GROUPS  IV.  AND  Y.  IN 
ALLOYS  (not  from  Bismuth,  Iron  and  Manganese). 

Proceed  as  in  173,  filter  off  the  precipitate,  and  convert  it  174 
by  ignition  into  antimony  tetroxide  according  to  §  125,  2. 
Results  only  approximate,  as  a  little  antimony  dissolves. 
Alloys  of  antimony  and  lead,  containing  the  former  metal  in 
excess,  should  be  previously  fused  with  a  weighed  quantity  of 
pure  lead  (YARRENTRAppf). 

5.  Methods  based  on  the  Precipitation  of  Tin  in 
Stannic  Salts  ~by  Normal  Salts  (e.g.,  Sodium  Sulphate) 
or  by  Sulphuric  Acid. 

TIN  FROM  THE  METALS  OF  GROUPS  I.,  II.,  Ill, ;  ALSO  FROM 
MANGANESE,  ZINC,  NICKEL  AND  COBALT,  COPPER,  CADMIUM 
(GOLD). 

Precipitate  the  hydrochloric  acid  solution,  which  must  175 
contain  the  tin  entirely  as  stannic  chloride,  according  to  §  126, 
1,  £»,  by  ammonium  nitrate  or  sodium  sulphate  (LOWENTHAL), 
or  by  sulphuric  acid,  which,  H.  ROSE  says,  answers  equally 
well.  Alloys  are  always  treated  as  follows  :  First,  oxidize  by 
digestion  with  nitric  acid  ;  when  no  more  action  takes  place, 
evaporate  the  greater  portion  of  the  nitric  acid  in  a  porcelain 
dish,  moisten  the  mass  with  strong  hydrochloric  acid,  and  after 
half  an  hour  add  water,  in  which  the  nietastannic  chloride  and 
the  other  chlorides  dissolve.  Alloys  of  tin  and  gold  are  dis- 
solved in  aqua  regia,  the  excess  of  acid  evaporated,  and  the 
solution  diluted  with  much  water,  before  precipitating  with 
sulphuric  acid. 

It  must  be  remembered  that  in  this  process  any  phosphoric 

*  Chem.  CentralbL,  1857,  929.         \  Dingier' s  polyt.  Journ.,  CLVIII,  316. 


708  SEPARATION.  [§  164. 

acid  that  may  be  present  is  precipitated  entirely  or  partially 
with  the  tin.  After  the  precipitate  has  been  well  washed  by 
decantation,  LOWENTHAL  recommends  to  boil  with  a  mixture 
of  1  part  nitric  acid  (sp.  gr.  1-2)  and  9  parts  water,  then  to 
transfer  to  the  -  filter,  and  wash  thoroughly.  Results  very 
satisfactory.  If  the  fluid  contains  a  ferric  salt,  a  portion  of 
the  iron  always  falls  down  with  the  tin.  Hence  the  stannic 
oxide  must  be  tested  for  iron  according  to  168,  /?,  which,  if 
present,  must  be  determined  and  deducted. 

6.  Method   based  on  the  Insolubility  of  Mercuric 
Sulphide  in  Hydrochloric  Add. 

MERCURY  FROM  ANTIMONY. 

Digest  the  precipitated  sulphides  with  moderately  strong  17(5 
hydrochloric  acid  in  a  distilling  apparatus.  The  sulphide  of 
antimony  dissolves,  while  the  mercuric  sulphide  remains 
behind.  Expel  all  the  hydrogen  sulphide,  then  add  tartaric 
acid,  dilute,  filter,  mix  the  filtrate  with  the  distillate  which 
contains  a  little  antimony,  and  precipitate  with  hydrogen 
sulphide.  The  mercuric  sulphide  may  be  weighed  as  such  (Fu. 
FIELD*). 

7.  Methods  based  upon  the  Conversion  of  Arsenic 
and  Antimony  into  Alkali  Ar senate  and  Antimonate. 

a.  ARSENIC  FROM  THE  METALS  OF  GROUPS  II. ,  IY.,  AND  Y. 

If  you  have  to  do  with  arsenites  or  arsenates,  fuse  with  3  177 
parts  of  sodium  and  potassium  carbonates  and  1  part  of  potas- 
sium nitrate ;  if  an  alloy  has  to  be  analyzed  it  is  fused  with  3 
parts  of  sodium  carbonate  and  3  parts  of  potassium  nitrate. 
In  either  case  the  residue  is  boiled  with  water,  and  the  solution, 
which  contains  the  arsenates  of  the  alkalies,  filtered  from  the 
undissolved  oxides  or  carbonates.  The  arsenic  acid  is  deter- 
mined in  the  filtrate  as  directed  §  127,  2.  If  the  quantity  of 
arsenic  is  only  small,  a  platinum  crucible  may  be  used,  other- 
wise a  porcelain  crucible  must  be  used,  as  platinum  would  be 
seriously  injured.  In  the  latter  case,  bear  in  mind  that  the 
fused  mass  is  contaminated  with  silicic  acid  and  alumina,  If 
the  alloy  contains  much  arsenic  a  small  quantity  may  be  readily 
lost  by  volatilization,  even  though  the  operation  be  cautiously 

*  Quart.  Journ.  Chem.  Soc.,  xu,  32. 


§164.]  METALS    OF   GROUP   VI.  709 

c<  ni ducted.  In  such  a  case,  therefore,  it  is  better  first  to  oxidize 
with  nitric  acid,  then  to  evaporate,  and  to  fuse  the  residue  as 
above  directed  with  sodium  carbonate  and  potassium  nitrate. 

b.  ARSENIC    AND    ANTIMONY    FROM    COPPER   AND    IRON, 
especially  in  ores  containing  sulphur. 

Diffuse  the  very  finely  pulverized  mineral  through  pure  178 
solution  of  potassa,  and  conduct  chlorine  into  the  fluid  (comp. 
p.  467).     The  iron  and  copper  separate  as  oxides,  the  solution 
contains   sulphate,    arsenate,    and    antimonate   of    potassium 
(RivoT,  BEUDANT,  and  DAGUIN*). 

c.  ARSENIC  AND  ANTIMONY  FROM  COBALT  AND  NICKEL. 
Dilute  the  nitric  acid  solution,  add  a  large  excess  of  potassa,  179 

heat  gently,  and  conduct  chlorine  into  the  fluid  until  the  pre- 
cipitate is  black.  The  solution  contains  the  whole  of  the 
arsenic  and  antimony,  the  precipitate  the  nickel  and  cobalt  as 
sesquioxides  (RivoT,  BEUDANT,  and  DAGUIN,  loc.  cit.) 

8.  Methods   based  upon   the  Volatility   of  certain 
Chlorides  or  Metals. 

a.  TIN,  ANTIMONY,  ARSENIC  FROM  COPPER,  SILVER,  LEAD, 
COBALT,  NICKEL. 

Treat  the  sulphides  with  a  stream  of  perfectly  dry  chlorine,  180 
proceeding  exactly  as  directed  in  160.  In  presence  of  anti- 
mony, fill  E  and  F  (Fig.  116)  with  a  solution  of  tartaric 
acid  in  water,  mixed  with  hydrochloric  acid.  The  metals  may 
be  also  separated  by  this  method  in  alloys.  The  alloy  must 
be  very  finely  divided.  Arsenical  alloys  are  only  very  slowly 
decomposed  in  this  way.  In  separating  arsenic  and  copper 
the  temperature  must  not  exceed  200°,  and  chlorine  water 
should  be  put  into  the  receiver  (PARNELLf).  If  tin  and  copper 
are  separated  in  this  manner,  according  to  the  experience  of 
H.  ROSE,;);  a  small  trace  of  tin  remains  with  the  copper  chloride. 

b.  STANNIC  OXIDE,  ANTLMONIOUS  OXIDE  (AND  ALSO 
ANTTMONIC  ACID).  ARSENOUS  AND  ARSENIC  ACIDS,  FROM 
ALKALIES  AND  ALKALINE  EARTHS. 

Mix  the  solid  compound  with  5  parts  of  pure  ammonium  181 
chloride  in  powder,  in  a  porcelain  crucible,  cover  this  with  a 

*  Compt.  rend.,  1853,  835;  Journ.  f.  prakt.  C7iem.,-Lxi,  133. 
f  Chem.  News,  xxi,  133.  j  Pogg.  Annal,  cxn,  169. 


710  SEPARATION.  [§  164. 

concave  platinum  lid,  on  which  some  ammonium  chloride  is 
sprinkled,  and  ignite  gently  until  all  ammonium  chloride  is 
driven  off ;  mix  the  contents  of  the  crucible  with  a  fresh  por- 
tion of  that  salt,  and  repeat  the  operation  until  the  weight 
remains  constant.  In  this  process,  the  chlorides  of  tin,  anti- 
mony, and  arsenic  escape,  leaving  the  chlorides  of  the  alkalies 
and  alkali-earth  metals.  The  decomposition  proceeds  most 
rapidly  with  alkali  salts.  With  regard  to  salts  of  alkali-earth 
metals  it  is  to  be  observed  that  those  which  contain  antimonic 
acid  or  stannic  oxide  are  generally  decomposed  completely  by 
a  double  ignition  with  ammonium  chloride  (magnesium  alone 
cannot  be  separated  perfectly  from  antimonic  acid  by  this 
method).  The  arsenates  of  the  alkali-earth  metals  are  the 
most  troublesome  to  decompose ;  barium,  stroiiium,  and  cal- 
cium salts  usually  require  to  be  subjected  5  times  to  the  opera- 
tion, before  they  are  free  from  arsenic,  and  magnesium  arsenate 
it  is  impossible  thoroughly  to  decompose  in  this  way  (H. 
ROSE*).  According  to  SALKowsKif  barium  arsenate  may  be 
converted  into  chloride  quite  free  from  arsenic  by  one  ignition 
with  ammonium  chloride ;  however  calcium  arsenate  was  found 
to  leave  a  residue  containing  arsenic  acid  even  after  six  igni- 
tions with  ammonium  chloride. 

c.  MERCURY  FROM  GOLD  (SILVER,  AND  GENERALLY  FROM 
THE  NON-VOLATILE  METALS). 

Heat  the  weighed  alloy  in  a  porcelain  crucible,  ignite  till  182 
the  weight  is  constant,  and  determine  the  mercury  from  the 
loss.  If  it  is  desired  to  estimate  it  directly,  the  apparatus 
(Fig.  88)  may  be  used.  In  cases  where  the  separation  of  mer- 
cury from  metals  that  oxidize  on  ignition  in  the  air  is  to  be 
effected  by  this  method,  the  operation  must  be  conducted  in 
an  atmosphere  of  hydrogen  (Fig.  83).  50). 

9.    Methods  based    on  the  Volatility  of  Arsenous 
Sulphide. 

ARSENIC  ACID  FROM  THE  OXIDES  OF  MANGANESE,  IRON, 
ZIMC,  COPPER,  NICKEL,  COBALT  (NOT  so  WELL  FROM  OXIDE  oir. 
LEAD,  AND  NOT  FROM  OXIDES  OF  SILVER,  ALUMINIUM,  OR  MAG- 
NESIUM). 

Mix  the  arsenic-acid  compound  (no  matter  whether  it  has  183 


1.  Annal.,  LXXIII   582;  LXXIV,  578;  cxu,  173. 
Journ.  f.  prakl.  Chem  ,  civ,  138. 


§  164.]  METALS   OF  GROUP   VI.  711 

been  air-dried  or  gently  ignited)  with  sulphur,  and  ignite 
under  a  good  draught  in  an  atmosphere  of  hydrogen  (Fig. 
83)  ;  the  perforated  lid  must  in  this  case  be  of  porcelain  ; 
platinum  would  not  answer).  The  whole  of  the  arsenic  vola- 
tilizes, the  sulphides  of  manganese,  iron,  zinc,  lead,  and  copper 
remain  behind  ;  they  may  be  weighed  directly.  After  weigh- 
ing, add  a  fresh  quantity  of  sulphur  to  the  residue,  ignite  as 
before,  and  weigh  again  ;  repeat  this  operation  until  the  weight 
remains  constant.  Usually,  if  the  compound  was  intimately 
mixed  with  the  sulphur,  the  conversion  of  the  arsenate  into 
sulphide  is  complete  after  the  first  ignition.  Results  very  good. 
In  separating  nickel  the  analyst  will  remember  that  the 
residue  cannot  be  weighed  directly,  since  it  does  not  possess  a 
constant  composition  ;  hence  the  ignition  in  hydrogen  may  be 
saved  ;  nickel  arsenate  loses  all  its  arsenic  on  being  simply 
mixed  with  sulphur  and  heated.  The  heat  should  be  moderate 
and  continued  till  no  more  red  sulphide  of  arsenic  is  visible 
on  the  inside  of  the  porcelain  crucible.  It  is  advisable  to  repeat 
the  operation.  The  separation  of  arsenic  from  cobalt  cannot 
be  completely  effected  in  this  manner  even  by  repeated  treat- 
ment with  sulphur,  but  it  can  be  effected  by  oxidizing  the  resi- 
due with  nitric  acid,  evaporating  to  dryness,  mixing  with  sul- 
phur, and  reigniting.  Smaltine  and  cobaltine  must  be  treated 
in  the  same  manner  (II.  ROSE*).  1  should  not  forget  to  men- 
tion that  Ei5ELMEN,f  a  long  while  ago,  noticed  the  separation 
of  arsenic  acid  from  sesquioxide  of  iron  by  ignition  in  a  stream 
of  hydrogen  sulphide. 

10.   Method  based  upon  the  Separation  of  Arsenic 
as  Mercurous  Arsenate. 

ARSENIC  ACID  FROM  ALKALIES,   ALKALI  EARTHS,  ZINC, 
COBALT,  NICKEL,  LEAD,  COPPER,  AND  CADMIUM. 

Proceed  exactly  as  in  separating  phosphoric  acid  by  mer-  184 
cury  (§  134,  5,  y).     The  arsenic  acid  can  not  be  determined 
in  the  insoluble  residue  as  is  done  with  phosphoric  acid.      If 
it  is  to  be  estimated  directly,  it  must  be  separated  from  the 


*  Zetischr.  /.  analyt.  Chem.,  i,  413. 

f  Annal.  de  Chim.  et  de  Phys.  (3),  xxv,  98. 


712  SEPARATION.  [§  164 

mercurous  mercury  by  one  of  the  methods  given  in  this  sec- 
tion.    Treat  the  filtrate  as  in  §  135,  k,  a  (H.  KOSE). 

1 1 .  Method  'based  upon  the  Separation  of  Arsenic  as 
Ammonium  Magnesium  Arsenate. 

ARSENIC  ACID  FROM  COPPER,  CADMIUM,  FERRIC  IRON,  MAN- 
GANESE, NICKEL,  COBALT,  ALUMINIUM. 

Mix  the  hydrochloric  acid  solution,  which  must  contain  18& 
the  whole  of  the  arsenic  in  the  form  of  arsenic  acid,  with 
enough  tartaric  acid  to  prevent  precipitation  by  ammonia,  pre- 
cipitate the  arsenic  acid  according  to  §  127,  2,  as  ammonium 
magnesium  arsenate,  allow  to  settle,  filter,  wash  once  with  a 
mixture  of  3  parts  water  and  1  part  ammonia,  redissolve  in 
hydrochloric  acid,  add  a  very  minute  quantity  of  tartaric  acid, 
supersaturate  again  with  ammonia,  add  some  more  magnesium 
chloride  and  ammonium  chloride,  allow  to  deposit,  and  deter- 
mine the  now  pure  precipitate  according  to  §  127,  2.  In  the 
filtrate  the  "bases  of  Groups  IV.  and  V.  may  be  precipitated  by 
ammonium  sulphide  ;  if  aluminium  is  present,  evaporate  the 
filtrate  from  the  sulphides  with  addition  of  sodium  carbonate 
and  a  little  nitre  to  dryness,  fuse,  and  estimate  the  aluminium 
in  the  residue.  The  method  is  more  adapted  to  the  separation 
of  rather  large  than  of  very  small  quantities  of  arsenic  from  the 
above-named  metals,  since  in  the  case  of  small  quantities  the 
minute  portions  of  ammonium  magnesium  arsenate  that  remain 
in  solution  may  exercise  a  considerable  influence  on  the  accu- 
racy of  the  result. 

12.  Method  based  upon  the  Separation  of  Arsenic  as 
Ammonium  Arsenio-molybdate. 

ARSENIC  ACID  FROM  ALL  METALS  OF  GROUPS  I. — Y. 

Separate  the  arsenic  acid    as  directed  in  §127,  2,  b ;  long  18$ 
continued  heating  at  100°  is  indispensable.  The  determination 
of  the  basic  metals  is  most  conveniently  effected  in  a  special 
portion. 


§  164.]  METALS    OF   GROUP   VI.  713 

13.  Method  based  upon  the  Insolubility  of  Ferric 
Ar  senate. 

AKSENIC  ACID  FROM  THE  METALS  OF  GROUPS  I.  AND  II., 

AND  FROM  ZlNC,  MANGANESE,  NlCKEL,  AND  COBALT. 

Mix  the  hydrochloric-acid  solution  with  a  sufficient  quantity  187 
of  pure  ferric  chloride,  neutralize  the  greater  part  of  the  free 
acid  with  sodium  carbonate,  and  precipitate  the  iron  and  arse- 
nic acid  together  with  barium  carbonate  in  the  cold  or  with 
sodium  acetate  at  a  boiling  heat.  The  precipitate  should  be  so 
basic  as  to  have  a  brownish-red  color.  The  method  is  espe- 
cially suitable  for  the  separation  of  arsenic  acid  when  its  esti- 
mation is  not  required.  However,  the  precipitate  may  be  dis- 
solved in  hydrochloric  acid  and  the  arsenic  determined  by 
precipitation  with  hydrogen  sulphide. 

14.  Methods   based  upon  the  Insolubility  of  some 
Chlorides. 

a.  SILVER  FROM  GOLD. 

Treat  the  alloy  with  cold  dilute  nitrohydrochloric  acid,  188 
dilute,  and  filter  the  solution  of  auric  chloride  from  the  undis- 
solved  silver  chloride.     This  method  is  applicable  only  if  the 
alloy  contains  less  than  15  per  cent,  of  silver ;  for  if  it  contains 
a  larger  proportion,  the  silver  chloride  which  forms  protects 
the  undecomposed  part  from  the  action  of  the  acid.     In  the 
same  way  silver  may  be  separated  also  from  platinum. 

b.  MERCURY  FROM  THE  OXYGEN  COMPOUNDS  OF  ARSENIC 
AND  ANTIMONY. 

Precipitate  the  mercury  from  the  hydrochloric  solution  by  189 
means  of  phosphorous  acid  as  mercurous  chloride  (§  118,  2). 
The  tartaric  acid,  which  in  the  presence  of  antimony  must  be 
added,  does  not  interfere  with  the  reaction  (H.  ROSE*). 

15.  Methods  based  upon  the  Insolubility  of  certain 
Sulphates  in  Water  or  Alcohol. 

a.  ARSENIC  ACID  FROM  BARIUM,  STRONTIUM,  CALCIUM,  AND 
LEAD. 

Proceed  as  for  the  separation  of  phosphoric  acid  from  the  190 
same  metals  (§  135,  I).    The  compounds  of  these  basic  radicals 
with  arsenous  acid  are  first  converted  into  arsenates,  before 

*  Pogg.  Annal.,  ex,  536. 


714  SEPARATION.  [§  164. 

the  sulphuric  acid  is  added ;  this  conversion  is  effected  by 
heating  the  hydrochloric  acid  solution  with  potassium  chlo- 
rate or  by  means  of  bromine. 

b.  ANTIMONY  FROM  LEAD. 

Treat  the  alloy  with  a  mixture  of  nitric  and  tartaric  acids.  191 
The  solution  of  both  metals  takes  place  rapidly  and  with  ease. 
Precipitate  the  greater  part  of  the  lead  as  sulphate  (§  116,  3), 
filter,  precipitate  with  hydrogen  sulphide,  and  treat  the  sul- 
phides according  to  168,  with  ammonium  sulphide,  in  order 
to  separate  the  antimony  from  the  lead  left  unprecipitated  by 
the  sulphuric  acid  (A.  STRENQ  *). 

16.  Method  based  upon  the  Separation  of  Copper  as 
Cuprous  Sulphocyanate. 

COPPER  FROM  ARSENIC  AND  ANTIMONY. 

From  the  properly  prepared  solution  precipitate  the  cop-  192 
per  by  §  119,  3,  5,  as  cuprous  sulphocyanate,  allow  to  settle, 
filter,  wash  with  water  containing  ammonium  nitrate  (to  pre- 
Tent  the  washings  being  muddy),  and  determine  antimony 
and  arsenic  in  the  filtrate,  preciptating  first  with  hydrogen 
sulphide.  Results  good. 

The  following  method,  depending  upon  the  precipitation 
of  the  copper  as  an  iodide,  is  not  good.  Dissolve  in  nitric  or 
sulphuric  acid,  taking  care  to  have  a  slight  excess  of  acid, 
dilute  with  pure  water  (if  antimony  is  present  use  water  con- 
taining some  tartaric  acid),  add  sulphurous  acid,  and  precipi- 
tate the  copper  with  potassium  iodide  as  cuprous  iodide. 
Arsenic  and  antimony  remain  in  solution  (FLAJOLOT). 
Results  are  approximate,  because  the  liquid  retains  some 
cuprous  iodide  in  solution  in  consequence  of  the  presence  of 
the  sulphurous  acid.  It  is  impracticable  to  use  stannous 
chloride  for  reducing  the  copper  as  recommended  by 
FLEISCHER, f  because  then  the  separation  of  the  tin  from  the 
arsenic  and  antimony  would  be  too  difficult. 

17.   Method  based  upon  the  Precipitation  of  Cop- 
per as  an  Oxalate. 
COPPEE  FBOM  AKSENIC. 

Add  ammonia  to  the  nitric-acid  solution  until  the  blue  193 
precipitate  no  longer  redissolves,  and  then  effect  solution  by 

*  Dingl.  polyt.  Journ.,  CLI,  389.        -\Zeitschr.f.  analyt.  Chem.,  ix,  256. 


§  165.]  METALS   OF   GROUP   VI.  715 

an  excess  of  ammonium  oxalate.  Now  add  very  cautiously 
hydrochloric  or  nitric  acid  to  acid  reaction,  and  allow  to 
stand.  The  copper  separates  almost  completely  as  oxalate, 
which  is  converted  into  oxide  by  ignition  in  the  air.  Make 
the  filtrate  animoniacal  and  precipitate  the  trace  of  copper 
still  in  solution  by  adding  a  few  drops  of  ammonium-sul- 
phide solution  (F.  FIELD  *). 

18.   Method  based  upon   the  different  Deportment 
with  Potassium  Cyanide. 

GOLD  FEOM  LEAD  AND  BISMUTH. 

These  metals  may  be  separated  in  solution  by  potassium  194 
cyanide  in  the  same  way  in  which  the  separation  of  mercury 
from  lead  and  bismuth  is  effected  (see  147).  The  solution  of 
the  double  cyanide  of  gold  and  potassium  is  decomposed  by 
boiling  with  aqua  regia,  and,  after  expulsion  of  the  hydro- 
cyanic acid,  the  gold  determined  by  one  of  the  methods 
given  in  §  123. 

II.  SEPAEATION  or  THE  METALS  OF  THE  SIXTH  GEOUP 

FEOM    EACH  OTHEE. 

§165. 

INDEX.     (The  numbers  refer  to  those  in  the  margin.) 
Platinum  from  gold,  195,  214,  215. 

tin,  antimony,  and  arsenic,  196. 
Gold  from  platinum,  195,  214,  215. 

tin,  196,  213. 

"          antimony  and  arsenic,  196. 
Tin  from  platinum,  196. 

gold,  175,  196,  213. 

arsenic,  199,  206,  207,  208,  211,  212,  216,  217. 
««          antimony,  197,  201,  208,  209,  210,  212,  216. 

Tin  in  staunous  from  tin  in  stannic  compounds,  221. 
Antimony  from  platinum  and  gold,  196. 

arsenic,  200,  201,  202,  203,  204,  206,  207. 
tin,  197,  201,  208,  209,  210,  212,  216. 
Antimony  of  antimonous  compounds  from  antimonic 

acid,  220. 
Arsenic  from  platinum  and  gold,  196. 

tin,  199,  206,  207,  208,  211,  212,  216,  217. 
antimony,  200,  201,  202,  203,  204,  206,  207,  218. 
Arsenous  acid  from  arsenic  acid,  198,  205,  219. 

*  Chem.  Gaz.,  1857,  313;  Journ.f.  prakt.  Chem.,  LXXU,  183. 


716  SEPAEATION.  [§  165. 

1.  Method  based  upon  the  Precipitation  of  Plati- 
num as  Potassium  Platinic  Chloride. 

PLATINUM  FROM  GOLD. 

Precipitate  from  the  solution  of  the  chlorides  the  plati-  195 
num  as  directed  §  124,  £,  and  determine  the  gold  in  the  filtrate 
as  directed  §  123,  I. 

2.  Methods  based  upon  the  Volatility  of  the  Chlo- 
rides of  the  inferior  Metals. 

a.  PLATINUM  AND  GOLD  FROM  TIN,  ANTIMONY,  AND  ARSENIC. 

Heat  the  finely  divided  alloy  or  the  sulphides  in  a  stream  196 
of  chlorine  gas.     Gold  and  platinum  are  left,  the  chlorides  of 
the  other  metals  volatilize  (compare  160). 

1).  ANTIMONY  FROM  TIN. 

The  tin  should  be  present  wholly  as  a  stannous  salt.  197 
Precipitate  with  hydrogen  sulphide,  filter  (preferably  through 
an  asbestos  filtering  tube),  dry  the  precipitate,  and  pass  through 
it  a  current  of  dry  hydrochloric  gas  at  the  ordinary  tempera- 
ture. The  sulphides  are  converted  into  the  corresponding 
chlorides ;  the  chloride  of  antimony  alone  escapes,  and  may 
be  received  in  water.  Dissolve  the  residual  stannous  chloride 
in  water  containing  hydrochloric  acid,  and  estimate  the  tin 
according  to  §  126  (C.  TOOKEY*).  The  method  can  only  be 
used  in  rare  cases,  as  it  is  difficult  to  obtain  a  precipitate  quite 
free  from  stannic  sulphide. 

c.   ARSENOUS  ACID  FROM  ARSENIC  ACID. 

The  amount  of  substance  taken  should  not  contain  more  198 
than  0*2  grm.  arsenous  acid.  Heat  with  45  grm.  sodium 
chloride,  135  grm.  sulphuric  acid  (free  from  arsenic)  of  1*61 
sp.  gr.,  and  30  grm.  water  in  a  tubulated  retort  containing  a 
spiral  of  platinum,  and  provided  with  a  thermometer.  The 
temperature  should  rise  to  about  125°.  In  order  to  condense 
the  arsenous  chloride  in  the  products  of  distillation,  a  LIEBIG'S 
condenser  is  connected  with  the  retort ;  a  tubulated  receiver 
is  connected  with  the  condenser ;  a  TJ-tube  is  connected  with 
the  receiver,  and  finally  a  calcium  chloride  tube  containing 
fragments  of  glass  moistened  with  weak  soda  solution  is  fixed 

*  Jour ii.  Chem.  Koc.,  xv,  462. 


§   165.]  METALS    OF   GROUP  VI.  717 

upright  in  the  exit  end  of  the  U-tube.  In  the  receiver  and 
U-tube  water  is  placed.  I  can  recommend  the  apparatus 
shown  in  Fig.  7" 8.  At  the  end  of  the  operation  rinse 
the  calcium  chloride  tube,  and  mix  with  the  contents  of  the 
receiver.  Determine  the  arsenic  in  the  distillate  according  to 
§  127,  4,  a,  in  the  residue  according  to  §  127,  4,  I.  The  sul- 
phide obtained  from  the  former  corresponds  to  the  arsenous 
acid,  from  the  latter  to  the  arsenic  acid.  Results  satisfactory 
(RiECKHEU*).  If  the  substance  given  is  a  dilute  fluid,  render 
slightly  alkaline  with  sodium  carbonate,  and  concentrate  to 
.about  20  c.c.,  finally  in  a  tubulated  retort. 

3.  Methods   based  upon  the  Volatility  of  Arsenic 
and  Arsenous  Sulphide. 

a.  ARSENIC  FROM  TIN  (H.  ROSE). 

Convert  into  sulphides  or  oxides,  dry  at  100°,  and  heat  a  199 
weighed  portion  with  addition  of  a  little  sulphur  in  a  bulb- 
tube,  gently  at  first,  but  gradually  more  strongly,  conducting 
a  stream  of  dry  hydrogen  sulphide  gas  through  the  tube 
during  the  operation.  Sulphur  and  arsenous  sulphide  vola- 
tilize ;  sulphide  of  tin  is  left.  The  arsenous  sulphide  is 
received  in  U-tubes  containing  dilute  ammonia,  which  are 
connected  with  the  bulb-tube  in  the  manner  described  in  160. 
"When  upon  continued  application  of  heat  no  sign  of  further 
sublimation  is  observed  in  the  colder  part  of  the  bulb-tube, 
drive  off  the  sublimate  which  has  collected  in  the  bulb,  allow 
the  tube  to  cool,  and  then  cut  it  off  above  the  coating.  Divide 
the  separated  portion  of  the  tube  into  pieces,  and  heat  these 
with  a  little  solution  of  soda  until  the  sublimate  is  dissolved; 
unite  the  solution  with  the  amrnoniacal  fluid  in  the  receivers, 
add  hydrochloric  acid,  then,  without  filtering,  potassium 
chlorate,  and  heat  gently  until  the  arsenious  sulphide  is  com- 
pletely dissolved.  Filter  from  the  sulphur,  and  determine  the 
arsenic  acid  as  directed  §  127,  2.  The  quantity  of  tin  cannot 
be  calculated  at  once  from  the  blackish-brown  sulphide  of  tin 
in  the  bulb,  since  this  contains  more  sulphur  than  SnS.  It  is 
therefore  weighed,  and  the  tin  determined  in  a  weighed  por- 
tion of  it,  by  converting  it  into  stannic  oxide,  which  is  effected  . 
by  moistening  with  nitric  acid,  and  roasting  (£  12t>,  1,  c\ 
*  Plniri/i.  (  i  ntntllmlle,  XI,  1)2. 


718  SEPARATION.  [§  165. 

Tin  and  arsenic  in  alloys  are  more  conveniently  converted 
into  oxides  by  cautious  treatment  with  nitric  acid.  If,  how- 
ever, it  is  wished  to  convert  them  into  sulphides,  this  may 
readily  be  effected  by  heating  1  part  of  the  finely  divided 
alloy  with  5  parts  of  sodium  carbonate  and  5  parts  of  sulphur, 
in  a  covered  porcelain  crucible  until  the  mass  is  in  a  state  of 
calm  fusion.  It  is  then  dissolved  in  water,  the  solution  filtered 
from  the  ferrous  sulphide,  &c.,  which  may  possibly  have 
formed,  and  then  precipitated  with  hydrochloric  acid. 

If  the  tin  only  in  the  alloy  is  to  be  estimated  directly, 
while  the  arsenic  is  to  be  found  from  the  difference,  convert 
as  above  directed  into  sulphides  or  oxides,  mix  with  sulphur 
and  ignite  in  a  porcelain  crucible  with  perforated  cover  in 
a  stream  of  hydrogen  sulphide.  The  residual  arsenic-free 
stannous  sulphide  is  to  be  converted  into  stannic  oxide  and 
weighed  as  such. 

b.  ARSENIC  FROM  ANTIMONY  IN  ALLOYS. 

Heat  a  weighed  portion  of  the  substance  with  two  parts  200 
of  sodium  carbonate  and  two  parts  of  potassium  cyanide  in  a 
bulb-tube  through  which  dry  carbonic  acid  is  being  trans- 
mitted. Heat  at  first  gently,  then  more  and  more  strongly, 
and  until  no  more  arsenic  volatilizes.  (Take  great  care  not 
to  inhale  the  escaping  fumes.  It  is  advisable  to  insert 
the  open  end  of  the  tube  into  a  flask  in  which  sublimed 
arsenic  will  condense.)  After  cooling,  treat  the  contents  of 
the  bulb  first  with  a  mixture  of  equal  volumes  of  alcohol  and 
water,  then  with  water  alone,  and  finally  weigh  the  residual 
antimony.  The  arsenic  is  found  from  the  loss.  The  results 
are  only  approximate.  If  it  is  desired  to  fuse  the  alloy, 
itself  not  under  a  slag,  in  a  current  of  carbonic-acid  gas, 
the  heating  must  be  very  carefully  done,  otherwise  much 
antimony  will  volatilize.  H.  EOSE  recommends  the  latter 
process. 

4.   Methods  based  upon  the  Insolubility  of  Sodium 
Metantimonate . 

a.   ANTIMONY  FROM  TIN  AND  ARSENIC  (H.  KOSE). 
If  the   substance  is   metallic,  oxidize  the  finely  divided  201 
weighed   sample  in  a  porcelain  crucible  with  nitric  acid  of 


§  165.]  METALS    OF    GKOUP    VI.  719 

1-4  sp.  gr.,  adding  the  acid  gradually.  Dry  the  mass  on  the 
water-bath,  transfer  to  a  silver  crucible,  rinsing  the  last  par- 
ticles adhering  to  the  porcelain  into  the  silver  crucible  with 
solution  of  soda,  dry  again,  add  eight  times  the  bulk  of  the 
mass  of  solid  sodium  hydroxide,  and  fuse  for  some  time. 
Allow  the  mass  to  cool,  and  then  treat  with  hot  water  until 
the  undissolved  residue  presents  the  appearance  of  a  fine 
powder ;  dilute  with  some  water,  and  add  one-third  the  volume 
of  alcohol  of  0*83  sp.  gr.  Allow  the  mixture  to  stand  for  24: 
hours,  with  frequent  stirring;  then  filter,  transfer  the  last 
adhering  particles  from  the  crucible  to  the  filter  by  rinsing 
with  dilute  alcohol  (1  vol.  alcohol  to  3  vol.  water),  and  wash 
the  undissolved  residue  on  the  filter,  first  with  alcohol  diluted 
with  twice  its  volume  of  water,  then  with  a  mixture  of  equal 
volumes  of  alcohol  and  water,  and  finally  with  a  mixture  of 
3  vol.  alcohol  and  1  vol.  water.  Add  to  each  of  the  alcoholic 
fluids  used  for  washing  a  few  drops  of  solution  of  sodium 
carbonate.  Continue  the  washing  until  the  color  of  a  portion 
of  the  fluid  running  off  remains  unaltered  upon  being  acidi- 
fied with  hydrochloric  acid  and  mixed  with  hydrogen- sul- 
phide water. 

Rinse  the  sodium  metantimonate  from  the  filter,  wash  the 
latter  with  a  mixture  of  hydrochloric  and  tartaric  acids,  dis- 
solve the  metantimonate  in  this  mixture,  precipitate  with 
hydrogen  sulphide,  and  determine  the  antimony  as  directed 
in  §  125,  1.  In  presence  of  much  tin  it  is  well  to  fuse  the 
metantimonate  again  with  caustic  soda,  etc. 

To  the  filtrate,  which  contains  the  tin  and  arsenic,  add 
hydrochloric  acid,  which  produces  a  precipitate  of  stannic 
arsenate;  conduct  now  into  the  unfiltered  fluid  hydrogen 
sulphide  for  some  time,  allow  the  mixture  to  stand  at  rest 
until  the  odor  of  that  gas  has  almost  completely  gone  off,  and 
separate  the  weighed  sulphides  of  the  metals  which  contain 
free  sulphur,  as  in  199. 

If  the  substance  contains  only  antimony  and  arsenic,  the 
alcoholic  filtrate  is  heated,  with  repeated  addition  of  water, 
until  it  scarcely  retains  the  odor  of  alcohol ;  hydrochloric  acid 
is  then  added,  and  the  arsenic  acid  determined  as  magnesium 
pyroarsenate  (§  127,  2),  or  as  arsenous  sulphide  (§  127,  4,  5). 


720  SEPARATION.  [§  165. 

~b.  Small  quantities  of  the  sulphides  of  arsenic  and  anti-  202 
mony  mixed  with  sulphur  are  often  obtained  in  mineral 
analysis.  The  two  metals  may  in  this  case  be  conveniently 
separated  as  follows:  Exhaust  the  precipitate  with  carbon 
disulphide,  oxidize  with  chlorine-free  red  fuming  nitric  acid, 
evaporate  the  solution  nearly  to  dryness;  mix  the  residue 
with  a  copious  excess  of  sodium  carbonate,  add  some  sodium 
nitrate,  and  treat  the  fused  mass  as  given  in  201,  a.  If,  on 
the  other  hand,  you  have  a  mixture  of  sulphides  of  tin  and 
antimony  to  analyze,  oxidize  it  with  nitric  acid  of  1*5  sp.  gr., 
and  treat  the  residue  obtained  on  evaporation  as  given  in 
201,  a. 

.   c.  DETERMINATION  OF  AKSENIC  SULPHIDE  IN  COMMERCIAL 
ANTIMONY  SULPHIDE  (WACKENRODER). 

Deflagrate  20  grm.  of  the  finely  triturated  antimony  sul-  203 
phide  with  40  grm.  sodium  nitrate  and  20  grm.  sodium  car- 
bonate by  projecting  the  mixture  in  portions  into  a  red-hot 
Hessian  crucible,  then  extract  the  strongly  ignited  mass  by 
repeated  treatment  with  water,  acidulate  the  filtrate  with 
hydrochloric  acid,  add  some  sulphurous  acid,  and  precipitate 
the  arsenic  together  with  a  small  part  of  the  antimony  by 
means  of  hydrogen  sulphide.  Digest  the  still  moist  precipi- 
tate with  ammonium  carbonate,  filter,  acidulate  the  filtrate, 
conduct  in  hydrogen  sulphide,  and  determine  the  arsenic  as 
arsenic  sulphide  according  to  §  127,  4. 

5.   Methods  "based  upon  the  Precipitation  of  Arsenic 
as  Ammonium  Magnesium  Ar senate. 

a.  ARSENIC  -FROM  ANTIMONY. 

Oxidize  the  metals  or  sulphides  with  nitrohydrochloric  204 
acid,  with  hydrochloric  acid  and  potassium  chlorate,  with 
bromine  dissolved  in  hydrochloric  acid,  or  with  chlorine  in 
alkaline  solution,  page  568,  &;  add  tartaric  acid,  a  large 
quantity  of  ammonium  chloride,  and  then  ammonia  in  excess. 
(Should  the  addition  of  the  latter  reagent  produce  a  precipi- 
tate, this  is  a  proof  that  an  insufficient  quantity  of  ammo- 
nium chloride  or  of  tartaric  acid  has  been  used,  which  error 
must  be  corrected  before  proceeding  with  the  analysis.) 


§  165.]  METALS    OF   GROUP   VI.  721 

Then  precipitate  the  arsenic  acid  as  directed  in  §  127,  2,  and 
determine  the  antimony  in  the  filtrate  as  directed  in  §  125,  1. 
As  basic  magnesium  tartrate  might  precipitate  with  the 
ammonium  magnesium  arsenate,  the  precipitate  should 
always,  after  slight  washing,  be  redissolved  in  hydrochloric 
acid,  and  reprecipitated  with  ammonia  with  addition  of  a 
little  magnesia  mixture.  An  excellent  method. 

b.  ARSEXOUS  ACID  FROM  ARSENIC  ACID. 

Mix  the  sufficiently  dilute  solution  with  a  large  quantity  205 
of  ammonium  chloride,  precipitate  the  arsenic  acid  as  directed 
in  §  127,  2,  and  determine  the  arsenous  acid  in  the  filtrate  by 
precipitation  with  hydrogen  sulphide  (§127, 4).  LUDWIG*  has 
observed  that  if  the  solution  is  too  concentrated,  magne- 
sium arsenite  falls  clown  with  the  ammonium  magnesium 
arsenate,  hence  it  is  necessary  to  dissolve  the  weighed  magne- 
sium precipitate  in  hydrochloric  acid  and  test  the  solution  with 
hydrogen  sulphide.  The  presence  of  arsenous  acid  will  be 
betrayed  by  the  immediate  formation  of  a  precipitate. 

c.  TIN  AND  ANTIMONY  FROM  ARSENIC  ACID. 

LKXSSEN!  separated  tin  from  arsenic  acid  with  good  206 
results  by  digesting  the  oxides  obtained  by  oxidation  with 
nitric  acid  with  ammonia  and  yellow  ammonium  sulphide,  and 
precipitating  the  arsenic  acid  from  the  clear  solution  accord- 
ing to  127,  2,  as  ammonium  magnesium  arsenate.  On  acidify- 
ing the  filtrate  the  tin  separates  as  stannic  sulphide.  The 
method  can  only  give  good  results  when  the  whole  of  the 
arsenic  was  present  as  arsenic  acid  before  the  addition  of 
ammonium  sulphide,  for  the  arsenic  in  a  solution  of  arsenous 
acid  in  yellow  ammonium  sulphide  is  not  thrown  down  by 
magnesia  mixture.  The  method  also  answers  for  separating 
antimony  from  arsenic. 

*  Archivfur  Pharm.,  xcvn,  24.        \  Annal.  d.  Chem.  u.  Pharm.,  cxiv,  116. 


722  SEPARATION.  [§  165. 

6.  Methods  based  on  the  different  behavior  of  the 
freshly  Precipitated  Sulphides  towards  Solution  of 
Potassium  Hydrogen  Sulphite  or  Oxalic  Acid. 

a.  ARSENIC  FROM  ANTIMONY  AND  TIN  (BUNSEN*). 

If  freshly  precipitated  arsenous  sulphide  is  digested  with  207 
sulphurous  acid  and  potassium  sulphite,  the  precipitate  is  dis- 
solved ;  on  boiling,  the  fluid  becomes  turbid  from  separated 
sulphur,  which  turbidity  for  the  most  part  disappears  again  on 
long  boiling.  The  fluid  contains,  after  expulsion  of  the  sul- 
phurous acid,  potassium  arsenite  and  thiosulphate.  The  sul- 
phides of  antimony  and  tin  do  not  exhibit  this  reaction.  Both 
therefore  maybe  separated  from  arsenous  sulphide  by  diluting 
the  solution  of  the  three  sulphides  in  potassium  sulphide  to  about 
500  c.c.  and  precipitating  with  a  large  excess  (about  a  litre)  of 
saturated  aqueous  sulphurous  acid,  digesting  the  whole  for 
some  time  in  a  water-bath,  and  then  boiling  till  one-third  of 
the  water  and  the  whole  of  the  sulphurous  acid  are  expelled 
and  the  sulphur  has  disappeared  ;  this  will  take  about  an  hour 
and  a  half.  The  residuary  sulphide  of  antimony  or  tin  is  arsenic- 
free,  the  filtrate  contains  the  whole  of  the  arsenic  and  maybe 
immediately  precipitated  with  hydrogen  sulphide.  BUNSEN 
determines  the  arsenic  by  oxidizing  the  dried  sulphide  together 
with  the  filter  with  fuming  nitric  acid,  diluting  the  solution 
a  little,  warming  gently  with  a  little  potassium  chlorate  (in 
order  to  oxidize  more  fully  the  substances  formed  from  the 
paper),  and  finally  precipitating  as  ammonium  magnesium 
arsenate. 

With  regard  to  the  separation  of  stannic  sulphide  from 
the  solution  of  potassium  arsenite,  it  is  to  be  observed  that  the 
stannic  sulphide  must  be  washed  with  concentrated  solution 
of  sodium  chloride,  as,  if  water  wrere  used,  the  fluid  would  run 
through  turbid.  As  soon  as  the  precipitate  is  thoroughly 
washed  with  the  sodium  chloride,  the  latter  is  displaced  by 
solution  of  ammonium  acetate,  containing  a  slight  excess  of 
acetic  acid.  These  last  washings  must  not  be  added  to  the 
first,  as  the  ammonium  acetate  hinders  the  complete  precipita 
tion  of  the  arsenous  acid  by  hydrogen  sulphide, 

*  Annal.  d.  Chem.  u.  Pharm.,  en,  3. 


§165.]  METALS    OF   GROUP  VI.  723 

The  test-analyses  adduced  by  BUNSEN  show  very  satisfac- 
tory results. 

b.  TIN  FBOM  ABSENIO  AND  ANTIMONY  (F.  W.  CLAKKE  *). 

Moist  freshly  precipitated  tin  disulphide  completely  dis- 
solves on  boiling  for  a  moderate  length  of  time  with  excess  of 
oxalic  acid,  and  therefore  tin  in  the  form  of  dichloride  is  not 
thrown  down  by  hydrogen  sulphide  from  a  hot  solution  con- 
taining excess  of  oxalic  acid.  The  sulphides  of  arsenic  are 
barely  aifected  by  boiling  with  oxalic  acid,  and  hydrogen  sul- 
phide immediately  reprecipitates  the  traces  dissolved.  Anti- 
mony sulphide  dissolves  more  copiously  on  boiling  with  oxalic 
acid,  but  hydrogen  sulphide  reprecipitates  the  antimony 
from  the  solution. 

CLARKE  hence  recommends  adding  to  the  solution  of  the 
three  metals  (and  in  which  the  tin  is  present  in  the  form  of 
dioxide)  oxalic  acid  in  twenty  times  the  weight  of  the  tin. 
The  solution  must  be  so  concentrated  that  on  cooling  the  oxalic 
acid  crystallizes  out.  Now  conduct  into  the  solution,  main- 
tained at  a  boiling  heat,  hydrogen-sulphide  gas  for  20  min- 
utes, let  stand  for  half  an  hour  in  a  warm  place  and  filter. 
According  to  CLARKE  all  the  arsenic  and  antimony,  free  from 
tin  sulphide,  or  at  least  nearly  so,  is  thus  precipitated,  while 
all  the  tin  remains  in  solution.  The  tin  is  obtained  by  mak- 
ing the  solution  weakly  alkaline  with  ammonia  and  adding 
ammonium  sulphide  until  the  precipitate  first  formed  redis- 
solves ;  then  decompose  the  sulphosalt  with  an  excess  of  acetic 
acid,  set  aside  in  a  warm*  place  for  the  tin  disulphide  to  settle, 
and  determine  it  according  to  §  126,  1,  c.  Acids  which  are 
stronger  than  acetic,  and  which  liberate  oxalic  acid,  must  not 
be  employed.  CLARKE  recommends,  in  order  to  secure  very 
accurate  results,  to  again  dissolve  the  precipitate  of  antimony 
and  arsenic  sulphides  in  an  alkaline  sulphide  solution,  add  an 
excess  of  oxalic  acid,  and  boil  with  hydrogen-sulphide  water, 
thereby  obtaining  the  last  portions  of  tin  in  solution. 
According  to  the  investigations  made  by  FR.  PHILLIPS  in  my 
laboratory,  this  last  operation  would  appear  to  be  absolutely 
required  in  order  to  obtain  any  proper  results  whatever. 

*  CJiem.  News^  XXT,  124  ;  Zetischr.  f.  analyt.  Chem.,  ix,  487. 


724  SEPARATION.  [§  165. 

The  very  unfavorable  results  obtained  by  G-.  C.  "WITTSTEIN  * 
.are  perhaps  referable  to  the  fact  that  the  solution  used  by 
him  contained  too  much  free  hydrochloric  acid,  whereby  the 
precipitation  was  .rendered  incomplete  at  the  boiling  heat, 
arid  he  was  compelled  to  complete  it  in  the  cold.  In  the  ex- 
periments made  by  PHILLIPS  the  free  hydrochloric  acid  was 
neutralized  as  nearly  as  possible  with  potassa. 

[CLARKE'S  method,  with  some  important  modifications,  has 
been  successfully  applied  to  the  separation  of  tin  from  anti- 
mony in  alloys  by  F.  P.  DEWEY,f  who  proceeds  as  follows: 

Dissolve  in  a  mixture  of  1  part  strong  nitric  acid,  -i  parts 
strong  hydrochloric  acid,  and  5  parts  water.  Since  even  small 
quantities  of  free  mineral  acids  prevent  complete  precipitation 
of  antimony,  they  are  removed  by  evaporating  to  dryness  on 
a  water-bath,  with  previous  addition  of  enough  potassium 
chloride  to  form  double  salts  with  the  tin  and  antimony  chlo- 
rides present.  The  presence  of  the  potassium  chloride  entirely 
prevents  loss  of  tin  and  antimony  by  volatilization  as  chlorides 
during  the  evaporation.  Add  to  the  salts  thus  obtained  a 
large  quantity  of  pure"  oxalic  acid  (at  least  20  parts  crytallized 
acid  to  1  part  tin)  and  dilute  with  water  to  about  125  c.  c.  per 
O'l  grm.  antimony  present.  The  salts  dissolve  readily.  Boil 
and  pass  H3S  through  the  boiling  solution  half  an  hour.  Filter 
immediately  while  hot,  and  wash  the  greater  part  of  the  soluble 
matter  out  of  the  precipitate  with  hot  water.  The  precipi- 
tated antimonous  sulphide  will  contain  a  little  stannic  sulphide. 
Dissolve  in  ammonium  sulphide,  avoiding  an  unnecessary 
quantity  of  the  sol  vent,  and  pour  the  solution  into  a  strong  hot 
solution  of  oxalic  acid.  A  liberal  excess  of  oxalic  acid  should 
be  present  after  decomposition  of  the  sulphur  salts.  Heat  the 
oxalic  solution  with  the  suspended  precipitate  of  antimonous 
sulphide  to  boiling  and  pass  H2S  gas  ten  minutes.  Collect 
the  Sb,S3  now  free  from  tin  on  a  weighed  filter,  wash  with 
hot  water,  and  proceed  to  determine  the  antimony  as  directed 
in  §  125,  1,  J.  To  recover  tin  from  the  filtrate,  evaporate 
nearly  to  dryness,  add  strong  sulphuric  acid,  and  heat  till  all 

*  Vierteljahresschr.  f.  prakt.  Pharm,    xix,  551. 
\  Am.  Chem.  Journ.,  i,  244. 


§  165.]  METALS    OF  GROUP  VI.  726 

the  oxalic  acid  present  is  decomposed  and  removed.  Dilute 
largely  and  precipitate  the  tin  \vith  hydrogen  sulphide 
according  to  §  126,  1,  <?.] 

7.  Methods  'based  upon  the  Separation  of  the  Metals 
themselves,  or,  as  the  case  may  be,  on  their  different 
Deportment  with  Acids. 

a.  TIN  FROM  ANTIMONY  (MODIFIED  GAY-LUSSAC). 

Heat  a  weighed  portion  of  the  finely  divided  alloy  (or  other  209 
compound)  with  hydrochloric  acid,  add  potassium  chlorate  in 
small  portions  until  solution  is  effected,  and  then  divide  the 
liquid  into  two  parts,  a  and  b.  Precipitate  both  metals  in  a 
by  means  of  a  zinc  rod,  wash  them  rapidly  with  hot  water 
containing  some  hydrochloric  acid,  wash  with  alcohol,  then 
with  ether,  dry  at  100°,  and  weigh.  To  b  add  a  rather 
large  quantity  of  hydrochloric  acid,  immerse  a  strip  of  tin, 
and  heat  for  a  long  time.  By  this  operation  the  antimony 
is  completely  precipitated  as  a  black  powder,  while  the 
stannic  chloride  is  reduced  to  stannous  chloride.  Wash  off 
the  antimony  from  the  tin  with  moderately  dilute  hydro- 
chloric acid,  collect  it  on  a  weighed  filter,  wash  first  with 
diluted  hydrochloric  acid,  then  with  alcohol,  and  finally  with 
ether,  then  dry  at  100°  and  weigh.  The  quantity  of  tin  is 
found  by  the  difference.  Since,  according  to  investigations 
made  by  A.  "W.  CLASEN,*  precipitated  metallic  antimony  is 
very  perceptibly  soluble  in  hydrochloric  acid,  either  hot  or 
cold,  of  various  strengths,  a  loss  of  antimony  is  scarcely 
avoidable. 

b.  TIN   FROM    ANTIMONY    (TooKET,f    improvements    by 
OLASEN  (loc.  cit.)  and  ATTFIELD  ;).). 

The  hydrochloric  solution  should  be  oxidized  if  necessary  210 
with  a  few  drops  of  nitric  acid  or  a  little  potassium  chlorate. 
Heat  nearly  to  boiling  and  add  iron  so  long  as  it  dissolves. 
Either  hoop-iron  or  fine  bright  wire  will  answer  the  purpose; 
it  should  dissolve  in  dilute  hydrochloric  acid,  leaving  little  or 
no  residue.  The  antimony  will  be  thrown  down,  the  tin 


*  Journ.f,  prakt.  Chem..  xcu,  477,  Zeitschr.  f.  analyt.  Chem.,  iv,  440. 
\Journ.  Chem,  Soc.,  xv,  462.  Journ.f  prakt.  Chem.,  LXXXVIII,  435. 
$Zeitschr.f.  analyt.  Chem.,  ix.  107. 


726  SEPARATION.  [§  165. 

reduced  to  stannous  chloride.  As  soon  as  all  antimony  ap- 
pears to  be  precipitated  and  the  iron  to  be  dissolved,  add 
more  hydrochloric  acid,  allow  to  deposit,  decant,  and  test 
whether  iron  produces  any  further  precipitate.  In  this  way 
you  will  ensure  the  absence  of  any  metallic  iron  and  the  com- 
plete precipitation  of  the  antimony.  Wash  the  antimony 
with  hot  water,  which  should  be  at  first  acidified,  then  with 
alcohol,  finally  with  ether,  drying  at  100°.  Throw  down  the 
tin  with  hydrogen  sulphide  (§  126,  1,  c).  With  care  the  re- 
results  are  good.  Compare  CLASEN  (loc.  cit.). 

c.  DETERMINATION  OF  ARSENIC  IN  METALLIC   TIN  (GAY- 
LUSSAC  *). 

Dissolve  the  laminated  or  granulated  metal  in  a  mixture  211 
of  1  eq.  nitric  acid  and  9  eq.  hydrochloric  acid  with  the  aid 
of  a  gentle  heat.     Solution  ensues  without  the  disengagement 
of  any  gas,  stannous  chloride  and  ammonium  chloride  being 
formed.     The  arsenic  remains  behind  as  a  powder. 
2H1TO3  +  18HC1  +  8  Sn  =  8  SnCla  +  2NH4C1  +  6H2O. 

The  nitrohydrochloric  acid  must,  hence,  not  be  employed  in 
much  larger  proportion  than  will  give  2  eq.  of  HNO3  and  18 
eq.  of  HC1  to  8  eq.  of  metal. 

d.  MUCH  TIN  FROM  LITTLE  ANTIMONY  AND  ARSENIC. 

If  an  alloy  of  the  three  metals  is  treated  in  a  very  finely  212 
divided  condition  in  a  stream  of  carbonic  acid  with  strong 
hydrochloric  acid,  the  whole  of  the  tin  dissolves  to  stannous 
chloride.  A  part  of  the  arsenic  and  antimony  escapes  as 
arsenetted  and  antimonetted  hydrogen,  while  the  rest  remains 
behind  in  the  state  of  metal,  or,  as  the  case  may  be,  of  a  solid 
combination  with  hydrogen.  Conduct  the  gas  through  several 
U-tubes  containing  a  little  chlorine-free  red  fuming  nitric 
acid,  whereby  the  arsenic  and  antimony  will  be  oxidized. 
When  the  solution  is  effected,  dilute  the  contents  of  the  flask 
with  air-free  water  to  a  certain  volume,  mix,  allow  to  settle, 
and  determine  the  tin  in  an  aliquot  part  either  gravimetrically 
or  volumetrically.  Filter  the  rest  of  the  fluid,  wash  the  pre- 
cipitate thoroughly,  dry  the  filter  with  its  contents  in  a  porce- 

*  Annal.  de  Chim.  et  de  PJiys.,  xxm,  228. 


§  165.]  METALS    OF   GROUP   VI.  727 

lain  crucible,  add  the  contents  of  the  U-tubes,  evaporate  to 
dryness,  and  in  the  residue  separate  the  antimony  and  arsenic 
as  directed  in  201.  It  is  well  to  treat  an  aliquot  part  of  the 
.hydrochloric  solution  with  iron  (210)  to  find  and  if  necessary 
estimate  traces  of  antimony  which  may  have  passed  into  the 
hydrochloric-acid  solution. 

e.  TIN  FROM  GOLD. 

Gold  may  be  separated  from  excess  of  tin  by  boiling  the  213 
finely  divided  alloy  with  only  slightly  diluted  sulphuric  acid 
to  which  hydrochloric  acid  has  been  cautiously  added.  The 
tin  dissolves  as  stannous  chloride.  Heat  is  applied  till  the 
sulphuric  acid  begins  to  volatilize  copiously.  Stannic  oxide 
is  formed  which  dissolves  in  the  concentrated  sulphuric  acid, 
while  the  gold  remains  behind.  On  addition  of  much  water 
the  stannic  oxide  falls,  mixed  with  finely  divided  gold,  in  the 
form  of  a  purple-red  precipitate.  On  warming  with  concen- 
trated sulphuric  acid  the  stannic  oxide  finally  redissolves, 
while  the  gold  is  left  pure  (H.  ROSE  *). 

/.  PLATINUM  FROM  GOLD. 

The  aqua-regia  solution  is  freed  so  far  as  possible  from  214 
nitric  acid  by  evaporation  with  hydrochloric  acid,  and  treated 
with  a  solution  of  ferrous  chloride,  the  gold  being  determined 
as  directed  in  §  123,  Z>.     The  platinum  may  be  precipitated 
from  the  filtrate  by  hydrogen  sulphide  according  to  §  124,  c. 

8.  Method    based  on  the  Extraction   of   Gold   ~by 
Mercury. 

DETERMINATION  OF  GOLD  IN  PLATINUM  ORE. 

Treat  the  mineral  for  several  hours  with  small  quantities  215 
of  pure,  boiling  mercury,  pour  off  and  repeat  the  operation ; 
then  wash  thoroughly  with  boiling  mercury  and  distil  off  all 
the  mercury  very  cautiously.  The  gold  remains  behind 
(DEVILLE  and  DEBRAV).  Prudence  requires  that  the  residue 
should  be  tested. 


Pogg.  AnnaL,  cxn,  172. 


728  SEPARATION.  [§  165. 

9.  Method  based  on  the  Precipitation  of  the  Indi- 
vidual Metals  as  Sulphides  ~by  Sodium  Thiosulphate. 

AKSENIC  AND  ANTIMONY  FROM  TIN. 

Add  an  excess  of  hydrochloric  acid  to  the  solution,  heat  21$ 
to  boiling,  and  add  sodium  thiosulphate  until  the  precipitate 
is  no  longer  orange  or  yellow,  but  white,  and  the  liquid  is 
opalescent  from  separated  sulphur.  Arsenic  and  antimony 
are  completely  precipitated,  while  all  the  tin  remains  in  solu- 
tion (YoHL*).  Estimate  the  former,  if  one  alone  of  the 
metals  is  present,  according  to  §  125,  1  and  §  127,  4.  If 
both  together  are  present,  separate  according  to  201  or  204. 
The  tin  in  the  filtrate  is  best  determined  according  to  §  126, 
1,  c.  LENSSEN  f  employed  this  method  with  apparently  good 
results.  My  experience  has  not,  however,  been  so  favor- 
able. As  tin  is  also  precipitated  by  sodium  thiosulphate 
unless  free  hydrochloric  acid  is  present,  the  separation  can  be 
successful  only  when  hydrochloric  acid  present  prevents  pre- 
cipitation of  tin,  while  not  hindering  that  of  the  antimony. 

10.  Method  based  upon  the  Precipitation  of  Tin 
as  Stannic  Ar senate. 

TIN  FROM  AKSENIC. 

ED.  HAFFELY  ^  has  proposed  the  following  method  of  deter-  217 
mining  both  the  tin  and  the  arsenic  in  commercial  sodium 
stannate,  which  often  contains  a  large  admixture  of  sodium 
arsenate.  Mix  a  weighed  sample  with  a  known  quantity  of 
sodium  arsenate  in  excess,  add  nitric  acid  also  in  excess, 
boil,  filter  off  the  precipitate,  which  has  the  composition 
2SnO,*AsaO,  -f-  10H3O,  and  wash;  expel  the  water  by  igni- 
tion and  weigh  the  residue,  which  consists  of  2SnOa*A.s2O6. 
In  the  filtrate  determine  the  excess  of  arsenic  acid  as  directed  in 
§  127,  2.  The  amount  of  the  stannic  oxide  is  found  from  the 
weight  of  the  precipitate^  that  of  the  arsenic  acid  is  obtained 
by  adding  the  quantity  in  the  precipitate  to  the  quantity  in 
the  filtrate  and  deducting  the  quantity  added. 

*  Annal.  d.  Chem.  u.  Pharm.,  xcvi,  240.  f  75. ,  cxiv,  118. 

\Phil.  Mag.,  x,  220  ;  Journ.  f.  prakt.  Chem.,  LXVII,  209. 


§  165.]  METALS    OF   GROUP  VI.  729 

1 1 .   Method  based  on  the  Separation  of  A  rsenic  and 
Antimony  from  their  Hydrogen  Compounds. 

To  determine  both  metals  in  a  mixture  of  arsenic  and  218 
antimony  hydrides,  conduct  the  gas  into  a  solution  of  neutral 
silver  nitrate.  Antimony  hydride  yields  silver  antimonide, 
whereas  arsenic  goes  into  solution  as  arsenous  acid,  with  reduc- 
tion of  silver.  This  method  was  recommended  by  A.  W. 
HOFMAN  *  for  the  qualitative  detection  of  arsenic  and  anti- 
mony. Filter  off  the  precipitate,  consisting  of  silver  and  silver 
antimonide,  and  wash  it.  To  the  solution  add  a  slight  excess 
of  hydrochloric  acid,  filter  off  the  silver  chloride,  and  pre- 
cipitate with  hydrogen  sulphide.  The  precipitate  is  arsenous 
sulphide  containing  a  small  quantity  of  antimonous  sulphide, 
which  is  to  be  separated  according  to  202  or  207.  The  pre- 
cipitate of  silver  and  silver  antimonide  heat  with  tartaric 
acid  and  a  very  little  nitric  acid,  and  determine  the  antimony 
according  to  §  125,  1. 

All  methods  of  determining  antimony  and  arsenic  in  solu- 
tions, based  on  treating  the  solution  with  zinc  and  hydro- 
chloric acid,  passing  the  gas  into  silver-nitrate  solution,  etc., 
are  unreliable,  because  only  a  certain  part  of  the  arsenic  and 
antimony  are  evolved  as  hydrides,  while  the  balance  remains  * 
in  the  flask  in  the  form  of  metals. 

1 2 .    Volumetric  Methods. 

a.  ARSENOUS  FROM  ARSENIC  ACID. 

Convert  the  whole  of  the  arsenic  in  a  portion  of  the  sub-  219 
stance  into  arsenic  acid  and  determine  the  total  amount  of  this 
as  directed  §  127,  2 ;  determine  in  another  portion  the  arsen- 
ous acid  as  directed  in  §  12T,  5,  #,  and  calculate  the  arsenic 
acid  from  the  difference. 

1}.  ANTIMONY  OF   ANTIMONOUS  COMPOUNDS  FROM  ANTIMONIO 
ACID. 

Determine  in  a  sample  of  the  substance  the  total  amount  220 
of  the  antimony  as  directed  §  125,  1,  in  another  portion  esti- 
mate the  antimony  present  as  an    antimonous  compound  as 

*  Annal.  d.  Chem.  u.  Pharm.,  cxv,  287. 


730  SEPARATION".  [§  166. 

directed  §  125,  3,  and  calculate  the  antimonic  acid  from  the 
difference. 

c.  TIN  OF  STANNOUS,  FROM  TIN  OF  STANNIC  Ccmporxns. 

In  one  portion  of  the  substance  convert  the  whole  of  the  221 
stannons  into  stannic  salts  by  digestion  with  chlorine  water  or 
some  other  means,  and  determine  the  total  quantity  of  tin  as 
directed  §  126,  1,  b  ;  in  another  portion,  which,  if  necessary, 
is  to  be  dissolved  in  hydrochloric  acid  in  a  stream  of  carbonic 
acid,  determine  the  stannous  tin  according  to  §  126,  2. 

II.    SEPARATION   OF  THE   ACIDS  FROM  EACH   OTHER. 

It  must  not  be  forgotten  that  the  following  methods  of 
separation  proceed  generally  upon  the  assumption  that  the 
acids  exist  either  in  the  free  state,  or  as  alkali  salts ;  compare  the 
introductory  remarks,  (p.  597.  Where  several  acids  are  to  be 
determined  in  one  and  the  same  substance,  we  very  often  use 
a  separate  portion  for  each.  The  methods  here  given  do  not 
embrace  every  imaginable  case,  but  only  the  most  important 
cases,  and  those  of  most  frequent  occurrence. 


First  Group. 

ARSENOUS     ACID ARSENIC    ACID — CHROMIC    ACID SULPHURIC    ACID — 

PHOSPHORIC     ACID BORIC     ACID OXALIC     ACID HYDROFLUORIC 

ACID — SILICIC    ACID CARBONIC    ACID. 

§  166. 

1.  ARSENOUS  ACID  AND  ARSENIC  ACID  FROM  ALL  OTHER 
ACIDS. 

Precipitate  the  arsenic  from  the  solution  by  hydrogen  sul-  222 
phide  (§  127,  4,  a  or  &),  filter,  and  determine  the  other  acids 
in  the  nitrate.  It  must  be  remembered,  that  the  arsenous 
sulphide  will  be  obtained  mixed  with  sulphur  if  chromic  acid, 
ferric  salts,  or  any  other  substances  which  decompose  hydro- 
gen sulphide  are  present.  The  estimation  of  sulphuric  acid 
in  the  nitrate  cannot  be  accurate  unless  air  is  excluded,  and 
oxidizers  such  as  chromic  acid  are  absent ;  sulphuric  acid  is, 
therefore,  best  estimated  in  a  separate  portion  (223).  From 
those  acids  which  form  soluble  magnesium  salts,  arsenic  acid 


§  166.]  ACIDS    OF   GROUP   I.  731 

may  be  separated  also  by  precipitation  as  ammonium  magne- 
sium arsenate  (§  127,  2). 

2.  SULPHURIC  ACID  FROM  ALL  THE  OTHER  ACIDS.* 

a.  from  Arsenous,  Arsenic,  Phosphoric, f  Boric,  Oxalic, 
and  Carbonic  Acids. 

Acidify  the  dilute  solution  strongly  with  hydrochloric  acid,  223 
mix  with  barium  chloride,  and  filter  the  barium  sulphate  from 
the  solution,  which  contains  all  the  other  acids.  Determine 
the  barium  sulphate  as  directed  §132.  If  acids  are  present 
\vkli  which  barium  forms  salts  insoluble  in  water  but  soluble 
in  acids,  the  barium  sulphate  is  apt  to  carry  down  with  it  such 
salts,  and  this  is  all  the  more  liable  to  happen,  the  longer  the 
precipitate  is  allowed  to  settle.  This  remark  applies  especially 
to  barium  oxalate,  and  tartrate,  and  the  barium  salts  of 
other  organic  acids  (H.  ROSE).  In  such  cases  I  would  recom- 
mend, after  washing,  to  stop  up  the  neck  of  the  funnel,  and 
digest  the  precipitate  with  a  solution  of  hydrogen  sodium  car- 
bonate, then  to  wash  with  water,  with  dilute  hydrochloric 
acid,  and  again  with  water.  In  every  case,  however,  the 
purity  of  the  weighed  barium  sulphate  must  be  tested  as 
directed  §  132,  1. 

In  the  fluids  filtered  from  the  barium  sulphate  the  other 
acids  are  determined  according  to  the  directions  of  the  Fourth 
Section,  after  the  removal  of  the  excess  of  barium  chloride. 
Or  the  other  acids  may  be  estimated  in  separate  portions  of 
the  substance,  which  is  indeed  usually  the  best  way,  and  for 
carbonic  acid  is  of  course  the  only  way. 

1).  From  Hydrofluoric  Acid. 

of.  When  sulphuric  acid  and  hydrofluoric  acid  are  present  224 
in  the  free  state  in  aqueous  solution,  it  is  best  to  estimate  the 
acidity  in  one  portion  by  means  of  standard  soda  (§  215),  and 
the  sulphuric  acid  in  another  (§  132,  I.,  1),  finding  the  hydro- 
fluoric acid  by  difference.  The  barium  sulphate  should  be 
purified  by  fusion  with  sodium  carbonate  (§  132,  I.,  1). 

*  With  respect  to  the  separation  of  sulphuric  acid  from  selenic  acid,  comp. 
WOHLWILL  (Annal.  d.  C/iem.  u.  Fharm.,  cxiv,  183). 

f  If  metaphosphoric  acid  is  present,  it  must  first  be  converted  into  ortho- 
phosphoric  by  fusion  with  alkali  carbonate. 


732  SEPARATION.  [§  166, 

/?.  To  estimate  both  acids  in  minerals  or  other  dry  sub-  225 
stances,  it  is  safest,  provided  the  fluoride  can  be  decomposed 
by  sulphuric  acid,  to  determine  the  fluorine  in  one  portion 
according  to  §  138,  3,  #,  and  to  fuse  another  portion  for  a 
long  time  with  four  times  its  amount  of  sodium  carbonate, 
which  will  decompose  the  sulphate  thoroughly,  the  fluoride 
generally  but  partially.  The  fused  mass  is  soaked  in  water, 
the  solution  filtered,  acidified  with  hydrochloric  acid  and  pre- 
cipitated with  barium  chloride.  The  barium  sulphate  thus 
obtained  generally  contains  barium  fluoride,  and  must  be 
purified  according  to  §  132,  I.,  1,  by  fusion  with  sodium  car- 
bonate, &c. 

y.  An  actual  separation  of  both  acids  may  be  effected,  226 
when  both  are  in  the  form  of  alkali  salts,  by  adding  sodium   ' 
carbonate  if  necessary,  and  then   precipitating   the   fluorine 
according  to  §  138,  I.,  adding  the  calcium  chloride  cautiously 
in  very  slight  excess.     The  sulphuric  acid  is  for  the  most  part 
found  in  the  filtrate  from  the  calcium  carbonate  and  fluoride, 
a  very  small  part   is   generally  also   found   in   the   calcium 
acetate  filtered  from  the  calcium  fluoride.     Both  filtrates  are 
acidified  and  precipitated  with  barium  chloride  (§  132,  I.,  1. 
H.  EOSE). 

ft.  Insoluble  compounds  may  also  be  decomposed  by  fusion  227 
with  six  parts  of  sodium  and  potassium  carbonates,  and  two 
parts  of  silica.  The  fused  mass,  after  cooling,  is  treated  with 
water,  the  solution  is  mixed  with  ammonium  carbonate,  and 
heated,  more  ammonium  carbonate  is  added  to  replace  what 
evaporates,  the  silicic  acid  thrown  down  is  filtered  off  and 
washed  with  water  containing  ammonium  carbonate,  a  solu- 
tion of  zinc  oxide  in  ammonia  is  added  to  precipitate  the 
remaining  silica,  the  fluid  is  evaporated  till  all  ammonia  is 
driven  oif,  filtered  and  the  process  concluded  as  in  y.  The 
precipitate  produced  by  the  zinc  should  be  tested  for  sulphuric 
acid. 

c.  From  Chromic  Acid. 

Boil  the   dry  compound  with    strong   hydrochloric   acid  228 
(p.  357,  ft)  and  estimate  the  chromic  acid  from  the  evolved 
chlorine.     Neutralize  some  of  the  acid  with  ammonia,  dilute 
and  precipitate  the  sulphuric  acid  by  long  boiling  writh  excess 
of   barium   chloride.      The   barium    sulphate   thus   obtained 


§  166.]  ACIDS    OF   GKOTJP   I.  733 

retains  chromic  oxide  (H.  ROSE)  and  must  always  be  fused 
with  sodium  carbonate,  &c.  (§  132,  I.,  1). 

d.  From  Hydrqfluosilicic  Acid. 

First  throw  down  the  hydrofluosilicic  acid  according  to  229 
§  133,  as  potassium  silicofluoride,  then  the  sulphuric  acid  in 
the  filtrate  with  barium  chloride. 

e.  From  Silicic  Acid. 
Compare  242. 

3.   PHOSPHORIC  ACID  FROM  THE  OTHER  ACIDS. 

* 

a.  From  the  acids  of  arsenic,  see  222 ;   from  sulphuric  230 
add,  see  223 ;   from  silicic  acid,  see  242. 

b.  from  Chromic  Acid. 

Precipitate  the  phosphoric  acid  by  adding  ammonium 
nitrate  and  ammonia,  and  then  magnesium  nitrate,  and  deter- 
mine the  chromic  acid  in  the  nitrate  as  directed  §  130,  L,  a, 
ft  or  I.,,  b. 

c.  From  Boric  Acid. 

Precipitate  the  phosphoric  acid  with  a  solution  of  double  231 
chloride  of  magnesium  and  ammonium  (§  134,  6,  or),  wash  the 
precipitate  partially,  redissolve  it  in  hydrochloric  acid,  repre- 
cipitate  with  ammonia,  adding  a  little  magnesium  and  ammo- 
nium chloride,  and  estimate  the  phosphoric  acid  as  magnesium 
pyrophosphate.  In  the  filtrate  estimate  the  boric  acid  as 
magnesium  borate  (§  136,  L,  1,  6). 

d.  From  Oxalic  Acid. 

a.  If  the  two  acids  are  to  be  determined  in  one  portion,  232 
the  aqueous  or  hydrochloric  solution  is  mixed  with  sodium 
auric  chloride  in  excess,  heat  applied,  and  the  oxalic  acid  cal- 
culated from  the  reduced  gold  (§  137,  c).  The  gold  added  in 
excess  is  separated  from  the  nitrate  by  hydrogen  sulphide,  and 
the  phosphoric  acid  then  precipitated  by  double  chloride  of 
magnesium  and  ammonium. 

/3.  If  there  is  enough  of  the  substance,  the  oxalic  acid  is  233 
determined  in  one  portion  according  to  §  137,  b,  or  d,  and  the 
phosphoric  acid  in  another  portion.     If  the  substance  is  solu- 


734  SEPARATION.  [§  166. 

ble  in  water,  and  the  quantity  of  oxalic  acid  inconsiderable, 
the  phosphoric  acid  may  be  precipitated  at  once  with  magne- 
sium chloride,  ammonium  chloride,  and  ammonia :  if  not,  the 
substance  is  ignited  with  potassium  carbonate  and  sodium  car- 
bonate, and  the  oxalic  acid  being  thus  destroyed,  the  phos- 
phoric acid  is  determined  in  the  nitric  acid  solution  of  the 
residue  according  to  §  134,  I.,  5,  ft. 

e.  From  Hydrofluoric  Acid. 

a.  Phosphates  and  fluorides  are  frequently  found  together  234 
in  minerals.  In  the  analysis  of  phosphorites,  for  instance,  we 
have  to  estimate  small  quantities  of  fluorine,  often,  too  in  the 
presence  of  aluminium  and  iron,  which  increase  the  difficulty. 
According  to  my  own  experience,*  it  is  always  safest  in  such 
cases  to  estimate  in  one  portion  the  fluorine  as  silicon  fluoride 
(§  138,  II.,  3,  #•),  and  in  another  portion  the  phosphoric  acid. 
Regarding  the  first  estimation,  it  must  be  mentioned  that  car- 
bonic acid  if  present  must  first  be  removed.  To  this  end  heat 
the  finely  powdered  weighed  substance  with  water,  add  acetic 
acid  in  slight  excess,  and  also,  if  the  fluoride  present  is  soluble 
in  water,  some  calcium  acetate  ;  evaporate  to  dryness  on  a  water 
bath,  treat  with  water,  filter,  wash  the  insoluble  matter,  dry, 
separate  as  far  as  possible  from  the  filter,  add  the  filter  ash, 
weigh,  test  a  small  portion  for  carbonic  acid  by  heating  with 
hydrochloric  acid,  and  weigh  the  rest  for  the  fluorine  estima- 
tion. For  the  estimation  of  the  phosphoric  acid,  dissolve  the 
finely  powdered  substance  in  hydrochloric  acid,  evaporate  to 
dryness  on  a  water-bath,  moisten  with  a  little  hydrochloric 
acid,  add  nitric  acid,  warm,  dilute,  filter,  evaporate  filtrate  and 
washings  to  dryness,  dissolve  in  nitric  acid,  and  proceed 
according  to  §  134,  I.,  &,  ft. 

ft.  Where  you  have  an  alkali  phosphate  and  an  alkali  235 
fluoride  together  in  aqueous  solution  the  phosphoric  acid  may 
be  separated  according  to  §  135,  II.,  d,  ft,  as  silver  phosphate, 
or  according  to  §  135,  II.,  &,  as  mercurous  phosphate.  The 
fluoride  will  be  all  in  the  filtrate.  If  the  former  method  is 
adopted  the  silver  is  removed  from  the  filtrate  by  sodium 
chloride,  and  the  fluorine  estimated  as  calcium  salt  (§  138, 1.). 

*  Zeitschr.  f.  analyt.  Chem.,  v,  190,  and  vi,  403. 


§  166.]  ACIDS   OF   GROUP   I.  735 

If  the  latter  method  is  adopted,  as  the  solution  is  always  acid, 
the  use  of  glass  and  porcelain  must  be  avoided.  The  mercury 
is  remove')  f'-om  the  filtrate  by  neutralizing  with  sodium  car- 
bonate {in-— without  filtering — passing  hydrogen  sulphide. 
The  fluorine  is  estimated  in  the  filtrate  as  calcium  salt,  accord- 
ing to  §  138,  I.  (II.  KOBE). 

stances  which  are  insoluble  in  water,  and  cannot  236 
be  decomposed  by  acids,  are  fused  with  sodium  carbonate  and 
silica  (227),  the  fused  mass  is  treated  with  water,  and  Jie 
solution  with  ammonium  carbonate.  In  this  way  all  the 
fluorine  and  all,  or  nearly  all,  the  phosphoric  acid  will  be 
brought  into  solution.  The  solution  is  treated  as  in  235,  and 
any  remainder  of  phosphoric  acid  in  the  undissolved  residue 
is  estimated  according  to  234. 

#.  In  compounds  decomposable  by  water,  fluorine  may  237* 
be  occasionally  estimated  indirectly  also.  Dissolve  in  hydro- 
chloric acid,  evaporate  with  a  slight  excess  of  sulphuric  acid 
until  all  the  hydrofluoric  acid  has  escaped  (the  amount  must 
not  be  increased  to  a  point  where  the  sulphuric  acid  will  be 
driven  off,  otherwise  some  phosphoric  acid  will  also  escape), 
and  determine  in  the  residue  the  phosphoric  acid  on  the  one 
hand ;  on  the  other  the  oxides.  If  now  the  proportion  be- 
tween the  phosphoric  acid  and  the  bases  in  the  compound 
investigated  is  known,  the  escaped  fluorine  may  be  calculated 
from  the  excess  of  bases.  It  is  assumed,  of  course,  that  other 
acids  must  not  be  present,  or  must  be  estimated  in  separate 
portions. 

4.   HYDROFLUORIC  ACID  FROM  OTHER  ACIDS. 

a.  Fluorides  from  B  orates. 

Mix  the  solution  containing  alkali  borate  and  fluoride  with  238 
some  sodium  carbonate,  and  add  calcium  acetate  in  excess.  A 
precipitate  is  formed,  which  contains  the  wrhole  of  the  fluorine 
MS  calcium  fluoride,  and  besides  this,  calcium  carbonate  and 
some  calcium  borate;  the  greater  portion  of  the  latter  having 
been  redissolved  by  the  excess  of  the  calcium  salt  added. 
Determine  the  calcium  fluoride  in  the  precipitate  as  directed 
§  138,  I.  The  small  quantity  ot  boric  acid  in  the  precipitate 
is,  in  this  process,  partly  volatilized,  partly  dissolved  after 
evaporating  the  mass  with  acetic  acid  and  extracting  with 


736  SEPARATION.  [§  166. 

water.  It  is  therefore  necessary  to  determine  the  boric  acid 
in  a  separate  portion  of  the  substance,  according  to  §  136,  I. 
2  (A.  STROMEYEK).* 

~b.  Fluorides  from  Silicic  Acid  and  Silicates. 

A  great  many  native  silicates  contain  fluorides :  care  must, 
therefore,  always  be  taken,  in  the  analysis  of  minerals,  not  to 
overlook  the  latter.  If  the  silicates  containing  .fluoride  are 
decomposable  by  acids — which  is  only  rarely  the  case — and 
the  silicic  acid  is  separated  in  the  usual  way  by  evaporation, 
the  whole  of  the  fluorine  may  volatilize. 

a.  BERZELIUS'S  method.  Fuse  the  elutriated  substance  239 
with  4  parts  of  sodium  carbonate  for  some  time  at  a  strong 
red  heat,  digest  the  mass  in  water,  boil,  filter,  and  wash,  first 
with  boiling  water,  then  with  ammonium  carbonate.  The  fil- 
trate contains  all  the  fluorine  as  sodium  fluoride,  and,  besides 
this,  sodium  carbonate,  silicate,  and  aluminate.  Mix  the  fil- 
trate with  ammonium  carbonate  and  heat  the  mixture,  replac- 
ing the  ammonium  carbonate,  which  evaporates.  Filter  off 
the  precipitate  of  hydrate  of  silicic  acid  and  aluminium 
hydroxide,  and  wash  with  ammonium  carbonate.  To  separate 
the  last  portions  of  silica  from  the  filtrate  add  a  solution  of 
zinc  oxide  in  ammonia,  evaporate  till  no  more  ammonia 
escapes,  and  filter  off  the  precipitate  of  zinc  silicate  and  oxide. 
Determine  the  silica  in  this  precipitate  by  dissolving  in  nitric 
acid,  evaporating  to  dryness,  taking  up  with  nitric  acid,  and 
filtering  off  the  undissolved  silica.  In  the  alkaline  filtrate 
estimate1  the  fluorine  as  calcium  salt  (§  138,  I.).  The  residue, 
insoluble  in  water,  and  the  precipitate  produced  by  ammonium 
carbonate  are  finally  treated  with  hydrochloric  acid  according 
to  §  140,  II.,  a,  in  order  to  separate  the  silica. 

fi.  In  substances  readily  decomposed  by  sulphuric  acid  you  240 
may  also  separate  and  weigh  the  silica  according  to  239  in  one 
portion,  and  determine  the  fluorine  in  another  portion  accord- 
ing to  §138,  II.,  3,  a. 

c.  Fluorides,  Silicates  and  Phosphates  together. 

Compounds  of  this  kind  are  not  rare  in  nature,  and  may  241 
be  decomposed  according  to  239.      We  cannot  alway  rely  on 

*  Annal,  d.  Chem.  u.  Pharm.,  c,  91. 


§  166.]  ACIDS   OF   GROUP  I.  737 

complete  decomposition  of  the  phosphate,  as,  for  instance,  cal- 
cium phosphate  is  but  partially  decomposed  on  fusion  with 
sodium  carbonate.  The  solution,  obtained  after  separation  of 
the  silica  by  ammonium  carbonate  and  the  zinc  solution,  is 
made  up  to  a  definite  volume,  and  a  portion  is  tested  for  phos- 
phoric acid  with  molybdic  solution.  If  none  is  present  the 
fluorine  is  estimated  in  the  measured  remainder  of  the  fluid  as 
fluoride  of  calcium  (§  138,  I.).  If  on  the  other  hand  phos- 
phoric acid  is  still  present,  treat  the  measured  remainder  of  the 
fluid  according  to  235.  In  the  original  residue  and  the  ammo- 
nium carbonate  precipitate  estimate  the  principal  amounts  of 
the  silicic  and  phosphoric  acids  and  the  basic  metals.  In  the 
zinc  precipitate  estimate  the  remainder  of  the  silicic  acid,  and 
in  the  filtrate  from  the  latter  estimate  the  portion  of  the 
phosphoric  acid  which  was  thrown  down  by  zinc  oxide. 

As  the  phosphoric  acid  is  so  divided  by  this  method,  it  is 
well  to  make  a  direct  estimation  of  it  in  another  portion  of  the 
substance,  especially  when  only  a  small  quantity  is  present. 
For  this  purpose  decompose  the  silicate  with  hydrofluoric  and 
hydrochloric  acids,  add  enough  but  not  too  large  an  excess  of 
sulphuric  acid,  and  evaporate  till  all  the  fluorine  has  escaped  as 
silicon  fluoride  and  hydrofluoric  acid.  Do  not  increase  the  heat 
to  the  escape  of  sulphuric  acid,  or  phosphoric  acid  may  be  lost, 
•Take  up  the  residue  with  nitric  acid,  dilute,  filter,  and  estimate 
the  phosphoric  acid  in  the  filtrate  by  the  molybdic  method, 
page  446. 

If  the  substance  can  be  easily  decomposed  with  sulphuric 
acid,  the  fluorine  may  of  course  also  be  expelled  as  silicon 
fluoride  and  estimated  according  to  §  138,  II.,  3,  a. 

5.  SILICIC  Aero  FKOM  ALL  OTHER  ACIDS. 

a.  In  'compounds  which  are  decomposed  ~by  hydrochloric 
acid. 

Decompose  the  substance  by  digestion  with  hydrochloric  242 
or  nitric  acid,  evaporate  the  whole  on  the  water  hath  to  dryness 
(§  140,  II.,  #),  treat  with  water,  hydrochloric  acid  or  nitric  acid 
according  to  circumstances,  filter  off  the  silica,  and  estimate 
the  other  acids  in  the  filtrate.  The  following  points  require 
attention. 

a.  In  the  presence  of  borates  or  fluorides  this  method  cannot 
be  used;  employ  243. 


738  SEPARATION.  [§  166. 

/3.  In  the  presence  of  phosphoric  acid  the  silica  always 
retains  a  small  portion,  which  cannot  be  extracted  by  washing 
with  acidified  water  (H.  ROSE,  W.  SKEY*).  After  washing 
the  silica  with  water,  treat  it  repeatedly  with  ammonia,  which 
will  leave  only  a  very  minute  quantity  of  the  phosphoric  acid. 
Evaporate  the  ammoniacal  fluid,  finally  adding  a  little  hydro- 
chloric acid,  dissolve  in  water  with  addition  of  a  little  nitric 
acid,  filter  off  the  small  amount  of  silica  which  was  taken  up 
by  the  ammonia,  and  estimate  the  remainder  of  the  phosphoric 
acid  in  the  filtrate. 

1).  In  compounds  which  are  not  decomposed  l>y  hydrochlo- 
ric acid. 

Fuse  with  carbonate  of  potash  and  soda  (p.  511)  and  treat  243 
the  residue  either  at  once  cautiously  with  dilute  hydrochloric 
or  nitric  acid,  in  order  to  proceed  with  the  solution  according 
to  242  (not  applicable  in  presence  of  boric  acid  or  fluorine) ; 
or  taking  the  fluid  obtained  by  boiling  the  residue  with  water, 
precipitate  the  dissolved  silica  by  warming  with  ammonium 
carbonate,  and  throw  down  the  last  portion  of  silica  from  the 
filtrate  of  zinc  oxide  dissolved  in  ammonia  (239). 

The  silicic  acid  is  then  found  partly  in  the  residue  left  un- 
dissolved  by  water,  partly  in  the  precipitate  produced  by 
ammonium  carbonate,  and  partly  in  the  precipitate  produced 
by  the  zinc  solution.  Separate  it  according  to  §  140,  II.,  a. 
Boric  acid  and  fluorine  will  be  found  entirely  in  the  last  alka- 
line filtrate  (239).  Regarding  phosphoric  acid  see  241. 
Sulphuric  acid  passes  for  the  most  part  into  the  last  alkaline 
filtrate,  yet  it  is  well  also  to  examine  the  acid  filtrates  from 
the  silica. 

6.   CARBONIC  ACID  FROM  ALL  OTHER  ACIDS. 

When  carbonates  are  heated  with  stronger  acids,  the  car-  244 
bonic  acid  is  expelled  ;  the  presence  of  carbonates,  therefore, 
does  not  interfere  with  the  estimation  of  most  other  acids. 
And  as,  on  the  other  hand,  the  carbonic  acid  is  determined  by 
the  loss  of  weight  or  by  combination  of  the  expelled  gas,  the 
presence  of  salts  of  non-volatile  acids  does  not  interfere  with 
the  determination  of  the  carbonic  acid.  Accordingly,  with 
compounds  containing  carbonates,  sulphates,  phosphates,  &c., 

*  Zeitschr.  f.  analyt.  Chem.,  vm,  70. 


§  167.]  ACIDS   OF  GROUP  II.  739 

either  the  carbonic  acid  is  determined  in  one  portion,  and  the 
other  acids  in  another,  or  both  estimations  are  pei  formed  on 
one  portion.  In  the  latter  case  the  process  described  on  p.  500, 
£,  or  p.  493,  </,  may  be  used  with  advantage,  the  other  acids 
being  determined  in  the  solution  remaining  in  the  decomposing 
flask.  In  presence  of  fluorides,  one  of  the  weak  non-volatile 
acids,  such  as  tartaric  acid  or  citric  acid,  must  be  employed  to 
expel  the  carbonic  acid  ;  since,  were  sulphuric  or  hydrochloric 
acid  used,  part  of  the  liberated  hydrofluoric  acid  would  escape 
with  the  carbonic  acid.  If,  as  will  occasionally  happen  in  an 
analysis,  a  mixed  precipitate  of  calcium  fluoride  and  calcium, 
carbonate  is  thrown  down  from  a  solution,  the  two  salts  may 
be  separated  by  evaporating  with  acetic  acid  to  dryness,  and 
extracting  the  residue  with  water ;  the  calcium  acetate  formed 
from  the  carbonate  is  dissolved,  the  calcium  fluoride  is  left 
behind. 

Second  Group. 

CHLORINE BROMINE — IODINE CYANOGEN SULPHUR. 

I.  SEPARATION  OF  THE  ACIDS  OF  THE  SECOND  GROUP  FROM  THOSE 

OF  THE  FIRST. 

§167. 

a.  All  the  Acids  of  the  Second  Group  from  those 
of  the  First. 

Mix  the  dilute  solution  with  nitric  acid,  add  silver  nitrate  245 
in  excess,  and  filter  off  the  insoluble  chloride,  bromide,  iodide, 
&c.,  of  silver.     The  filtrate  contains  the  whole  of  the  acids  of 
the  first  group,  the  silver  salts  of  these  acids  being  soluble  in 
water  or  nitric  acid.     Carbonic  acid  must,  under  all  circum- 
stances, be  determined  in  a  separate  portion  (§  139,  d,  e,  or  g). 
If  method  d  or  g  is  used,  the  remarks  on  p.  489   must  be 
borne  in  mind. 


740  SEPARATION.  [§  167. 

5.  Some  of  the  Acids  of  the  Second  Group  from 
Acids  of  the  First  Group. 

As  it  is  often  inconvenient  for  the  further  separation  of  246 
the  acids  of  the  second  group  to  have  them  all  in  the  form  of 
insoluble  silver  compounds,  the  analysis  is  sometimes  effected 
by  separating  first  the  acid  of  the  first  group,  then  that  of  the 
second.  If  the  quantity  of  substance  is  large  enough,  the 
most  convenient  way  generally  is  to  determine  the  several 
acids,  e.g.,  sulphuric  acid,  phosphoric  acid,  chlorine,  sulphur, 
&c.,  in  separate  portions. 

Of  the  infinite  number  of  combinations  that  may  present 
themselves  we  will  here  consider  only  the  most  important. 

1.  SULPHURIC  ACID  may  be  readily  separated  from  chlorine,  247 
bromine,  iodine,  and  cyanogen,  by  precipitation  with  a  barium 
salt.     If  the  acids  of  the  second  group  are  to  be  determined 

in  the  same  portion,  barium  nitrate  or  acetate  is  used  instead 
of  barium  chloride.  In  presence  of  hydrogen  sulphide,  sul- 
phuric acid  cannot  be  determined  in  this  way,  as  part  of  the 
hydrogen  sulphide  would  be  converted  into  sulphuric  acid  by 
the  oxygen  of  the  air.  The  error  thus  introduced  into  the 
process  may  be  very  considerable  (FRESENIUS*)  The  hydrogen 
sulphide  must,  therefore,  first  be  removed  by  cupric  chloride, 
and  the  sulphuric  acid  determined  in  the  filtrate ;  or,  the 
hydrogen  sulphide  must  be  completely  oxidized  into  sulphuric 
acid  by  chlorine  or  bromine,  and  a  corresponding  deduction 
afterwards  made  in  calculating  the  quantity  of  the  sulphuric 
acid.  In  other  cases  it  is  well  to  expel  the  hydrogen  sulphide 
according  to  §  148,  0,  by  heating  with  hydrochloric  acid,  and 
to  estimate  the  sulphuric  acid  in  the  residual  fluid. 

2.  PHOSPHORIC  ACID  may  be  precipitated  by  ammonium  248 
magnesium    nitrate,   after   addition   of   ammonium    nitrate; 
OXALIC  ACID  by  calcium  nitrate  ;  chlorine,  bromine,  iodine,  &c.. 

are  determined  in  the  filtrate. 

3.  CHLORINE  IN  SILICATES. 

a.  If  the  silicates  dissolve  in  dilute  nitric  acid,  precipitate  249 
the  highly  dilute  solution  with  silver  nitrate,  without  applying 
heat,  remove  the  excess  of  silver  from  the  filtrate  by  dilute 

*  Journ.f.  pmkt.  Chem.,  LXX,  9. 


§  167.J  ACIDS   OF   GROUP   II.  741 

hydrochloric  acid,  still  without  applying  heat,  and  then  sepa- 
rate the  silicic  in  the  usual  way. 

b.  If  the  silicate  becomes  gelatinous  upon  decomposition 
with  nitric  acid,  dilute,  allow  to  deposit,  filter,  wash  the  sepa- 
rated silicic  acid,  and  treat  the  filtrate  as  in  a. 

In  the  processes  a  and  5  the  silver  chloride  may  contain 
silica.  Reduce  the  weighed  silver  salt  by  hydrogen  and  treat 
with  nitric  acid ;  the  silica  will  remain  behind. 

c.  If  nitric  acid  fails  to  decompose  the  silicates,  mix  the 
substance  with  sodium  and  potassium  carbonates,  moisten  the 
mass  with  water,   dry  in  the  crucible,  fuse,  boil  with  water, 
remove  the  dissolved  silicic  acid  by  ammonium  carbonate  and 
zinc  oxide  dissolved  in  ammonia  (239),  and  then  precipitate, 
after  addition  of  nitric  acid,  with  silver  nitrate. 

d.  If  the  silicates  are  readily  decomposed  by  acids,  the 
chlorine  may  be  estimated  by  heating  them  with  moderately 
concentrated  sulphuric  acid  and  collecting  the  hydrochloric- 
acid  gas  in  receivers,  the  first  of  which  contains  water,  the 
second  water  containing    ammonia.      During  the    operation 
conduct  a  current  of  air  through  the  apparatus  and  heat  until 
copious  furnes  of  sulphuric  acid  pass  over.     If  suitable  pro- 
vision be  made  for  the  current  of  air,  the  apparatus,  Fig.  78, 
will  answer  the  purpose  (H.  ROSE).      The  hydrochloric  acid 
in  the  receivers   is  estimated  according  to  §  141,   a.      As 
methods  a  and  J  may  perhaps  yield  a  silver  chloride  contain- 
ing silicic  acid,  it  is  advisable  to  reduce  by  ignition  in  a  cur- 
rent of  hydrogen  and  to  treat  the  residue  with  nitric  -acid. 
Any  silicic  acid  remains  behind. 

4.  CHLOBIDES  IN  PRESENCE  OF  FLUORIDES. 

If  the  substance  is  soluble  in  water,  the  separation  may  be  250 
effected  as  directed  in  245  ;  but  it  is  more  convenient  to  precipi- 
tate the  fluorine  with  calcium  nitrate,  and  the  chlorine  in  the 
filtrate  with  silver  nitrate.     Insoluble  compounds  are  fused 
with  sodium  carbonate  and  silicic  acid,  and  treated  as  in  251. 

5.  CHLORIDES  IN  PRESENCE  OF  FLUORIDES  IN  SILICATES. 
Proceed  as  directed  in  239.      Saturate  the  alkaline  filtrate  251 

nearly  with  nitric  acid,  precipitate  with  calcium  nitrate,  sepa- 
rate the  calcium  fluoride  and  carbonate  as  directed  in  244,  and 
precipitate  the  chlorine  in  the  filtrate  by  silver  nitrate. 


742  SEPARATION.  [§  168. 

6.  SULPHIDES  IN  SILICATES. 

If  the  substance  is  decomposable  by  acids,  reduce  it  to  the  252 
very  finest  powder  and  treat  with  fuming  nitric  acid  free  from 
sulphuric  acid  (§148,  II. ,  2,  #),  or  with  rather  dilute  nitric 
acid  in  sealed  tubes  at  120°-150°  (CARIUS  *).  When  the  sul- 
phur is  completely  oxidized,  rinse  the  contents  of  the  flask  or 
tube  into  a  dish,  evaporate  on  the  water-bath,  treat  with  hydro- 
chloric or  nitric  acid,  dilute,  filter  off  the  silica,  and  determine 
in  the  filtrate  the  sulphuric  acid  formed.  If,  on  the  contrary, 
the  substance  is  not  decomposable  by  acids,  fuse  with  4  parts 
of  sodium  carbonate  and  1  part  of  potassium  nitrate,  boil  the 
fused  mass  with  water,  filter,  remove  the  dissolved  silicic  acid 
from  the  filtrate  by  acidifying  with  hydrochloric  or  nitric  acid 
a-nd  evaporating,  and  proceed  as  above  directed. 

7.  SULPHIDES  IN  PRESENCE  OF  CARBONATES. 

If  you  have  to  estimate  sulphur  in  sulphides,  which  can  253 
easily  be  decomposed  by  acids  (e.g.,  calcium  sulphide),  in  pres- 
ence of  carbonates,  decompose  the  substance  by  heating  with 
hydrochloric  acid,  dry  the  evolved  mixture  of  hydrogen  sul- 
phide and  carbonic  acid,  take  up  the  hydrogen  sulphide  by 
tubes  filled  with  pumice  prepared  with  cupric  sulphate  (p. 
561),  and  the  carbonic  acid  by  soda-lime  tubes  (p.  493).  For 
details  see  "  Analysis  of  Black  Ash  "  in  the  Special  Part. 


Supplement. 

ANALYSIS  OF  COMPOUNDS,  CONTAINING  ALKALI  SULPHIDES,  CARBON- 
ATES, SULPHATES,  AND  THIOSULPHATES. 

§  168. 

The  following  method  was  first  employed  by  G.  WERTHER  t  254 
in  the  examination  of  gunpowder  residues.     N.  FEDOROW  £  has 
shown  that  the  original  process  included  an  error,  which  has 
been  put  right  in  the  method  described  below. 

Put  the  substance  into  a  flask,  add  water,  in  which  a  suf- 
ficient quantity  of  cadmium  carbonate  §  is  suspended  ;  cork, 
and  shake  the  vessel  well.  The  alkali  sulphide  decomposes 

*Comp.  "  The  Determination  of  Sulphur  in  Organic  Substances." 
\  Journ.  f.  praM.  Chem.,  LV,  22.        \Zeitschr.  f.  analyt.  Chem.,  ix,  127. 
§  To  obtain  the  cadmium  carbonate  free  from  alkali,  ammonium  carbonate 
must  be  used  as  precipitant. 


§  168.]  ACIDS    OF  GROUP  II.  743 

completely  with  the  cadmium  carbonate.  Filter  the  yellowish 
precipitate  off,  and  treat  it  with  dilute  acetic  acid  (not  with 
hydrochloric  acid) ;  the  cadmium  carbonate  dissolves,  the 
cadmium  sulphide  is  left  tmdissolved.  Oxidize  the  latter  with 
potassium  chlorate  and  nitric  acid  (p.  567),  or  with  bromine 
(p.  568),  and  precipitate  with  barium  chloride  the  sulphuric 
acid  formed  from  the  sulphide. 

Heat  the  fluid  filtered  from  the  yellow  precipitate,  and 
mix  with  solution  of  neutral  silver  nitrate.  The  precipitate 
consists  of  silver  carbonate  and  silver  sulphide  (K2S2O3-|- 
2A-N03+TI20=Iv2SO,+Ag2S+2PI^03).  Filter  it  off,  and 
wash  with  carbonic  acid  water,  then  remove  the  silver 
carbonate  by  ammonia  and  precipitate  the  silver  from  the 
ammoniacal  solution  by  acidifying  with  nitric  acid  and  adding 
sodium  chloride.  -2  mol.  silver  chloride  so  obtained  corre- 
spond to  1  mol.  carbonate.*  Dissolve  the  silver  sulphide  in 
dilute  boiling  nitric  acid,  determine  the  silver  in  the  solution 
as  silver  chloride,  and  calculate  from  the  result  the  quantity 
of  the  thiosulphuric  acid;  1  mol.  AgCl  corresponds  to  1  at. 
sulphur  in  thiosulphuric  acid,  or  2  AgCl  correspond  to  K2S2O3. 

From  the  fluid  filtered  from  the  silver  sulphide  and 
carbonate  remove  first  the  excess  of  silver  by  means  of 
hydrochloric  acid,  and  then  precipitate  the  sulphuric  acid  by 
a  barium  salt.  From  the  sulphuric  acid  found  you  have,  of 
course,  to  deduct  the  quantity  of  that  acid  resulting  from  the 
decomposition  of  the  thiosulphuric  acid,  and  accordingly  for  1 
part  of  silver  chloride  formed  from  the  sulphide,  0-279  parts  of 
sulphuric  anhydride  (SO3).  The  difference  gives  the  amount 
of  sulphuric  acid  originally  present  in  the  analyzed  compound. 
By  way  of  control,  you  may  determine,  in  the  fluid  filtered, 
from  the  barium  sulphate,  the  alkali  as  sulphate  (§  97  or  §  98). 
Compare  also  "  Analysis  of  Black  Ash  "  and  the  "  Lye  from 
Soda  Residues  "  in  the  Special  Part. 

*  A  quantity  equivalent  to  the  sulphur  found  existing  as  sulphide  has  to  be 
deducted  from  this  (K2S  -f-  CdCO3  =  CdS  -j-  KaCO3).  On  the  other  hand,  a  quan- 
tity equivalent  to  the  sulphide  of  silver  precipitated  by  the  thiosulphate  must  be 
added,  for  each  mol.  of  sulphide  of  silver  from  the  thiosulphate  gives  2  njol. 
HNO3,  which  decomposes  1  mol.  carbonate  of  silver.  This  correction  was  over- 
looked by  WERTHER. 


744  SEPARATION.  [§  169. 

II.  SEPARATION  OF  THE  ACIDS  OF  THE  SECOND  GEOUP  FEOM 

EACH  OTHEE. 

§169. 

1.  CHLOEINE  FEOM  BEOMINE. 

All  the  methods  of  direct  analysis  hitherto  proposed  to 
effect  the  separation  of  chlorine  from  bromine  are  defective. 
The  bromine  is  therefore  always  determined  in  a  more  indi- 
rect way. 

a.  Precipitate  with  silver  nitrate,  wash  the  precipitate,  255' 
wash  it  from  the  filter  into  a  porcelain  dish,  extract  the  filter 
with  hot  ammonia,  evaporate  the  ammonia  in  a  weighed  porce- 
lain crucible,  add  the  principal  quantity  of  the  precipitate, 
dry,  fuse,  and  weigh.  Transfer  an  aliquot  part  of  the  mixed 
silver  chloride  and  bromide  to  a  light  weighed -bulb-tube  of 
nard  glass,*  fuse  in  the  bulb,  let  the  mass  cool,  and  weigh. 
This  operation  gives  both  the  total  weight  of  the  tube  with 
its  contents,  and  the  weight  of  the  portion  of  mixed  silver 
chloride  and  bromide  in  the  bulb.  The  greatest  accuracy  in 
the  several  weighings  is  indispensable.  Now  transmit  through 
the  tube  a  slow  stream  of  dry  pure  chlorine  gas,  heat^the  con- 
tents of  the  bulb  to  fusion,  and  shake  the  fused  mass  occasion- 
ally about  in  the  bulb.  After  the  lapse  of  about  20  minutes,, 
take  off  the  tube,  allow  it  to  cool,  hold  it  in  an  oblique  posi- 
tion, that  the  chlorine  gas  may  be  replaced  by  atmospheric 
air,  and  then  weigh.  Heat  once  more  for  about  10  minutes  in 
a  stream  of  chlorine  gas,  and  weigh  again.  If  the  two  last 
weighings  agree,  the  experiment  is  terminated ;  if  not,  the 
operation  must  be  repeated  once  more.  The  loss  of  weight 
suffered,  multiplied  by  4*2218  (which  may  be  taken  as  4 '222), 
gives  the  quantity  of  the  silver  bromide  decomposed  by  the 
chlorine.  For  the  proof  of  this  rule,  see  "  Calculation  of 
Analyses,"  §  200. 

Tliis  method  gives  very  accurate  results,  if  the  proportion 
of  bromine  present  is  riot  too  small ;  but  most  uncertain 

*  The  best  way  of  effecting  the  removal  of  the  fused  mass  from  the  crucible 
is  to  f  use  again  and  then  pour  out. 


§  169.]  ACIDS  OF  GROUP  II.  745 

results  in  cases  where  mere  traces  of  bromine  have  to  be 
determine9  in  presence  of  large  quantities  of  chlorides,  as,  for 
instance,  in  salt-springs.  To  render  the  method  available  in 
such  cases,  the  great  point  is  to  produce  a  silver  compound 
containing  nil  the  bromine,  and  only  a  small  part  of  the 
chlorine.  This  end  may  be  attained  in  several  ways.  In 
these  processes  the  quantity  of  chlorine  is  found  by  completely 
precipitating  a  separate  portion  with  silver  solution,  and 
deducting  the  silver  bromide  found  from  the  weight  of  the 
precipitate. 

a.  Mix  the  solution  with  sodium  carbonate  in  excess  (if  a 
precipitate  is  formed,  do  not  filter),  evaporate  to  dryness, 
powder  the  residue,  extract  with  hot  absolute  alcohol;  the 
solution  contains  the  whole  of  the  alkali  bromide,  and  only  a 
small  portion  of  the  alkali  chloride ;  add  a  drop  of  soda  solu- 
tion, and  evaporate,  dissolve  the  residue  in  water,  acidify  with 
nitric  acid,  and  precipitate  with  silver  solution. 

/j.  FKJILING'S  method.*  Mix  the  solution  cold  with  a  256 
quantity  of  solution  of  silver  nitrate  not  nearly  sufficient  to 
effect  complete  precipitation,  shaking  the  mixture  vigorously, 
and  leave  the  precipitate  for  some  time  in  the  fluid,  with 
repeated  shaking.  If  the  amount  of  the  precipitate  produced 
corresponds  at  all  to  the  quantity  of  bromine  present,  the 
whole  of  the  latter  substance  is  obtained  in  the  precipitate. 

FEHLING  gives  the  following  rule  : 

If  the  fluid  contains  1  bromine  to  1000  chlorine  use  \  or  -J- 
the  quantity  of  silver  nitrate  that  would  be  required  to  effect 
complete  precipitation ;  if  the  fluid  contains  10,000  times  as 
much  chlorine  as  bromine,  use  -j^;  if  50,000,  use  ^ ;  if 
100,000,  use  -gV 

Wash  the  mixed  precipitate  of  silver  chloride  and  bromide 
thoroughly,  dry,  ignite,  weigh,  and  treat  with  chlorine  as 
above. 

y.  MARCH  ANDf  has  slightly  modified  FEHLING'S  method.  257 
He  reduces  with  zinc  the  mixed  precipitate  of  silver  chloride 
and  bromide  obtained  by  FEHLIXG'S  fractional  precipitation, 
decomposes  the  solution  of  zinc  chloride  and  bromide  with 
sodium  carbonate,  evaporates  to  dry  ness,  and  extracts  the  resi- 


*Journ.f.  prakt.  Cliem.,  XLV,  269.  f  Ib.,  XLVII, 


746  SEPARATION.  [§  169. 

due  with  absolute  alcohol,  which  dissolves  all  the  sodium 
bromide  with  only  a  little  of  the  sodium  chloride ;  he  then 
evaporates  the  solution  to  dryness,  takes  up  the  residue  with 
water,  precipitates  again  with  silver  nitrate,  and  subjects  a 
part  of  the  weighed  precipitate  to  the  treatment  with  chlorine. 

d.  If  a  fluid  containing  chlorides  in  presence  of  some 
bromide  is  heated  in  a  retort  with  hydrochloric  acid  and 
manganese  dioxide,  the  whole  of  the  bromine  passes  over  before 
any  of  the  chlorine.  Upon  this  circumstance  MOHR  *  bases 
the  following  method  for  effecting  the  concentration  of 
bromine : — Distil  as  stated,  and  conduct  the  vapors,  through  a 
doubly  bent  tube,  into  a  wide  WOULF'S  bottle,  which  contains 
some  strong  ammonia.  Dense  fumes  form  in  the  bottle,  fill- 
ing it  gradually.  Conduct  the  excess  of  vapors  from  the  first 
into  a  second  bottle,  with  narrow  neck,  containing  ammoniated 
water.  Both  bottles  must  be  sufficiently  large  to  allow  no 
vapors  to  escape.  When  the  whole  of  the  bromine  is  evolved, 
which  may  be  distinctly  seen  by  the  color  of  the  space  above 
the  liquid  in  the  retort  and  tubes,  raise  the  cork  of  the  flask 
to  prevent  the  receding  of  ammonium-bromide  fumes.  Let 
the  apparatus  cool,  and  unite  the  contents  of  the  two  bottles ; 
the  fluid  contains  the  whole  of  the  bromine,  with  a  rela- 
tively small  portion  of  the  chlorine. 

5.  Instead  of  treating  the  silver  chloride  and  bromide  in  258 
a  current  of  chlorine  as  in  &,  the  mixture  may  be  reduced  to 
metallic  silver  in  a  current  of  hydrogen.  After  the  weight 
of  the  silver  has  been  accurately  ascertained,  calculate  its 
equivalent  in  silver  chloride,  subtract  from  this  the  weight  of 
the  silver  chloride  and  bromide  which  had  been  reduced,  and 
thus  obtain  the  difference  which,  as  in  #,  served  as  a  point 
of  departure  for  calculation  (WACKENRODER).  The  method 
possesses  no  advantage  over  #,  because  a  long-continued  and 
very  strong  heat  is  required  in  the  current  of  hydrogen  in 
order  to  completely  reduce  the  silver  bromide.  It  will  be 
seen  that  one  and  the  same  portion  of  silver  bromide  and 
chloride  may  be  treated  first  as  in  #,  and  then,  for  control,  as 
in  b.  The  difference  found  direct  in  a,  and  calculated  in  5, 

*  Annal.  d.  Chem.  u.  Pharm.,  xcm,  80. 


§169.]  ACIDS    OF   GROUP  II.  747 

between  the  silver  chloride  and  bromide,  must  be  equal  to 
the  silver  chloride  equivalent  to  it. 

c.  FR.   MOHB*  recommends  precipitating  the    bromine  259 
and  a  part  of  the  chlorine  by  a  known  quantity  of  silver,  and 
then  weighing  the  precipitate  of  silver  chloride.      It  is  seen 
that  this  method  will  yield  the  same  data  for  calculation  as  in 

Z>.  The  known  quantity  of  silver  used  for  precipitating  is 
weighed  either  directly  and  is  dissolved  in  nitric  acid,  or  it  is 
added  in  the  form  of  standard  silver  solution.  This  method 
is  more  convenient  than  that  given  under  #,  but  I  do  not  con- 
sider it  quite  so  accurate,  particularly  if  only  small  quantities 
of  bromine  are  present.  It  is  assumed  that  from  a  weighed 
quantity  of  silver  the  absolutely  correct  quantity  of  silver 
chloride  equivalent  to  it  is  obtained;  and  this  assumption 
cannot  be  realized  in  practice.  Errors  to  the  extent  of  some 
milligrammes  cannot  be  avoided,  hence  the  difference  might 
be  calculated  as  bromine,  even  when  none  is  present  at  all. 
The  method  given  under  a  is  not  nearly  so  likely  to  afford 
errors,  or  at  least  to  the  same  extent.  Further,  one  can 
ascertain  without  trouble  whether,  on  carefully  heating  silver 
chloride  in  a  current  of  chlorine,  any  change  in  weight  takes 
place,  and  thereby  rendering  an  error  of  0*5  milligramme  less 
excusable  than  one  of  2  milligrammes  incurred  by  converting 
2  or  3  grm.  silver  into  chloride;  and  this  is  scarcely  avoid- 
able, particularly  if  a  filter  is  used  in  the  process,  as  is  re- 
quired in  a  partial  precipitation,  in  which  case  the  precipi- 
tate always  subsides  less. 

d.  FisANi'sf  method  may  be  regarded  as  a  modification  260 
of  0,  wherein  a  known  quantity  of  silver  solution  is  added  in 
slight  excess,  the  precipitate  filtered  off,  and  the  silver  in  the  fil- 
trate estimated  with  starch  iodide  (page  349) .    The  precipitate  is 
weighed  as  in  c.     This  method  precludes  partial  precipitation. 

e.  Determine  in  a  portion  of  the  solution   the  chlorine  261 
-f-bromine  (by  precipitating  with  silver),  either  gravimetrically 

or  volu metrically ;  in  another  portion  the  bromine,  either  by 
the  colorimetric  method  (§  143,  L,  J,  or  or  /?)  or  volumetrically 
(§  143,  I.,  £>,  y}.  Calculate  the  chlorine  from  the  difference. 

*  Annal.  d.  Chem.  u.  Pharm.,  xcm,  76. 

f  Compt.  rend.,  XLIV.  352  ;  Journ.f.  prakt.  Chem.,  LXXII,  266. 


748  SEPARATION.  [§  169. 

The  method  is  very  suitable  for  an  expeditious  analysis  of 
mother-liquors. 

/.  Compare  also  271  and  272. 

2.   CHLORINE  FROM  IODINE. 

a.  Add  to  the  solution  palladious  nitrate,  and  determine  262 
the  precipitated  palladious  iodide  as  directed  §145,  I.,  a,  fi. 
Conduct  hydrogen  sulphide  into  the  filtrate  to  remove  excess 
of  the  palladium,  destroy  the  excess  of  hydrogen  sulphide  by 
solution  of  ferric  sulphate,  and  precipitate  the  chlorine  finally 
with  solution  of  silver.  It  is  generally  found  more  simple 
and  convenient  to  precipitate  from  one  portion  the  iodine,  by 
means  of  palladious  chloride,  as  directed  §  145,  I.,  «,  ytf,  from 
another  portion  the  chlorine  and  iodine  jointly  with  silver 
nitrate,  and  to  calculate  the  chlorine  from  the  difference.  If 
you  have  no  solution  of  palladious  nitrate  ready,  and  the 
chlorine  and  iodine  must  be  determined  in  one  portion  of  the 
solution  under  examination,  add  a  measured  quantity  of  a 
solution  of  palladious  chloride,  determine  the  amount  of  chlo- 
rine in  this  and  in  another  exactly  equal  portion  of  the  same 
solution,  and  deduct  it.  The  results  are  accurate.  In  the  case 
of  fluids  containing  a  large  proportion  of  alkali  chlorides  to  a 
small  quantity  of  iodide — and  such  cases  often  occur — the 
iodide  is  concentrated  by  adding  sodium  carbonate  to  the  fluid, 
evaporating  to  dryness,  extracting  the  residue  with  hot  alcohol, 
evaporating  the  alcoholic  solution  with  addition  of  a  drop  of 
solution  of  soda,  and  taking  the  residue  up  with  water. 

J.  Proceed  exactly  as  for  the  indirect  determination  of  263 
bromine  in  presence  of  chlorine  (255).  The  greatest  care 
must  be  taken  that  as  little  as  possible  of  the  mixed  silver  chlo- 
ride and  iodide  adheres  to  the  filter,  for  silver  iodide  dissolves 
only  very  slightly  in  ammonia.  Any  particles  of  silver  iodide 
remaining  attached  to  the  filter  may  be  saved  by  incinerating 
the  filter  and  evaporating  the  ash  with  a  drop  of  nitric  acid 
and  a  drop  of  hydriodic  acid.  The  loss  of  weight  suffered  by 
the  silver  precipitate  on  fusion  in  chlorine  multiplied  by  2*569 
gives  the  amount  of  silver  iodide  present.  The  methods 
given  under  259  and  260  are  also  applicable.  These  methods 
give  still  better  results  than  in  the  separation  of  bromine  from 
chlorine,  inasmuch  as  the  difference  between  the  atomic 
weights  of  iodine  and  chlorine  is  far  greater  than  the  differ- 


§  169.]  ACIDS   OF  GROUP   II.  749 

ence  between  those  of  bromine  and  chlorine.     Regarding  the 
concentration  of  the  iodide,  if  necessary,  see  262. 

c.   Liberate  the  iodine  by  nitrous  acid,  take  it  up  with  car-  264 
bon  disulphide,  wash  the  latter,  and  then  estimate  the  iodine 
in  it  by  sodium  thiosulphate  (p.  537,  /?). 

In  this  process  the  chlorine  is  determined  either  in  the 
fluid  separated  from  the  violet  carbon  disulphide,  or  with 
greater  accuracy  by  precipitating  the  chlorine  -f-  iodine  in 
a  second  portion  with  silver  and  deducting  the  weight  of 
silver  iodide  corresponding  to  the  iodine  already  found  from 
the  weight  of  the  precipitate.  A  good  and  approved  method. 

If  the  quantity  of  iodine  is  small,  the  following  method 
may  also  be  used  with  advantage  for  estimating  it: 

The  carbon  disulphide  should  be  thoroughly  washed,  and 
covered  with  a  layer  of  water  in  a  stoppered  bottle.  Add 
drop  by  drop,  with  shaking,  dilute  chlorine  water  (of  unknown 
strength)  till  the  coloration  has  just  vanished  and  all  the 
iodine  is  consequently  converted  into  IC16.  Separate  the  solu- 
tion from  the  disulphide,  add  potassium-iodide  solution  in  suf- 
ficient excess,  and  determine  the  free  iodine  after  §  146.  Six 
parts  of  the  iodine  found  correspond  to  1  part  originally  pres- 
ent. If  the  analyst  would  avoid  the  trouble  of  pouring  off  the 
fluid  from  the  disulphide,  and  of  washing  the  latter,  he  may 
transfer  the  mixture,  after  the  addition  of  chlorine  to  decolora- 
tion, to  a  somewhat  narrow  measuring  cylinder,  note  the  vol- 
ume occupied  by  the  iodine-pentachloride  solution,  take  out 
a  portion  with  a  pipette,  and  proceed  as  above  directed. 

Instead  of  carbon  disulphide  MOEIDE*  uses  benzene,  while 
RoGERf  employs  chloroform;  and  instead  of  nitrous  acid, 
the  latter  uses  iodic  acid  for  liberating  the  iodine,  as  pre- 
viously recommended  by  LIEBIG,  a  dilute  solution  of  the 
reagent  being  added  to  the  dilute  fluid  acidulated  with  sul- 
phuric acid.  From  the  equation  5HI  +  HIO,  =  61  +  3H2O, 
it  follows  that  only  £  of  the  iodine  found  belonged  to  the 
iodide  originally  present. 

d.   Determine  in  one  portion  chlorine  and  iodine  as  in  265 
§  141,  I.,  5,  a-,  and  in  another  portion  the  iodine  alone  as  in 
§  145,  I.,  5,  y^  tf,  or  e.      The  chlorine  is  found  by  difference. 

*Compt.  rend.,  xxxv,  789;  Journ.  f.  prakt.  Cfom.,  LVIII,  317. 
•\Journ.  de  P/tarm.,  xxxvn,  410. 


750  SEPAKATIOH.  [§  169. 

The  method  in  §  145,  I.,  5,  £  (PisANi's)  is  very  rapid,  and 
still  gives  approximately  accurate  results  in  the  presence  of 
small  quantities  of  chloride;  if  much  chloride  is  present, 
however,  the  results  are  altogether  inaccurate  (see  page  540). 
The  method  in  §  145,  I.,  Z>,  y  (REINIGE'S)  cannot  be  em- 
ployed if  the  solution  contains  any  organic  or  other  sub- 
stances capable  of  reducing  potassium  permanganate.  The 
method  in  §  145,  I.,  5,  e  is  inapplicable  if  the  fluid  contains 
chlorates,  nitrites,  or  nitrates. 

e.  For  technical  purposes  the  following  method  is  also  266 
suitable.  It  was  recommended  by  WALLACE  and  LAMONT  * 
for  the  estimation  of  iodine  in  kelp.  The  kelp-lye  is  nearly 
neutralized  with  nitric  acid,  evaporated  to  dryness,  and  the 
residue  fused  in  a  platinum  vessel  to  oxidation  of  all  the  sul- 
phides. Treat  with  water,  filter,  add  silver  nitrate  till  the 
precipitate  appears  perfectly  white,  wash,  digest  with  strong 
ammonia,  and  weigh  the  residual  silver  iodide.  Finally 
add  to  the  weight  of  the  latter  the  amount  which  passes  into 
solution  in  the  ammonia;  it  is  -5-^3-  of  the  aqueous  am- 
monia (sp.  gr.  0*89)  used.  See  also  268,  271,  and  272. 

3.  CHLORINE,  BROMINE,  AND  IODINE  FROM  EACH  OTHER. 

a.  The  three  acid  radicals  are  determined  jointly  in  u  por-  267 
tion  of  the  fluid  by  precipitating  with  solution  of  silver 
nitrate  (§  141,  I.,  a  or  £,  a).  To  determine  the  iodine,  another 
portion  is  precipitated  with  palladious  chloride  in  the  least  pos- 
slble.exeess  (§  145,  L,  a,  ft).  The  fluid  filtered  from  the  pre- 
cipitate is  freed  from  palladium  by  hydrogen  sulphide  and  the 
excess  of  the  latter  removed  by  means  of  ferric  sulphate  ;  the 
chlorine  and  bromine  are  then  precipitated  jointly  either  com- 
pletely or  partially  with  silver  nitrate,  and  the  bromine  deter- 
mined as  directed  255. 

If  the  compound  contains  a  large  proportion  of  chlorine  to 
a  small  proportion  of  bromine,  the  iodine  may  be  precipitated 
also  by  palladious  nitrate,  as  there  is  no  danger,  in  that  case, 
of  palladious  bromide  being  coprecipitated.  The  filtrate  is 
treated  as  above. 

These  methods  give  accurate  results ;  but  they  are  appli- 

*Gliem.  Gaz  ,  1859,  137. 


§  169.]  ACIDS    OF   GROUP   II.  751 

cable  only  if  the  quantity  of  iodide  present  is  somewhat  con- 
siderable. 

1).  Mix  the  neutral  dilute  and  cold  solution  containing  alkali  268 
iodide  with  alkali  chloride  or  alkaki  bromide,  or  both,  with  a 
saturated  neutral  solution  of  thallium  nitrate,  stirring  well  till, 
on  repeated  trial,  you  obtain  a  transient  white  precipitate — 
the  first  and  permanent  precipitate  being  yellow.  It  is  best  to 
have  the  thallium  polution  in  a  burette,  so  that  you  can  easily 
add  it  by  drops.  If  the  white  precipitate  of  thallium  chloride 
or  bromide  does  not  at  once  disappear  on  stirring,  add  more 
water,  but  not  an  unnecessary  quantity,  or  some  of  the  thal- 
lium iodide  will  remain  in  solution. 

Allow  to  stand  eight  or  twelve  hours  in  a  cold  place,  pour 
ofl  the  clear  fluid  through  a  weighed  filter  dried  at  100°,  wash 
the  filter  a  little  so  that  no  more  water  than  necessary  may 
pass  through  the  precipitate,  turn  the  precipitate  on  to  the 
filter,  wash  with  as  little  water  as  you  can,  dry  at  100°,  and 
wreigh.  Precipitate  the  chlorine  and  bromine  in  the  filtrate 
by  silver  solution.  If  they  are  both  present,  the  mixed  silver 
precipitate  is  to  be  treated  according  to  255 .  Results  quite  satis- 
factory (HtJBNEB  and  SPEZIA,*  and  HUBNER  and  FKERiCHsf). 

c.  Remove  the  iodine  from  the  solution  by  carbon  disul-  269 
phide  or  chloroform,  as  in  264.     In  the  fluid  separated  from 

the  iodized  carbon  disulphide  determine  the  chlorine  and  bro- 
mine as  directed  in  255,  arid  in  the  iodized  carbon  disulphide, 
the  iodine  as  directed  in  §  145,  I.,  5,  /?.  This  method  is 
particularly  recommended  for  the  separation  of  small  quanti- 
ties of  iodine,  and  in  this  respect  is  supplementary  to  267. 

d.  Determine  in  a  portion  of  the  compound  the  chlorine,   270 
bromine,  and  iodine  jointly  by  adding  a  known  quantity  of 
standard  silver  solution  in  slight  excess,  filtering  and  deter- 
mining the  small  excess  of  silver  in  the  filtrate  by  iodide  of 
starch  (p.  349).      The  precipitate* is  weighed.     Compare  263. 
We  now  know  the  total  of  the  chloride,  bromide,  and  iodide 

of  silver  and  also  the  silver  therein  contained. 

Determine  the  iodine  separately  as  in  269,  calculate  the 

quantity  of  silver  iodide  and  of  silver  corresponding  to  the 

amount  found,  deduct  the  calculated  amount  of  silver  iodide 

from  the  mixed  iodide,  chloride,  and  bromide  of  silver,  that 

*  Zeitschr.  /.  analyt.  Chem.,  xi,  397.  f/6.,  xi,  400. 


752  SEPAEATION.  [§  169. 

of  the  silver  from  the  known  quantity  of  the  metal  contained 
in  the  mixed  compound ;   the  remainders  are  respectively  the 
joint  amount  of  chloride  and  bromide  of  silver  and  the  quan- 
tity of  the  metal  contained  therein ;    these  are  the  data  for . 
calculating  the  chlorine  and  bromine  (258). 

e.  On  the  fact  that  freshly  precipitated  silver  chloride  is  271 
converted  into  silver  bromide  by  a  solution  of  sodium 
bromide,  and  that  freshly  precipitated  silver  bromide  and 
chloride  are  converted  into  silver  iodide  by  potassium  iodide 
in  solution,  F.  FIELD*  has  based  the  following  method  of 
estimating  all  three  halogens,  if  present,  and  combined  with 
metals :  Introduce  three  weighed  portions  each  into  a  stop- 
pered flask,  add  to  each  about  30  c.  c.  water  and  an  excess 
of  silver  solution,  shake  vigorously,  and  thoroughly  wash  the 
precipitates  Nos.  I,  II,  and  III  with  water.  Dry  and  weigh 
No.  I ;  the  weight  represents  the  sum  of  the  silver  chloride, 
bromide,  and  iodide  present.  Then  digest  ~No.  II  with 
potassium-bromide  solution,  and  No.  Ill  with  potassium- 
iodide  solution,  for  10  hours,  taking  care  that  the  solutions 
are  dilute  and  not  added  in  too  great  excess,  and  avoiding 
warming,  otherwise  notable  quantities  of  silver  salts  will  be 
dissolved.  After  II  is  washed,  ignited,  and  weighed,  it  gives 
the  quantity  of  silver  bromide  and  iodide;  while  III  finally 
gives  pure  silver  iodide.  The  calculation  is  as  follows : 

a.  The  difference  between  the  equivalents  of  iodine  and 
chlorine  (=  91*4)  :  eq.  of  silver  chloride  (=  143*37)  :  :  dif- 
ference between  the  weights  of  I  and  II :  the  silver  chloride 
contained  in  I. 

P.  The  difference  between  the  equivalents  of  iodine  and 
bromine  (=  46-9)  :  eq.  of  silver  bromide  (=  187*87)  : :  dif- 
ference between  II  and  III  :  silver-bromide  content  of  II. 
On  deducting  the  silver  bromide  found  from  the  weight  of  the 
precipitate  II,  the  silver-iodide  Content  is  obtained. 

y.  Finally  on  subtracting  the  silver  chloride  found  in  a, 
together  with  the  silver  iodide  found  in  /?,  from  the  precipi- 
tate I,  the  weight  of  the  silver  bromide  is  obtained.  The 
method  is  of  interest  theoretically.  FIELD  obtained  quite 
satisfactory  results. 

The    method   was   later    on  thoroughly  investigated  by 

*  Quart.  Journ.  Chem.  Soc.,  x,  No.  39,  234;  Journ.  f.  prakt.  Chem.,  LXXIII, 
404;  also  Chem.  News,  n,  325. 


§  169.]  ACIDS  OF   GROUP   II.  753 

O.  HUSCHKE,*  and  also  by  M.  SiEWERT.f  The  former  used  a 
1 :  48  potassium-bromide  solution  and  a  1 :  34  potassium- iodide 
solution,  and  digested  with  a  moderate  excess  of  solution  for  1 
Lour.  He  obtained  5*248  and  5*206  grains  of  iodine  instead 
of  5-287;  3-313  and  3-349  grains  bromine  instead  of  3-333, 
and  1-477  and  1-496  grains  chlorine  instead  of  1-503  grains. 

SIEWERT  worked  with  both  cold  and  hot  solutions,  but 
obtained  less  satisfactory  results.  According  to  his  investiga- 
tions, the  conversion  of  silver  chloride  into  bromide  is  incom- 
plete, and  further,  on  boiling  silver  bromide  with  sodium- 
chloride  solution,  silver  chloride  is  found.  The  conversion 
of  silver  chloride  and  bromide  into  iodide,  however,  he 
found  to  be  perfectly  complete. 

FIELD'S  method,  hence,  can  at  most  be  used  only  when 
relatively  large  quantities  of  all  three  halogens  are  present, 
and  when  approximate  results  will  suffice.  The  method  is 
absolutely  inapplicable  in  the  analyses  of  mineral  waters  J  and 
more  particularly  when  only  very  small  quantities  of  iodides  and 
bromides  are  present  with  comparatively  large  quantities  of 
chlorides. 

f.  II.  HAGER'S  §  method  depends  upon  the  fact  that  freshly  272 
precipitated  silver  chloride  is  soluble  in  a  boiling  solution  of 
ammonium  carbonate,  while  only  traces  of  silver  bromide  dis- 
solve in  such  a  solution,  and  silver  iodide  is  almost  absolutely 
insoluble.  Regarding  the  details  of  this  method,  in  which 
the  silver  bromide  and  iodide  are  separated  by  ammonia,  I 
refer  to  the  original  paper.  The  method  can  be  employed 
only  when  approximate  results  suffice.  In  SONSTADT'S  J 
method  the  iodine  is  precipitated  as  barium  iodate. 

4.   ANALYSIS  OF  IODINE  CONTAINING  CHLORINE. 

a.  Dissolve  a  weighed  quantity   of  the   dried  iodine  in  273 
cold  sulphurous  acid,  precipitate  with  silver  nitrate,  digest  the 
precipitate  with  nitric  acid    to  remove  the   silver  sulphite 
which  may  have  coprecipitated,  and  weigh.     The  calculation 

*  Zeitschr.f.  analyt.  Chem.,  vu,  434. 

t  ZeitscJir.  f.  die  gesammt.  Naturwiss.,  1868,  No.  1;  Zeitschr.f.  analyt.  Chem., 
vii,  469. 

{J.  MITTEREGGER,  however,  employed  it  thus,  using  only  500  grin,  of 
mineral  water.  See  Chem.  Analyse  des  Radeiners  Sauerbrun,  by  Dr.  Jos.  Mn> 
TEREGGER,  Vienna,  1872,  published  W.  by  BRAUMULLER,  p.  5. 

%Pharm.  Centralbl.,  xn,  42  ;  Zeitschr.f.  analyt.  Chem.,  x,  341. 

||  Chem.  News,  xxvi,  173;  Zeitschr.  f.  analyt.  Chem.,  xii,  91. 


754  SEPARATION.  [§  169. 

of  the  iodine  and  chlorine  is  made  by  the  following  equations, 
in  which  A  represents  the  quantity  of  iodine  analyzed,  x  the 
iodine  contained  in  it,  y  the  chlorine  contained  in  it,  and  B 
the  amount  of  silver  chloride  and  iodide  obtained  : 

x  +  y  =  A 
and 


Now  as 
and 
we  have 


2-1935 


J.  If  you  have  free  iodine  and  free  chlorine  in  solution,  deter-  274 
mine  in  one  portion,  after  heating  with  sulphurous  acid,  the 
iodine  as  palladium  iodide  (§  145,  I.,  #,  ft\  and  treat  another 
portion  as  directed  §  146.  Deduct  from  the  apparent  amount 
of  iodine  found  by  the  latter  process,  the  actual  quantity  calcu- 
lated from  the  palladium  iodide  ;  the  difference  expresses  the 
amount  of  iodine  equivalent  to  the  chlorine  contained  in  the 
substance. 

5.  ANALYSIS  OF  BROMINE  CONTAINING  CHLORINE. 
a.  Proceed  exactly  as  in  273,  weighing  the  bromine  in  a  275 
small  glass  bulb.     Taking  A  to  be  equal  to  the  analyzed  bro- 
mine, B  to  the  silver  bromide  and  chloride  obtained,  x  to  the 
bromine  contained  in  A,  y  to  the  chlorine  contained  in  A,  the 
calculation  is  made  by  the  following  equations  : 

x  4-  y  =  A 

-i  '      *J 

and 

^g  -2  -34984  J. 
1-69444 

5.  Mix  the  weighed  anhydrous  bromine  with  solution  of  27$ 
iodide  of  potassium  in  excess,  and  determine  the  separated 
iodine  as  directed  §  146. 


§  169.]  ACIDS    OF   GROUP  II.  755 

From  these  data,  the  respective  quantities  of  bromine  and 
chlorine  are  calculated  by  the  following  equations.  Let  A 
represent  the  weighed  bromine,  i  the  iodine  found,  y  the 
chlorine  contained  in  A,  %  the  bromine  contained  in  A,  then 


_i  —  1-5866  A 
1-9907 

BUNSEN,  the  originator  of  methods  4  and  5,  has  experi- 
mentally proved  their  accuracy.* 

6.  CYANOGEN  FROM  CHLOKINE,  BKOMINE,  OR   IODINE. 

a.  Precipitate  with  solution  of  silver,  collect  the  precipi-  277 
tate  upon  a  weighed  filter,  and  dry  in  the  water-bath  until  the 
weight  remains  constant  ;  then  determine  the  cyanogen  by  the 
method  of  organic  analysis  ;  the  quantity  of  the  chlorine,  bro- 
mine, or  iodine  is  found  by  difference. 

J.  Precipitate  with  solution  of  silver  as  in  277,  dry  the  pre-  278 
cipitate  at  100°  and  weigh.  Heat  the  precipitate,  or  an  ali- 
quot part  of  it,  in  a  porcelain  crucible,  with  cautious  agitation 
of  the  contents,  to  complete  fusion  ;  add  dilute  sulphuric  acid 
to  the  fused  mass,  then  reduce  by  zinc,  filter  the  solution  from 
the  metallic  silver  and  silver  paracyanide,  and  determine  the 
chlorine,  iodine,  or  bromine  in  the  filtrate,  in  the  usual  way 
by  silver.  The  silver  cyanide  is  the  difference.  NEUBATJER 
and  KEENER  f  obtained  very  satisfactory  results  by  this 
method. 

c.  Precipitate  with  solution  of  silver  as  in  277,  weigh  the  pre-  279 
cipitate  and  heat  it,  or  an  aliquot  part,  with  nitric  acid  of  1'2 

sp.  gr.  in  a  sealed  tube  at  100°  for  several  hours,  or  at  150° 
for  one  hour.  The  silver  cyanide  is  completely  decomposed, 
while  the  chloride,  bromide,  and  iodide  are  unaffected.  Filter 
the  contents  of  the  tube,  wash  the  precipitate  and  w^eigh  it, 
the  loss  indicates  the  amount  of  silver  cyanide  (K.  KRAUT;):). 

d.  Determine  the  radicals  jointly  in  a  portion  by  precipi-  280 
tating  with  solution   of  silver,  and  the  cyanogen  in  another 
portion,  in  the  volumetric  way  (§  147,  I.,  5  or  c). 

*  Annal.  d.  Cliem.  u.  P7iarm.,  LXXXVI,  274,  276.  f  76.,  ci,  344. 

\  Zeitschr.f.  analyt.  Chem.,  n,  243. 


756  SEPARATION.  [§  169- 

7.  FERRO-  OK  FERRICYANOGEN  FROM   HYDROCHLORIC 
ACID. 

To  analyze  say  potassium  ferro-  or  ferricyanide,  mixed  with  281 
an  alkali  chloride,  determine  in  one  portion  the  ferro-  or  ferri- 
cyanogen  as  directed  "§  147,  II.,  g ;  acidify  another  portion 
with  nitric  acid,  precipitate  with  solution  of  silver,  wash  the 
precipitate,  fuse  with  4  parts  of  sodium  carbonate  and  1  part 
of  potassium  nitrate,  extract  the  fused  mass  with  water,  and 
determine  the  chlorine  in  the  solution  as  directed  in  §  141. 

8.  SULPHUR  (IN  SULPHIDES)  FROM  CHLORINE. 

The  old  method  of  separating  the  two  radicals  by  means  of  a  282 
metallic  salt  is  liable  to  give  false  results,  as  part  of  the  chlo- 
rine may  fall  down  as  chloride  with  the  sulphide.  We,  there- 
fore, precipitate  both  as  silver  compounds,  dry  the  precipitate 
at  100°,  weigh  it,  and  determine  the  sulphur  in  a  weighed 
portion  ;  or — and  this  is  usually  preferred — determine  in  a 
portion  of  the  solution  the  sulphur  as  directed  in  §  148,  I. ,  a,  J, 
or  <?,  in  another  portion  the  sulphur  -j-  chlorine  in  form  of  silver 
salts.  If  you  employ  a  solution  of  silver  nitrate  mixed  with 
excess  of  ammonia,  for  the  determination  of  the  sulphur,  you 
may,  after  filtering  off  the  silver  sulphide,  estimate  the  chlo- 
rine directly  as  silver  chloride,  by  adding  nitric  acid,  and,  if 
necessary,  more  neutral  silver  solution.  In  this  case  you  must 
take  care  that  the  silver  sulphide  is  pure ;  should  it  contain 
calcium  carbonate,  which  is  not  unlikely  if  calcium  is  present, 
you  remove  this  with  dilute  acetic  acid.  The  weighed  silver 
sulphide  should  be  reduced  by  hydrogen,  and  then  weighed 
again  by  way  of  control.  To  remove  hydrogen  sulphide  from 
an  .acid  solution,  in  order  that  chlorine  may  be  determined  in 
the  latter  by  means  of  silver  nitrate,  H.  ROSE  recommends  to 
add  solution  of  ferric  sulphate,  which  will  effect  the  separa- 
tion of  sulphur  alone;  the  separated  sulphur  is  allowed  to 
deposit,  and  then  filtered  off. 


§  170.]  ACIDS   OF   GROUP   III.  757 

• 

Third   Group. 

NITRIC    ACID — CHLORIC    ACID. 

I.  SEPARATION  OF   THE  ACIDS  OF  THE  THIRD  GROUP  FROM  THOSE 

OF    THE    FIRST    TWO    GROUPS. 

§170. 

a.  If  you  have  a  mixture  of  nitric  acid  or  chloric  acid  with  283 
another  free  acid  in  a  fluid  containing  no  bases,  determine  in 
one  portion  the  joint  amount  of  the  free  acid,  by  the  acidi- 
metric  method  (see  Special  Part),  in  another  portion  the  acid 
mixed  writh  the  chloric  or  nitric  acid,  and  calculate  the  amount 
of  either  of  the  latter  from  the  difference. 

1).  If  you  have  to  analyze  a  mixture  of  a  nitrate  or  chlorate  284 
with  some  other  salt,  determine  in  one  portion  the  nitric  or 
chloric  acid  volumetrically  (§  149,  II.,  d,  a,  /?,  or  ;/,  or  II.,  0, 
and  §  150),  or  the  nitric  acid  by  §  149,  II.,  <z,  J3 ;  and  in 
another  portion  the  other  acid.  I  think  I  need  hardly  remark 
that  no  substances  must  be  present  which  would  interfere  with 
the  application  of  these  methods. 

c.  From  the  chlorides  of  many  metals  whose  carbonates  or  285 
normal  phosphates  are  insoluble,  chlorates  and   nitrates  may 

be  separated  also  by  digesting  the  solution  with  recently  pre- 
cipitated thorojLighly  washed  silver  carbonate  or  normal  silver 
phosphate  in  excess,  and  boiling  the  mixture.  In  this  process, 
the  chlorides  react  with  the  carbonate  or  phosphate — silver 
chloride  and  carbonate  or  phosphate  of  the  metal  with  which 
the  chlorine  was  originally  combined  being  formed,  which 
both  separate,  together  with  the  excess  of  the  silver  carbon- 
ate or  phosphate,  whilst  the  chlorates  and  nitrates  remain  in 
solution  (H.  ROSE,  CHENEVIX,  LASSAIGNE*). 

d.  The  estimation  of  an  alkaline  chlorate,  in  presence  of  286 
a  chloride,  may  be  effected  also  by  precipitating  one  portion 

at  once,  and  another  portion  after  gentle  ignition,  with  solu- 
tion of  silver,  and  calculating  the  chloric  acid  from  the  differ- 
ence between  the  two  precipitates.  Or,  estimate  in  one  por- 
tion the  chlorine  content  by  means  of  silver  solution,  at  once, 

*  Journ.  de  Pharm.,  xvi,  289;  Pharm.  CentralbL,  1850,  121. 


758  SEPARATION.  [§  17(X 

and  in  another  portion  after  previous  reduction  of  the  chloric 
acid  with  nitrous  acid  or  ferrous  hydroxide  (§150,  II.,  o 
and  d). 

e.  Where  you  have  sodium-  or  potassium  nitrate  in  presence  287 
of  nitrate  or  carbonate,  as  for  instance  in  the  commercial 
alkali  nitrates,  estimate  in  one  portion  the  carbonate  by  stand- 
ard acid  (§  219),*  in  another  portion  the  nitrous  acid  by 
potassium  permanganate  or  chromate  (p.  433).  The  nitrate 
is  found  by  difference. 

For  estimating  nitric  and  nitrous  acids,  when  only  one 
base  is  present,  e.g.,  either  sodium  or  potassium,  an  indirect 
method  may  also  be  employed.  Intimately  mix  the  weighed 
substance  with  powered  ammonium  chloride,  heat  moderately 
in  a  porcelain  crucible  until  the  excess  of  ammonium  chloride 
and  the  decomposition  products  have  been  expelled,  dissolve 
the  residue  in  water,  and  titrate  the  sodium  chloride  formed 
(if  a  sodium  salt  has  been  present)  with  silver  solution  (§  141, 
I.,  5,  a).  After  the  necessary  corrections  have  been  made 
for  the  slight  quantity  of  sodium  carbonate  present,  i.e.,  de- 
ducting the  carbonate  fron  the  weight  of  the  substance  taken, 
and  the  weight  of  the  sodium  chloride  equivalent  to  it  from 
the  sodium  chloride  found,  the  necessary  data  for  calculation 
will  be  at  hand.  From  the  residual  sodium  chloride  calculate 
its  equivalent  in  sodium  nitrate,  and  deduct  from  this  the 
total  weight  of  the  sodium  nitrate  and  nitrite,  and  thus 
obtain  the  difference  corresponding  to  the  sodium  nitrite, 
according  to  the  following  proportions :  1 6  (the  difference 
between  the  equivalents  of  NalSTO,  and  NaNO9)  :  85'09 
(the  equivalent  of  JS"aNO3) : :  the  remainder  in  question  :#; 
x  being  the  quantity  of  sodium  nitrite  in  the  sub- 
stance taken.  On  finally  deducting  the  sodium  carbonate 
and  nitrite  from  the  substance  taken,  the  sodium  nitrate 
is  found.  Of  course  this  method  is  applicable  only  when 
no  other  substances  are  present  (compare  TicHBORNE,f 
and  my  own  report  on  this  method  :f).  A  similar  indirect 
method  is  also  based  on  the  expulsion  of  the  nitrous,  nitric 
(and  carbonic)  acids  by  vitrified  borax,  §  139,  II.,  c,  and 
§  149,  II.,  a,  /?,  and  also  on  the  different  oxidizing  action 

*  The  alkali  nitrates  do  not  react  alkaline. 

\Chem.  News,  1865,  No.  304.          \Zeitschr.f.  analyt.  Chem.,  iv,  446. 


§  170.]  ACIDS    OF   GROUP   III.  759 

of  nitrous  and  nitric  acids  on  ferrous  sulphate  solution  acidu- 
lated with  hydrochloric  acid  (p.  577),  compare  C.  D.  BKAUN.* 

II.   SEPARATION  OF  THE  ACIDS  or  THE  THIRD  GROUP  FROM 

EACH  OTHER. 

"We  have  as  yet  no  method  of  effecting  the  direct  separation  288 
of  nitric  acid  from  chloric  acid ;  the  only  practicable  way, 
therefore,  is  to  determine  the  two  acids  jointly  in  a  portion  of 
the  compound,  by  the  method  described  for  nitric  acid,  §  149, 
II.,  d,  /?,  bearing  in  mind  that  12  atoms  iron  are  converted 
from  a  ferrous  to  a  ferric  salt  by  2  mol.  chloric  acid  (HC1O3) 
or  1  mol.  chloric  anhydride  (01,0.)  §  150,  II.,  I).  In 
another  portion  estimate  the  chloric  acid  by  adding  sodium 
carbonate  in  excess,  evaporating  to  dryness,  fusing  the  residue 
until  the  chlorate  is  completely  converted  into  chloride,  and 
then  determining  the  chlorine  in  the  latter,  taking  care  that 
the  silver  chloride  contains  no  difficultly  soluble  nitrite.  2 
mol.  silver  chloride  produced  from  this  corresponds  to 
2HC1O,  or  ClaO§,  provided  there  was  no  chloride  originally 
present. 

*  Zeitschr.f.  analyt.  Chem.,  vi,  47. 


INDEX. 


PAGE 

\cid  boric,  determination 465 

as  potassium  borofluoride 466 

separation  from  basic  radicals 468 

arsenic,  separation  from  alkalies,  alkali  earths,  zinc,  etc 711 

from  arsenous  acid 716,  721, 729 

from  barium,  strontium,  calcium,  and  lead.. .   713 
from  copper,  cadmium   iron    (ic),  manganese, 

etc 712 

from  manganese,    iron,    zinc,    copper,    nickel, 

and  cobalt 710 

from  metals  of  groups  I  and  n 713 

from  metals  of  groups  i-rv.  - 712 

from  tin  and  antimony 721 

arsenous,  determination  gravimetricaHy,  indirectly 419 

by  Rose's  method 419 

by  Vohl's  method 419 

separation  from  arsenic  acid 716, 721,  729 

carbonic,  determination  by  Dietrich's  method 504 

by  Kolbe's  method 493 

by  measuring  the  gas 500 

by  Pettenkofer's  method 484 

by  Rose's  method 496 

by  Scheibler's  method 500 

gravimetrically 482 

volumetrically 483 

in  carbonates 487 

in  gases 479 

with  barium    chloride    or   calcium   chlo- 
ride and  ammonia 481 

with  calcium  hydroxide 480 

Geissler's  apparatus  for  determining 491 

separation  from  all  other  acids «, 738 

from  basic  radicals « c . . 487 

table  of  absorption  of.  . . 5£S 

761 


-      INDEX.  763 

PAGE 

A.tid  oxalic,  determination  as  carbonic  acid 471 

with  gold,  by  Rose's  method 470 

with  permanganate 470 

separation  from  basic  radicals 471 

phosphoric,  determination  as  ferric  phosphate 452 

as  lead  phosphate 445 

as  magnesium  phosphate  by  Schulze's 

method 453 

as  magnesium  pyrophosphate 445 

as  uranyl  pyrophosphate 451 

by  Chancel's  method 450 

by  Girard's  method 449 

by  Xeubauer's  method 454 

by  Reissig's  method 448 

by  Rose's  method 448 

by  Sonnenschein's  method 446 

by  Weeren's  method 452 

volumetrically 453 

•eparation  from  alkalies,  barium,  calcium,  lead,  and 

strontium 457 

from  all  bases 464 

from  aluminium  and  magnesium. 458 

from  basic  metals 462 

from  chromium 4€0 

from   metals    of    the    second,   third,   and 

fourth  groups 460 

of  fifth  and  sixth  groups  . .  462 

eelenous,  determination 429 

silicic 233 

determination 505 

by  fusion  with  alkali  carbonates 511 

by  Mitscherlich's  method 521 

by  Smith's  method 519 

in  compounds  decomposable  by  Hd  or  HXQ» .  509 

with  ammonium  fluoride 516 

with  barium  hydroxide  or  carbonate 517 

with      calcium      carbonate     and     ammonium 

chloride 

with  hydrofluoric  acid 513 

with  hydrogen  potassium  fluoride 516 

separation  from  all  other  acids 

from  basic  radicals 509 

sulphuric,  determination 434 

by  Rohli^s  method , 

by  Clemm's  method 436 


764  INDEX. 

PAGE 

Acid  sulphuric,  determination  by  Mohr's  method 435 

by  Wildenstein's  method 437 

in  presence  of  sulphates 442 

separation  from  all  other  acids 731 

from    barium,    calcium,    lead,    and    stron- 
tium    441 

from  mercury  in  mercurous  sulphate 442 

sulphurous 149 

determination 431 

thiosulphuric,  determination  of 432 

Acids  arsenous  and  arsenic,  separation  from  all  other  acids 730 

separation  of 739 

Air-baths 63 

Alkalies  in  f errocyanides,  determination  of 554 

Alumina 180 

Aluminium  hydroxide 179 

oxide ISO 

separation  from  alkali-earth  metals 623 

from  ammonium 622 

from  barium  and  strontium 627 

from  calcium 627 

from  chromium 630 

from  iron  (ic) 646 

from  iron  (ic  and  ous),  cobalt,  and  nickel 643 

from  magnesium  and  calcium 628 

from  potassium  and  sodium 622 

from  radicals  of  the  fourth  group 640 

from  uranyl 674 

from  zinc,  cobalt,  and  nickel 656 

Ammonia-iron  alum 147 

Ammonium,  arseno-molybdate 224 

carbonate 142 

chloride 143,  167 

determination  as  ammonia 253 

as  ammonium-platinic  chloride 252 

as  chloride 252 

as  nitrogen 256 

as  oxide 278 

-ferrous  sulphate 146 

-hydrogen  fluoride 142 

-magnesium  arsenate 222 

-magnesium  phosphate 177 

-manganese  phosphate 188 

molybdate 136 

nitrate 142 


INDEX.  765 


Ammonium,  phosphate 135 

phospho-molybdate 230 

-platiiiic  chloride 167 

separation  from  metals  of  the  fourth  group 631 

from  potassium  and  sodium 604 

from  sodium 603 

succinate 135 

Analysis,  volumetric 122 

Antimony 218 

determination  as  sulphide  (ous) 396 

as  tetroxide 398 

by  decomposing  the  sulphide 403 

by  Kessler's  method 400 

by  Mohr's  method 400 

by  Schneider's  method 403 

with  dichromate 401 

with  permanganate 402 

volumetrically 400 

separation  from  antimonic  acid 729 

from  arsenic „ 720 

from  lead 714 

from  mercury 708 

from  metals  of  groups  iv  and  v  in  alloys 707 

from  tin 716 

from  tin  and  arsenic 718 

sulphide 217 

tetroxide 218 

Arsenic,  see  also  acid  arsenous. 

determination  as  ammonium-magnesium  arsenate 412 

as  arsenate 411 

as  sulphide  (ous) 414 

as  uranyl  pyroarsenate 413 

by  Bunsen's  method 417 

by  Kessler's  method 417 

by  Mohr's  method 416 

by  Werther's  method. 413 

in  tin 726 

volumetrically 416 

f eparation  from  antimony 720 

from  antimony  and  tin 722 

from  antimony  in  alloys 718 

from  copper 714 

from  metals  of  groups  n,  iv,  and  v 708 

from  tin 717,  728 

sulphide,  determination  in  antimony  sulphide 720 


766  INDEX. 

PAGE 

Arsenous  acid,  see  acid  arsenous. 

oxide 149 

sulphide 221 

Asbestos  filters 120 

Aspirator,  Bunsen's 103 

Balance,  testing,  etc 12 

Barium  acetate 137 

carbonate 138, 170 

chloride 137 

chromate 226 

determination  as  carbonate 264 

as  sulphate 263 

separation  from  calcium 617 

from  potassium  and  sodium 607,  608 

from  strontium  and  calcium 616,  617' 

silicofluoride 171 

sulphate 168 

Belohoubeck's  method  of  determining  uranium 336 

Berzelius'  method  of  separating  phosphoric  acid  from  aluminium 459 

Berzelius-Rose's  method  of  determining  sulphur 564 

Bismuth 212 

and  copper,  separation  from  lead  and  cadmium 692 

carbonate 212 

chloride,  basic 212 

chromate 212 

determination  as  arsenate 387 

as  carbonate 383 

as  chromate 385 

as  metal 386 

as  trioxide 383 

as  trisulphide 383,  384 

by  Lowe's  method 385 

•eparation  from  all  other  metals 684,  690 

from  cadmium 694 

from  copper 692 

from  copper  cadmium,  and  mercury  (ic) 692 

from  lead  and  cadmium 0 692 

from  silver,  lead,  and  copper 696 

trioxide 211 

trisulphide 213 

Bohlig's  method  of  determining  chlorine 526 

of  determining  ferrocyanides 557 

of  determining  sulphuric  acid 436 

Borax .140 


INDEX.  767 

PAGE 

Boric  acid,  see  acid  boric. 

anhydride,  determination  of 465 

Bromine  containing  chlorine,  analysis  of 754 

determination  as  silver  bromide 532 

colorimetrically  by  Heine's  method 534 

gravimetrically  and  volumetrically 532 

in  free  state 536 

with   chlorine   water  and   chloroform  by  Rei- 

mann's  method 532 

with    chlorine    water    and    heat    by    Figuier's 

method 533 

separation  from  chlorine 744 

from  chlorine  and  iodine 750 

from  metals 535 

Bunsen's  aspirator 103 

method  of  determining  arsenic 417 

of  determining  chlorine  volumetrically 530 

of  determining  chromic  acid 424 

of  determining  sulphur 566 

Burette,  Gay-Lussac's 48 

Geissler's 49 

Mohr's 42 

Burettes 42 

Cadmium  carbonate 214 

determination  as  oxide 388 

as  sulphate 389 

as  sulphide 388 

oxide 213 

separation  from  copper 693 

separation  from  zinc 684 

sulphide 214 

Calcium 132 

carbonate 173 

chloride 155 

determination  as  carbonate 269 

as  oxide 269 

as  sulphate 269 

by  volumetric  methods 273 

fluoride 232 

oxalate 175 

separation  from  aluminium 627 

from  magnesium 618,  619 

from  nickel  and  cobalt 633.  638 

from  potassium  and  sodium 607,  609 

from  strontium 619 


768  INDEX. 

PAGE 

Calcium  sulphate 173 

Carbon  dioxide,  see  acid  carbonic. 

disulphide ,   128 

Carbonic  acid,  see  acid  carbonic. 

Chancel's  method  of  determining  phosphoric  acid 450 

Chlorates,  see  acid  chloric. 
Chloric  acid,  see  acid  chloric. 

Chlorides,  determining  in  presence  of  fluorides 741 

Chlorine • 143 

determining   alkalimetrically  by  Bohlig's  method 526 

as  chloride 521 

by  silver  nitrate  (volumetrically) 522 

gravimetrically 531 

in  free  state 529 

in  silicates 740 

volumetrically   with   potassium   iodide   by   Bun- 
sen's  method 530 

with  mercuric    nitrate,  by    Liebig's  method....   525 
with    silver    nitrate    and    starch    iodide,    by  Pi- 

sani's  method   524 

separation  from  bromine 744 

from  iodine  and  bromine 750 

from  iodine 748 

from  metals 527 

Chromic  acid,  see  acid  chromic. 
Chromium,  see  also  acid  chromic. 

determination  as  oxide 281 

hydroxide 181 

separation  from  alkali-earth  metals 628 

from  aluminium 630 

from  ammonium 622 

from  barium,  strontium  and  calcium 629 

from  metals  of  fourth  group 653 

from  potassium  and  sodium 622 

from  radicals  of  the  fourth  group 640 

Clemm's  method  of  determining  sulphuric  acid 436 

Clips 43,  44 

Cobalt 192 

and  nickel,  separation  from  barium  and  strontium 633,634,638 

from  manganese 651 

from  manganese  and  iron 651 

from  manganese  and  zinc -, 659 

from  zinc 659 

determination  as  hydroxide 306 

as  metal 306 

hydroxide  (ous) 191 


INDEX.  769 

PAGE 

Cobalt  separation  from  alkalies 632 

from  nickel 654,  656,  665 

from  nickel  manganese  and  zinc 655 

from  zinc 657 

sulphate  (ous) 193 

sulphide 192 

-tripotassium  nitrite 193 

Copper 133, 154,  208 

determination  as  metal 373 

as  oxide  (ic) 371 

as  sulphide 375,  379 

as  sulphocyanate  (ous) 376,  382 

by  De  Haen's  method 377 

by  Fleck's  modification  of  Parke's  method 378 

by  Fleischer's  method 382 

by  Fleitmann's  method 382 

by  Parkes'  method 378 

by  reduction  with  stannous  chloride 380 

by  Rivot's  method 376 

by  Schwarz's  method 381,  382 

by  de  Weil's  method 380 

electrolytically  .  .  .  . 375 

volumetrically 377 

oxide  (ic) 151,  208 

separation  from  arsenic 714 

from  arsenic  and  antimony 714 

from  bismuth 692 

from  cadmium 693,  694 

(ic)  from  (ous) 697 

(ous)  from  (ic) 697,  698 

from  iron 683 

from  mercury  (ic)  and  cadmium 691 

from  nickel 683 

from  other  metals 680 

from  zinc 683 

sulphide  (ic) 210 

sulphide  (ous) 21 1 

sulphocyanate  (ous) 210 

Cyanides,  see  cyanogen. 

Cyanogen,  determining  by  Liebig's  volumetric  method 549 

in      mercuric      cyanide      by      Rose-Finkener's 

method 552 

separation  from  chlorine,  bromine,  or  iodine 755 

from  the  metals 551 

volumetric  determination  by  Fordos-Gelis's  method 55G 


770  INDEX 

PAGE 

Decantation 93 

De  Haen's  method  of  determining  copper 377 

of  determining  f erro-  and  f  erri-cyanides 554 

Dessication 54 

Dessicators 56 

Dietrich's  method  of  determining  carbonic  acid 504 

Drying 54 

Drying-disk 67 

Duflos'  method  of  determining  iodine 540 

Dupasquier's  method  of  determining  hydrogen  sulphide 558 

Elutria-tion 53 

Erdmann's  Float 47 

Eudiometer 28 

Evaporation  .  .  , 81 

Ferricyanides,  determination  by  Lenssen's  method 556 

by  Bohlig's  method 557 

by  Rheineck's  method 557 

Perrocyanogen,  separation  from  hydrochloric  acid 756 

volumetric  determination  by  De  Haen's  method 554 

Figuier's  method  of  determining  bromine 533 

Filter,  Gooch 120 

Filtering 94 

Filters,  asbestos 120 

Fleck's  modification  of  Parkes'  method  of  determining  copper 378 

Fleischer's  method  of  determining  copper 382 

Fleitmann's  method  of  determining  copper 382 

Float,  Erdmann's 47 

Fluids,  measuring 36 

Fluorides,  determination  by  decomposition  with  alkali  carbonates....  474 

by  decomposition  with  sulphuric  acid 474 

from  silicon  fluoride  evolved 475 

Fluorine,  separation  from  metals 473 

Fordos-Gelis's  method  of  determining  cyanogen  volumetrically 550 

Fuch'e  method  of  determining  ferric  iron 334 

Gas-lamp 82 

Gases,  measuring 27 

reading-off 30 

Gay-Lussac's  burette 48 

method  of  determining  silver 342 

Geissler's  apparatus  for  determining  carbonic  acid 491 

burette 49 

Oirard's  method  of  determining  phosphoric  acid 449 


INDEX.  771 

PAGE 

Gold 215 

determination  as  metal 391 

as  sulphide  (ic) 393 

in  platinum  ore 727 

separation  from  lead  and  bismuth 715 

from  metals  of  group  1 705 

from  metals  of  groups  iv.  and  v.  in  alloys 703 

from  platinum 716,  727 

from  silver 713 

from  tin 727 

sulphide 215 

Gooch  filters 120 

Gunpowder  residues,  examination  of,  by  Werther's  method 742 

Harcourt's  method  of  determining  nitric  acid  as  ammonia 585 

Heine's  colorimetric  method  of  determining  bromine 534 

Hydrofluoric  acid,  see  hydrofluoric  acid. 

Hydrofluosilicic  acid,  see  acid  hydrofluosilicic. 

Hydrogen 143 

-ammonium  fluoride 142 

-potassium  fluoride 141 

sulphide,  determination  by  Mohr's  method 560 

with  iodine  by  Dupasquier's  method. .  558 

lodic  acid,  see  acid  iodic. 

Iodine 148 

containing  chlorine,  analysis  of 753 

determination  as  silver  iodide 536 

as  palladious  iodide  by  Lassaigne's  method 536 

colorimetrically  by  Struve's  method 541 

in  free  state  by  Schwarz's  method ..542,  543* 

with  ferric  chloride  by  Duflos'  method 540 

with  nitrous  acid  and  carbon  disulphide 537 

with  palladious  chloride  by  Kersting's  method.  .  540 

with  permanganate  by  Reinige's  method 538 

with     silver     solution     and     starch     iodide     by 

Pisani's  method 539 

volumetrically 537 

separation  from  chlorine 748 

from  chlorine  and  bromine 750 

from  metals 541 

Iron  acetate,  1  >asic  (ic) 197 

Iron-alum 147 

-ammonium  sulphate   (ous) 146 

arsenal?  (ic) 224 

converting  ferrous  into  ferric 311 


772  INDEX. 

PAGE 

Iron  ferric,  determination  as  oxide  or  hydroxide 323 

as  sulphide 323,  325 

by  Oudeman's  method 332 

by  reduction  with  hydrogen  sulphide 326 

by  reduction  with  stannous  chloride 327 

by  reduction  with  zinc 325 

volumetrically 325 

with  thiosulphate 331 

with  thiosulphate  and  copper  sulphate ......   332 

Fuch's  method  of  determining 334 

separation  from  aluminium 646,  652,  660 

from  aluminium  and  chromium    652 

from  barium  and  strontium 633,  634 

from  calcium  and  magnesium  .    633,  634 

from  ferrous  iron 664,  666 

from  ferroUo  iron,  zinc,  and  nickel 661 

from  manganese,  nickel,  cobalt,  and  zinc ....  644,  649 
from     manganese,     zinc,    cobalt,     nickel,    and 

ferrous  iron    647 

from  potassium  and  sodium      632 

from  radicals  of  the  fourth  group 640 

from  uranium 675 

ferrous,  determination 311 

as  metal    313 

by  Penny's  method 319 

volumetrically  .  312 

with  ammonium-ferrous  sulphate      315 

with  oxalic  acid         ; 316 

with  permanganate 313 

separation  from  ferric  iron 645 

formate,  basic  (ic) 197 

hydroxide  (ic) 194 

oxide  (ic) 195 

phosphate  (ic) 227 

separation  from  copper 683 

succinate,  basic  (ic) 196 

sulphide  (ous) 195 

Kersting's  method  of  determining  iodine  540 

Kessler's  method  of  determining  antimony    .  .  . . 400 

of  determining  arsenic 417 

Kolbe's  method  of  determining  carbonic  acid 493 

Lamp,  Haste's    82 

Lassaigne's  method  of  determining  iodine      536 

Lead  arsenate .  221 


INDEX.  773 

PAGE 

Lead  carbonate,  normal , 201 

chloride 203 

chromate 152,  225 

determination  as  chloride 357 

as  chromate 356 

as  metal 358 

as  oxide 353 

as  oxide  4-  lead 357 

as  sulphate 355 

as  sulphide 354 

by  Schwarz's  method 360 

volumetrically 359 

oxalate 202 

oxide 134,  202 

phosphate 227 

separation  from  antimony 714 

from  bismuth 697 

from  other  metals 689,  690 

from  silver 693 

sulphate 202 

sulphide 204 

Lenssen's  method  of  determining  f erricyanides 556 

of  determining  tin 408 

Levigation 52 

Liebig's  method  of  determining  chlorine 525 

of  determining  cyanogen  volumetrically 549 

Lime 132 

Liquids,  reading-off 46 

Lithium,  determination  of 258 

separation  from  other  alkalies 605 

Litmus,  tincture 145 

Lowe's  method  of  determining  bismuth 385 

Magnesium-ammonium  arsonate 222 

ammonium  phosphate 177 

chloride , 138 

determination  as  oxide 276 

as  pyrophosphate 275 

as  sulphate 275 

oxide 179 

phosphate 227 

pyroarsenate 223 

pyrophosphate 178 

separation  from  barium  and  strontium 617 

from  calcium 619 

from  potassium  and  sodium 610 


774  INDEX. 

PAGE 

Magnesium,  separation  from  uranium 674 

sulphate  .- 176 

Manganese-ammonium  phosphate 188 

carbonate ^  185 

determination  as  carbonate 293 

as  dioxide 294 

as  hydroxide 294 

as  protosesquioxide 293 

as  pyrophosphate 297 

as  sulphate 297 

as  sulphide 295 

volumetric-ally 298 

with  potassium  f erricyanide 298 

with  potassium  permanganate 300 

dioxide 186 

hydroxide  (ous) 186 

protosesquioxide 186 

pyrophosphate 189 

separation  from  alkalies 632 

from  aluminium  and  iron 665 

from  barium  and  strontium 633,  634,  635,  636 

from  cobalt  and  nickel 651 

from  lead,  bismuth,  cadmium,  and  copper 685 

from  nickel  and  cobalt 633,  638 

from  nickel  and  zinc 644 

from  zinc 665 

sulphate,  anhydrous  (ous) 188 

sulphide 187 

Marguerite's  method  of  ferrous  determination 312 

Maste's  Lamp 82 

Measuring 26 

Mechanical  division 51 

Mercury 205 

chloride  (ous) 205 

chromate  (ous) 226 

mercuric,  determination  as  choride  (ous) 366 

determination  as  metal 364 

as  oxide 367 

as  sulphide 366 

by  Scherer's  method 369 

volumetrically , 367 

separation  from  mercury  (ous)  copper,  cadmium,  and 

lead 688 

mercurous,  determination  as  chloride » 361 

determination  volumetrically 362 


INDEX.  775 

PAGE 

Mercury,  mercurous,  separation  from  mercury  (ic),  copper,  cadmium, 

bismuth,  and  lead 688 

oxide  (ic) 134,  207 

phosphate  (ous) 230 

separation  from  antimony 708 

from  arsenic  and  antimony  oxides 713 

from  gold  and  silver 710 

from  metals • 679 

from     silver,     bismuth,     copper,     cadmium,     and 

lead 694 

sulphide  (ic) 206 

Metals  in  cyanides,  determination  of 553 

Mitscherlich's  method  of  determining  silicic  acid 521 

Mohr's  burette 42 

method  of  determining  antimony 400 

of  determining  arsenic 416 

of  determining  hydrogen  sulphide- .  560 

of  determining  sulphuric  acid 435 

Moisture,  influence  of  upon  gases,  in  reading-off 34 

Molybdic  acid,  see  acid  molybdic. 

Mortreux's  method  of  determining  sulphur  in  free  state 570 

Mviller's    modified    Schulze's    method    of  determining  phosphoric  acid 

as  ferric  phosphate 452 

Neubauer's  method  of  determining  phosphoric  acid 454 

Nickel 190 

and  cobalt,  separation  from  barium  and  strontium 633,  634,  638 

determination  as  metal 304 

as  nickel  tripotassium  nitrate 307 

as  oxide  and  hydroxide 302 

as  sulphate •. 304,  308 

as  sulphide 303, 307 

volumetrically 305,  308 

hydroxide  (ous) 189 

oxide  (ous) 189 

separation  from  alkalies 632 

from  copper 683 

from  zinc 658 

sulphide,  hydrated  (ous) 190 

Nitric  acid,  see  acid  nitric. 

Nitrogen 168 

table  of  absorption . .  259 

table    of    weight    of     1    c.c.    at   different    temperatures    and 

pressures 260 

Nitrous  acid,  see  acid  nitrous. 


776  INDEX. 

PAGE 

Oil-baths 66 

Operations 11 

Otto's  method  of  separating  phosphoric  acid  from  aluminium 459 

Oudeman's  method  of  determining  ferric  iron 332 

Oxalic  acid,  see  acid  oxalic. 

Oxygen 153 

Palladium,  determination  as  chloride  (ic) 390 

as  metal 390 

iodide  (ous) : 237 

Pelouze's  method  of  determining  nitric  acid  with  ferrous  chloride.  .  .   573 
Penfield's     method     of     determining     silicon     fluorides     evolved    from 

fluorides 478 

Penny's  method  of  ferrous  iron  determination 319 

Pettenkofer's  method  of  determining  carbonic  acid 484 

Phosphates,  see  acid  phosphoric. 
Phosphoric  acid,  see  acid  phosphoric. 

Pinchcocks 43,  44 

Pipettes 39 

Pisani's  method  of  determining  chlorine 524 

of  determining  iodine 539 

of  determining  molybdic  acid 421 

of  determining  silver 349 

Vogel's  modification  of 351 

Platinum , 216 

and  gold,  separation  from  tin,  antimony,  and  arsenic 716 

determination  as  metal 393 

as  potassium-platinic  chloride 394 

as  sulphide  (ic) 395 

separation  from  gold 716,  727 

from  metals  of  groups  iv  and  v  in  alloys 705 

sulphide  (ic) 216 

Potassa 131 

fused 155 

solution 155 

Potassium  borofluoride 232 

chloride 162 

-cobaltic  nitrite 193 

cyanide 136 

determination  as  chloride 245 

as  nitrate 244 

as  potassium-platinic  chloride 245 

as  silicofluoride 248 

as  sulphate 243 

dichromate 156 

disulphate 141 


INDEX.  777 


Potassium  hydroxide •. 131 

-hydrogen  fluoride 141 

iodide 148 

nitrate 162 

permanganate 145 

-platinic  chloride 163 

separation  from  sodium 599,  604 

silicofluoride '  164 

sulphate 161 

Precipitates,  drying 110 

igniting 112 

washing 98 

Precipitation,  effecting 91 

.Pressure,  influence  of,  upon  gases  in  reading-off 33 

Radicals,  determination  of 239 

Reagents 127 

Reimann's  method  of  determining  bromine 532 

Reinige's  method  of  determining  iodine 538 

Reissig's  method  of  determining  phosphoric  acid 448 

Rheineck's  method  of  determining  ferrocyanides 557 

Rivot's  method  of  determining  copper 376 

Rivot-Beudant-Daguin's  method  of  determining  sulphur 568 

Rose's  method  of  determining  arsenous  acid 419 

carbonic  acid 496 

oxalic  acid 470 

phosphoric  acid 448 

Rose-Finkener's  method  of  determining  cyanogen  in  mercuric  cyanide  552 

Samples,  selection  of 50 

Scheibler's  method  of  determining  carbonic  acid 500 

Scherer's  method  of  determining  mercury  (ic) "...  .  .  369 

Schneider's  method  of  determining  antimony 403 

Schlosing's  method  of  determining  nitric  acid 579 

Schulze's  method  of  determining  nitric  acid 5S2 

nitric  acid  from  loss  of  hydrogen ....  588 
phosphoric  acid  as  magnesium  phos- 
phate    453 

Schwarz's  method  of  determining  chromic  acid 424 

copper 3S1 ,  3S2 

free  iodine 542,  543 

lead 360 

Selenium,  determination 429 

Selenous  acid,  see  acid  selenous. 

Sifting 53 

Silica  Csee  also  acid  silicic) 233 


778  INDEX. 

PAGE 

Silicic  acid,  see  acid  silicic. 

Siewert/s  method  of  determining  nitric  acid  as  ammonia .  « 587 

Silver 1 50, 198 

bromide 236 

chloride 198 

cyanide 201 

determination  as  chloride 338 

as  cyanide 341 

as  metal 341 

as  sulphide ,   340 

by  Gay-Lussac's  method 342 

by  Pisani's  method 349 

volumctrically 342 

oxide 236 

phosphate,  normal 230 

separation  by  cupellation 698* 

from  copper,  cadmium,  bismuth,  mercury,  and  lead..   686 

from  gold 713 

from  lead 693 

from  mercury  (ic),  copper,  and  cadmium 690 

from  metals 679 

sulphide 200 

Smith's  method  of  determining  silicic  acid 519 

Soda 131 

-lime 153, 154 

Sodium  carbonate 135 

anhydrous 166 

chloride 150,  165 

determination  as  carbonate 250 

as  chloride 250 

as  nitrate 249 

as  sulphate 249 

disulphate 141 

hydroxide 131 

nitrate 165 

-platinic  chloride 166 

separation  from  ammonium 603 

from  potassium 599,  604 

silicofluoride , 167 

sulphate,  anhydrous 164 

thiosulphate 135 

Solution  of  substances 79 

potassa 155 

stannous  chloride  for  ferric-iron  determination 329 

Sonnenschein's  method  of  determining  phosphoric  acid 446 

Starch  iodide  . 349 


INDEX.  779 

PAGE 

Stolba's  method  of  determining  hydrofluosilicic  acid 443 

Strontium  carbonate e    172 

determination  as  carbonate 267 

as  sulphate 266 

separation  from  calcium 619^  @21 

from  potassium  and  sodium 607,  609 

sulphate 17': 

Struve's  method  of  determining  iodine  colorimetrically 541 

Substances,  converting  into  weighable  forms 81 

Sulphides,  determining  in  presence  of  carbonates 742 

in  silicates 742 

Sulphur,  determination  as  hydrogen  sulphide 558,  562,  569 

by  Berzelius-Rose's  method 564 

by  Bunsen's  method 566 

by  Rivot-Beudant-Daguin's  method 568 

in  free  state  by  Mortreux's  method 570 

in  sulphides 562,  569 

in  sulphides,  separation  from  chlorine 756 

Sulphuric  acid,  see  acid  sulphuric. 
Sulphurous  acid,  see  acid  sulphurous. 

Table  of  absorption  of  carbonic  acid 508 

of  weight  of   1    c.c.   of   carbonic   acid  at  various  temperatures 

and  pressures 506 

Temperature,  influence  of  on  gases,  in  reading-off 33 

Tirmann-Schulze's  method  of  determining  nitric  acid 582 

Thiosulphuric  acid,  see  acid  thiosulphuric. 

Tin,  determination  as  oxide  (ic) 405 

as  stannic  (or  metastannic)  acid  ...... 405 

as  sulphide  (ous  or  ic) 406 

by  alkaline  iodine  solution 408 

by  Lenssen's  method 408 

with  ferric  chloride 408 

volumetrically 407 

oxide  (ic) 219 

phosphate  (ic) 230 

separation  from  antimony 716,  725 

from  antimony  and  arsenic 723,  726,  72S 

from  arsenic 717,  728 

from  gold 727 

from  metals  of  groups  i,  n,  and  in 707 

from  metals  of  groups  iv  and  v 706 

from  stannic  tin 730 

sulphide,  hydrated  (ic)  and  (ous) 220 

Titanium,  determination  of 2s4 

Tripotassium  cobaltic  nitrite 193 

Uranium  acetate : 139 


OF  THE 

UNIVERSITY 


780  INDEX. 

PAGE 

Uranium  determination 335 

by  Belohoubeck's  method 336 

separation  from  aluminium 674 

from  barium,  calcium,  and  strontium 673 

from  chromium 674 

from  cobalt,  nickel,  and  zinc 675 

from  iron  (ic) 675 

from  magnesium 674 

from  other  metals  of  groups  i-iv 672 

Uranyl  pyroarsenate 223 

pyrophosphate 229 

Vogel's  modification  of  Pisani's  method 351 

Vohl's  method  of  determining  arsenous  acid 419 

of  determining  chromic  acid .-423 

Wackenroder-Fresenius'   method  of  separating  phosphoric  acid  from 

aluminium 459' 

Water,  distilled 127 

estimating. 72 

Water-bath 58 

Weeren's  method  of  determining  phosphoric  acid  by  Miiller's  modi- 
fied Schulze's  method 452 

Weighing,  process  of 21,  70 

Weights,  testing,  etc .* 19 

Weil's  method  of  determining  copper 380 

Wells'  apparatus  for  determining  carbonic  acid 499 

Werther's  method  of  determining  arsenic  as  uranyl  pyroarsenate 413 

of  examining  gunpowder  residues 742 

Wildenstein's  method  of  determining  sulphuric  acid 437,  438 

Wohler's  method  of   determining   silicon   fluoride   evolved  from  fluo- 
rides     478 

Zinc 132 

carbonate,  basic 182 

oxide 183 

sulphide 184 

determination  as  carbonate 287 

as  oxide 287 

as  sulphide . . . .  .288,  289 

separation  from  aluminium  and  manganese 649 

from  barium  and  strontium 633,  634 

from  cadmium 684 

from  calcium 633,  634 

from  copper ' 683 

from  iron  in  alloys 660 

from  nickel,  cobalt,  and  manganese 650 

from  potassium  and  sodium 632 


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.-\ 

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by  Thomas  S.  Fiske. 

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12 


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18 


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OF  THE 

UNIVERSITY 

OF 


19 


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APR  2  1 1965 


JAN     7  REC'O 


Book  Slip-20m-9,'60(B3010s4)458 


193957 


F7 


5  7  3  7 


